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Snell's law

Snell's law (also known as the Snell–Descartes law, the ibn-Sahl law, and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air. In optics, the law is used in ray tracing to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a material. The law is also satisfied in meta-materials, which allow light to be bent "backward" at a negative angle of refraction with a negative refractive index.

The law states that, for a given pair of media, the ratio of the sines of angle of incidence ($$\theta_1 $$) and angle of refraction ($$\theta_2 $$) is equal to the refractive index of the second medium with regard to the first ($$n_{21}$$) which is equal to the ratio of the refractive indices ($$\tfrac{n_2}{n_1}$$) of the two media, or equivalently, to the ratio of the phase velocities ($$\tfrac{v_1}{v_2}$$) in the two media.

 * $$\frac{\sin\theta_1}{\sin\theta_2}= n_{21} = \frac{n_2}{n_1} = \frac{v_1}{v_2} $$

The law follows from Fermat's principle of least time, which in turn follows from the propagation of light as waves.

History

Ptolemy, in Alexandria, Egypt, had found a relationship regarding refraction angles, but it was inaccurate for angles that were not small. Ptolemy was confident he had found an accurate empirical law, partially as a result of slightly altering his data to fit theory (see: confirmation bias).

The law was eventually named after Snell, although it was first discovered by the Persian scientist Ibn Sahl, at Baghdad court in 984. In the manuscript On Burning Mirrors and Lenses, Sahl used the law to derive lens shapes that focus light with no geometric aberration.

Alhazen, in his Book of Optics (1021), came close to rediscovering the law of refraction, but he did not take this step.

The law was rediscovered by Thomas Harriot in 1602, who however did not publish his results although he had corresponded with Kepler on this very subject. In 1621, the Dutch astronomer Willebrord Snellius (1580–1626)—Snell—derived a mathematically equivalent form, that remained unpublished during his lifetime. René Descartes independently derived the law using heuristic momentum conservation arguments in terms of sines in his 1637 essay Dioptrique, and used it to solve a range of optical problems. Rejecting Descartes' solution, Pierre de Fermat arrived at the same solution based solely on his principle of least time. Descartes assumed the speed of light was infinite, yet in his derivation of Snell's law he also assumed the denser the medium, the greater the speed of light. Fermat supported the opposing assumptions, i.e., the speed of light is finite, and his derivation depended upon the speed of light being slower in a denser medium. Fermat's derivation also utilized his invention of adequality, a mathematical procedure equivalent to differential calculus, for finding maxima, minima, and tangents.

In his influential mathematics book Geometry, Descartes solves a problem that was worked on by Apollonius of Perga and Pappus of Alexandria. Given n lines L and a point P(L) on each line, find the locus of points Q such that the lengths of the line segments QP(L) satisfy certain conditions. For example, when n = 4, given the lines a, b, c, and d and a point A on a, B on b, and so on, find the locus of points Q such that the product QA*QB equals the product QC*QD. When the lines are not all parallel, Pappus showed that the loci are conics, but when Descartes considered larger n, he obtained cubic and higher degree curves. To show that the cubic curves were interesting, he showed that they arose naturally in optics from Snell's law.

According to Dijksterhuis, "In De natura lucis et proprietate (1662) Isaac Vossius said that Descartes had seen Snell's paper and concocted his own proof. We now know this charge to be undeserved but it has been adopted many times since." Both Fermat and Huygens repeated this accusation that Descartes had copied Snell. In French, Snell's Law is sometimes called "la loi de Descartes" or more frequently "loi de Snell-Descartes".

In his 1678 Traité de la Lumière, Christiaan Huygens showed how Snell's law of sines could be explained by, or derived from, the wave nature of light, using what we have come to call the Huygens–Fresnel principle.

With the development of modern optical and electromagnetic theory, the ancient Snell's law was brought into a new stage. In 1962, Nicolaas Bloembergen showed that at the boundary of nonlinear medium, the Snell's law should be written in a general form. In 2008 and 2011, plasmonic metasurfaces were also demonstrated to change the reflection and refraction directions of light beam.

Explanation
Snell's law is used to determine the direction of light rays through refractive media with varying indices of refraction. The indices of refraction of the media, labeled $$n_1$$, $$n_2$$ and so on, are used to represent the factor by which a light ray's speed decreases when traveling through a refractive medium, such as glass or water, as opposed to its velocity in a vacuum.

As light passes the border between media, depending upon the relative refractive indices of the two media, the light will either be refracted to a lesser angle, or a greater one. These angles are measured with respect to the normal line, represented perpendicular to the boundary. In the case of light traveling from air into water, light would be refracted towards the normal line, because the light is slowed down in water; light traveling from water to air would refract away from the normal line.

Refraction between two surfaces is also referred to as reversible because if all conditions were identical, the angles would be the same for light propagating in the opposite direction.

Snell's law is generally true only for isotropic or specular media (such as glass). In anisotropic media such as some crystals, birefringence may split the refracted ray into two rays, the ordinary or o-ray which follows Snell's law, and the other extraordinary or e-ray which may not be co-planar with the incident ray.

When the light or other wave involved is monochromatic, that is, of a single frequency, Snell's law can also be expressed in terms of a ratio of wavelengths in the two media, $$\lambda_1$$ and $$\lambda_2$$:

 * $$\frac{\sin\theta_1}{\sin\theta_2} = \frac{v_1}{v_2} = \frac{\lambda_1}{\lambda_2}$$

Derivations and formula
Snell's law can be derived in various ways.

Derivation from Fermat's principle
Snell's law can be derived from Fermat's principle, which states that the light travels the path which takes the least time. By taking the derivative of the optical path length, the stationary point is found giving the path taken by the light. (There are situations of light violating Fermat's principle by not taking the least time path, as in reflection in a (spherical) mirror.) In a classic analogy, the area of lower refractive index is replaced by a beach, the area of higher refractive index by the sea, and the fastest way for a rescuer on the beach to get to a drowning person in the sea is to run along a path that follows Snell's law.

As shown in the figure to the right, assume the refractive index of medium 1 and medium 2 are $$n_1$$ and $$n_2$$ respectively. Light enters medium 2 from medium 1 via point O.

$$\theta_1$$ is the angle of incidence, $$\theta_2$$ is the angle of refraction with respect to the normal.

The phase velocities of light in medium 1 and medium 2 are
 * $$v_1=c/n_1$$ and
 * $$v_2=c/n_2$$ respectively.

$$c$$ is the speed of light in vacuum.

Let T be the time required for the light to travel from point Q through point O to point P.
 * $$T=\frac{\sqrt{x^2 + a^2}}{v_1} + \frac{\sqrt{b^2 + (l - x)^2}}{v_2} = \frac{\sqrt{x^2 + a^2}}{v_1} + \frac{\sqrt{b^2 + l^2 -2lx + x^2}}{v_2}$$

where a, b, l and x are as denoted in the right-hand figure, x being the varying parameter.

To minimize it, one can differentiate :
 * $$\frac{dT}{dx}=\frac{x}{v_1\sqrt{x^2 + a^2}} + \frac{ - (l - x)}{v_2\sqrt{(l-x)^2 + b^2}}=0$$ (stationary point)

Note that $$\frac{x}{\sqrt{x^2 + a^2}} =\sin\theta_1$$

and $$\frac{ l - x}{\sqrt{(l-x)^2 + b^2}}=\sin\theta_2$$

Therefore,

 * $$\frac{dT}{dx}=\frac{\sin\theta_1}{v_1} - \frac{\sin\theta_2}{v_2}=0$$

 * $$\frac{\sin\theta_1}{v_1}=\frac{\sin\theta_2}{v_2}$$

 * $$\frac{n_1\sin\theta_1}{c}=\frac{n_2\sin\theta_2}{c}$$

 * $$n_1\sin\theta_1=n_2\sin\theta_2$$

Derivation from Huygens's principle
Alternatively, Snell's law can be derived using interference of all possible paths of light wave from source to observer—it results in destructive interference everywhere except extrema of phase (where interference is constructive)—which become actual paths.

Derivation from Maxwell's equations
Another way to derive Snell's Law involves an application of the general boundary conditions of Maxwell equations for electromagnetic radiation and induction.

Derivation from conservation of energy and momentum
Yet another way to derive Snell's law is based on translation symmetry considerations. For example, a homogeneous surface perpendicular to the z direction cannot change the transverse momentum. Since the propagation vector $$\vec{k}$$ is proportional to the photon's momentum, the transverse propagation direction $$(k_x,k_y,0)$$ must remain the same in both regions. Assume without loss of generality a plane of incidence in the $$z,x$$ plane $$k_{x\text{Region}_1} = k_{x\text{Region}_2}$$. Using the well known dependence of the wavenumber on the refractive index of the medium, we derive Snell's law immediately.

 * $$k_{x\text{Region}_1} = k_{x\text{Region}_2} \, $$

 * $$ n_1 k_0\sin\theta_1 = n_2 k_0\sin\theta_2 \, $$

 * $$ n_1\sin\theta_1 = n_2\sin\theta_2 \, $$

where $$k_0=\frac{2\pi}{\lambda_0}=\frac{\omega}{c}$$ is the wavenumber in vacuum. Although no surface is truly homogeneous at the atomic scale, full translational symmetry is an excellent approximation whenever the region is homogeneous on the scale of the light wavelength.

Vector form
Given a normalized light vector $$\vec{l}$$ (pointing from the light source toward the surface) and a normalized plane normal vector $$\vec{n}$$, one can work out the normalized reflected and refracted rays, via the cosines of the angle of incidence $$\theta_1$$ and angle of refraction $$\theta_2$$, without explicitly using the sine values or any trigonometric functions or angles:

 * $$\cos\theta_1 = -\vec{n}\cdot \vec{l}$$

Note: $$\cos\theta_1$$ must be positive, which it will be if $$\vec{n}$$ is the normal vector that points from the surface toward the side where the light is coming from, the region with index $$n_1$$. If $$\cos\theta_1$$ is negative, then $$\vec{n}$$ points to the side without the light, so start over with $$\vec{n}$$ replaced by its negative.

 * $$\vec{v}_{\mathrm{reflect}}=\vec{l} + 2\cos\theta_1 \vec{n}$$

This reflected direction vector points back toward the side of the surface where the light came from.

Now apply Snell's law to the ratio of sines to derive the formula for the refracted ray's direction vector:
 * $$\sin\theta_2 = \left(\frac{n_1}{n_2}\right) \sin\theta_1 = \left( \frac{n_1}{n_2} \right) \sqrt{ 1 - \left(\cos\theta_1 \right)^2 }$$
 * $$\cos\theta_2 = \sqrt{1-(\sin\theta_2)^2} = \sqrt{1 - \left( \frac{n_1}{n_2} \right)^2 \left( 1 - \left( \cos\theta_1 \right)^2 \right)}$$
 * $$\vec{v}_{\mathrm{refract}} = \left( \frac{n_1}{n_2} \right) \vec{l} + \left( \frac{n_1}{n_2} \cos\theta_1 - \cos\theta_2 \right) \vec{n}$$

The formula may appear simpler in terms of renamed simple values $$ r = n_1 / n_2 $$ and $$ c = -\vec{n}\cdot \vec{l}$$, avoiding any appearance of trig function names or angle names:
 * $$\vec{v}_{\mathrm{refract}} = r \vec{l} + \left( r c - \sqrt{1 - r^2 \left( 1 - c^2 \right)} \right) \vec{n}$$

Example:
 * $$\vec{l} = \{0.707107, -0.707107\}, ~ \vec{n} = \{0,1\}, ~ r = \frac{n_1}{n_2} = 0.9$$
 * $$c = \cos\theta_1=0.707107, ~ \sqrt{1 - r^2 \left( 1 - c^2 \right)} = \cos\theta_2 = 0.771362$$
 * $$\vec{v}_{\mathrm{reflect}}=\{0.707107, 0.707107\}

,~\vec{v}_{\mathrm{refract}}=\{0.636396, -0.771362\}$$

The cosine values may be saved and used in the Fresnel equations for working out the intensity of the resulting rays.

Total internal reflection is indicated by a negative radicand in the equation for $$\cos\theta_2$$, which can only happen for rays crossing into a less-dense medium ($$ n_2 < n_1$$).

Total internal reflection and critical angle

When light travels from a medium with a higher refractive index to one with a lower refractive index, Snell's law seems to require in some cases (whenever the angle of incidence is large enough) that the sine of the angle of refraction be greater than one. This of course is impossible, and the light in such cases is completely reflected by the boundary, a phenomenon known as total internal reflection. The largest possible angle of incidence which still results in a refracted ray is called the critical angle; in this case the refracted ray travels along the boundary between the two media.

For example, consider a ray of light moving from water to air with an angle of incidence of 50°. The refractive indices of water and air are approximately 1.333 and 1, respectively, so Snell's law gives us the relation

 * $$\sin\theta_2 = \frac{n_1}{n_2}\sin\theta_1 = \frac{1.333}{1}\cdot\sin\left(50^\circ\right) = 1.333\cdot 0.766 = 1.021,$$

which is impossible to satisfy. The critical angle θcrit is the value of θ1 for which θ2 equals 90°:

 * $$\theta_\text{crit} = \arcsin\left(\frac{n_2}{n_1}\sin\theta_2\right) = \arcsin\frac{n_2}{n_1} = 48.6^\circ.$$

Dispersion
In many wave-propagation media, wave velocity changes with frequency or wavelength of the waves; this is true of light propagation in most transparent substances other than a vacuum. These media are called dispersive. The result is that the angles determined by Snell's law also depend on frequency or wavelength, so that a ray of mixed wavelengths, such as white light, will spread or disperse. Such dispersion of light in glass or water underlies the origin of rainbows and other optical phenomena, in which different wavelengths appear as different colors.

In optical instruments, dispersion leads to chromatic aberration; a color-dependent blurring that sometimes is the resolution-limiting effect. This was especially true in refracting telescopes, before the invention of achromatic objective lenses.

Lossy, absorbing, or conducting media
In a conducting medium, permittivity and index of refraction are complex-valued. Consequently, so are the angle of refraction and the wave-vector. This implies that, while the surfaces of constant real phase are planes whose normals make an angle equal to the angle of refraction with the interface normal, the surfaces of constant amplitude, in contrast, are planes parallel to the interface itself. Since these two planes do not in general coincide with each other, the wave is said to be inhomogeneous. The refracted wave is exponentially attenuated, with exponent proportional to the imaginary component of the index of refraction.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Snell's law


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Éire

Éire is the Irish Gaelic name for "Ireland". Like its English counterpart, the term Éire is used for both the island of Ireland and the Republic of Ireland, the sovereign state that governs 85% of the island's landmass. The latter is distinct from Northern Ireland, which covers the remainder of the northeast of the island. The same name is also sometimes used in English.

Etymology
The modern Irish Éire evolved from the Old Irish word Ériu, which was the name of Ireland and of a Gaelic goddess. Ériu is generally believed to have been the matron goddess of Ireland, a goddess of sovereignty, or simply a goddess of the land. Ériu has been derived from reconstructed Archaic Irish *Īweriū, and further from the Proto-Celtic *Φīwerjon- (nominative singular Φīwerjū). This suggests descent from the Proto-Indo-European *piHwerjon- ("fertile land" or "abundant land"), from the adjective *piHwer- ("fat") – cognate with Ancient Greek píeira and Sanskrit pīvarī, ("fat, full, abounding"). The Archaic Irish form was borrowed from Ancient Greek. During his exploration of northwest Europe (circa 320 BC), Pytheas of Massilia called the island Ierne (written Ἰέρνη). The Pseudo-Aristotelian text On the Universe (393b) has: "Ἐν τούτῳ γε μὴν νῆσοι μέγισται τυγχάνουσιν οὖσαι δύο, Βρεττανικαὶ λεγόμεναι, Ἀλβίων καὶ Ἰέρνη." Translation: There are two very large islands in it, called the British Isles, Albion and Ierne. (modern Great Britain and Ireland). In his book Geographia (circa 150 AD), Claudius Ptolemaeus called the island Iouernia (written Ἰουερνία; ou represented /w/) and named a tribal group called the (Ἰούερνοι, Iouernoi or Iverni who lived in the southwest. This was borrowed into Latin as Hibernia.

The evolution of the word would follow as such:
 * Proto-Celtic *Φīwerjon- (nominative singular *Φīwerjū)
 * Archaic Irish *Īweriū
 * Old Irish Ériu
 * Modern Irish Éire

An Old Irish by-form of this placename was íriu, meaning "land, earth". In Irish mythology, Íth is the first of the Milesians (Irish) to see Ireland from Iberia. Later, he is the first to step ashore and praises the island's abundance, saying to the Tuatha Dé Danann: "You dwell in a good land. Abundant are its mast and honey and wheat and fish".

A 19th-century proposal, which does not follow modern standards of etymology, derives the name from Scottish Gaelic: ì (island) + thiar (west) + fónn (land), which together give ì-iar-fhónn, or "westland isle". The etymology fails in that tiar (the historical form) has no *téir forms which would allow the development of the *é of Éire; moreover, Old Irish í ("island") was a late loanword from Old Norse ey ("island"), and so did not exist in prehistoric Ireland.

Difference between Éire and Erin
While Éire is simply the name for the island of Ireland in the Irish language, and sometimes used in English, Erin is a common poetic name for Ireland, as in Erin go bragh. The distinction between the two is one of the difference between cases of nouns in Irish. Éire is the nominative case, the case that is used for nouns that are the subject of a sentence, i.e., the noun that is doing something. Erin derives from Éirinn, the Irish dative case of Éire, which has replaced the nominative case in Déise Irish and some non-standard sub-dialects elsewhere, in Scottish Gaelic (where the usual word for Ireland is Èirinn) and Manx (like Irish and Scottish Gaelic, a Goidelic Celtic language), where the word is spelled "Nerin," with the initial n- probably representing a fossilisation of the preposition in/an "in" (cf. Irish in Éirinn, Scottish an Èirinn/ann an Èirinn "in Ireland"). The genitive case, Éireann (e.g. stair na hÉireann "the history of Ireland, Ireland's history"), is found in the Gaelic forms of the titles of companies and institutions in Ireland e.g. Iarnród Éireann (Irish Rail), Dáil Éireann (Irish Parliament), Poblacht na hÉireann (The Republic of Ireland) or Tuaisceart Éireann (Northern Ireland).

As a state name
Article 4 of the Irish constitution adopted in 1937 by the government under Éamon de Valera states "Éire is the name of the state, or in the English language, Ireland". The Constitution's English-language preamble also describes the population as "We, the people of Éire". Despite the fact that Article 8 designated Irish as the "national" and "first official" language, Éire has to some extent passed out of everyday conversation and literature, and the state is referred to as Ireland or its equivalent in all other languages. The name "Éire" has been used on Irish postage stamps since 1922; on all Irish coinage (including Irish euro coins); and together with "Ireland" on passports and other official state documents issued since 1937. "Éire" is used on the seal of the president of Ireland.

Initially, after 1937 the United Kingdom insisted on using only the name "Eire" and refused to accept the name "Ireland". It adopted the Eire (Confirmation of Agreements) Act 1938 putting in law that position. At the 1948 Summer Olympics in London the organisers insisted that the Irish team march under the banner "Eire" notwithstanding that every other team was marching according to what their name was in English. The UK Government generally avoided all reference to "Ireland" in connection with the state and used what Senator Thomas O'Connell described as "sneering titles such as Eirish". However, the term "Eirish" was also used by some writers in the US, who referred to "the Eirish people". Using the genitive form Éireann as an adjective, the UK media would refer to "Eireann Ministers" and the "Eireann Army". The Ireland Act 1949 changed this to "Republic of Ireland". It was not until after the 1998 Good Friday Agreement that the UK government accepted the preferred name of simply "Ireland", at the same time as the Republic of Ireland dropped its territorial claim over Northern Ireland.

Before the 1937 Constitution, "Saorstát Éireann" (the Irish name of the Irish Free State) was generally used.

During the Emergency (as the Second World War was known), Irish ships had "EIRE" (and the Irish tricolour) painted large on their sides and deck, to identify them as neutrals.

In the 1947 Sinn Féin Funds case, a co-defendant was cited as "the Attorney General of Eire" in the High and Supreme Court cases, and there were similar cases where "Eire" was used in the late 1940s as a descriptor of the state in English.

In 1922–1938 the international plate on Irish cars was "SE". From 1938 to 1962 it was marked "EIR", short for Éire. Bernard Commons, a Clann na Talmhan TD, suggested to the Dáil in 1950 that the Government examine "the tourist identification plate bearing the letters EIR ... with a view to the adoption of identification letters more readily associated with this country by foreigners". "EIR" was indicated in the Road Traffic (Third Party Risks) (Visiting Motorists) Regulations 1952 and 1958. The Mechanically Propelled Vehicles (International Circulation) Order 1961 specified EIR but also permitted IRL. and by 1962 "IRL" had been adopted.

Under the 1947 Convention Irish-registered aircraft have carried a registration mark starting "EI" for Éire.

From January 2007, the Irish Government nameplates at meetings of the European Union have borne both Éire and Ireland, following the adoption of Irish as a working language of the European Union.

Spelling Eire rather than Éire
In 1938 the British government provided in the Eire (Confirmation of Agreements) Act 1938 that British legislation would henceforth refer to the Irish Free State as "Eire" (but not as "Ireland"). This was altered by the Ireland Act 1949, where the English-law name of the state was changed to "Republic of Ireland". The 1938 Act was repealed in 1981, and in 1996 a British journalist described Eire as "now an oddity rarely used, an out-of-date reference".

Within Ireland however, the spelling "Eire" was incorrect. When Irish language texts were printed in Gaelic type, both capital and lower-case letters were printed with diacritics (written accents). From the later 1940s, in conjunction with other reforms, printing switched to the same Latin alphabet used in English. There was some uncertainty about whether the síneadh fada (acute accent) should be written on upper-case letters. While it was preserved in all-Irish texts, it was often omitted when short fragments of Irish appeared alone or in English texts. Noel Davern asked in the Dáil in 1974 why Irish stamps had EIRE rather than ÉIRE. The reply from the Minister for Posts and Telegraphs was: "The accent has been omitted on most Irish stamps issued over the past ten years in the interests of artistic balance and in accordance with a common practice in the printing of Irish in Roman script for display purposes. This is a prevailing typographical convention and is common to several European languages, including French." Davern considered EIRE to be worse than a misspelling, because eire is a word in its own right, meaning "a burden, load or encumbrance". The minister stated, "The word on the stamp ... does not mean 'eire' and it is not understood to mean 'eire' by anybody except Davern." Stamps later used a Gaelic type with the accent preserved.

English rarely uses diacritics for English words, and often omits them from written loanwords from any source language; the acute accent is often omitted when ÉIRE is written in English—in that context, the omission or expression is regarded by English speakers who do not speak Irish as a negligible variation, reflecting two accepted spellings without further implication, in the same way as Mexico and México are seen as being the same. But for an Irish-speaker the diacritic changes the pronunciation.

Other uses
Éire has been incorporated into the names of Irish commercial and social entities, such as Eir (formerly Eircom and Telecom Éireann) and its former mobile phone network, Eircell. Ireland's postal code system is known as Eircode. In 2006 the Irish electricity network was devolved to EirGrid. The company "BetEire Flow" (eFlow), named as a pun on "better", is a French consortium running the electronic tolling system at the West-Link bridge west of Dublin. According to the Dublin Companies Registration Office in 2008, over 500 company names incorporate the word Éire in some form.

title: Éire


text:
Montreal Canadiens

The Montreal Canadiens (Les Canadiens de Montréal), officially le Club de hockey Canadien ( The Canadian Hockey Club) and colloquially known as the Habs, are a professional ice hockey team based in Montreal. The Canadiens compete in the National Hockey League (NHL) as a member of the Atlantic Division of the Eastern Conference. Since 1996, the team has played its home games at Bell Centre, originally known as Molson Centre. The Canadiens previously played at the Montreal Forum, which housed the team for seven decades and all but their first two Stanley Cup championships.

Founded in 1909, the Canadiens are the oldest continuously operating professional ice hockey team worldwide, and the only existing NHL club to predate the founding of the NHL. One of the oldest North American professional sports franchises, the Canadiens' history predates that of every other Canadian franchise outside the Toronto Argonauts, as well as every American franchise outside baseball and the National Football League's Arizona Cardinals. The franchise is one of the "Original Six", the teams that made up the NHL from 1942 until the 1967 expansion. The team's championship season in 1992–93 marked the last time a Canadian team won the Stanley Cup.

The Canadiens have won the Stanley Cup more times than any other franchise, having earned 24 championships, with 23 victories since the founding of the NHL, and 22 since 1927, when NHL teams became the only ones to compete for the Stanley Cup. The Canadiens also had the most championships by a team of any of the four major North American sports leagues until the New York Yankees won their 25th World Series title in 1999.

History
The Canadiens were founded by J. Ambrose O'Brien on December 4, 1909, as a charter member of the National Hockey Association, the forerunner to the National Hockey League. It was to be the team of the francophone community in Montreal, composed of francophone players, and under francophone ownership as soon as possible. The founders named the team "Les Canadiens," a term identified at the time with French speakers. The team's first season was not a success, as they placed last in the league. After the first year, ownership was transferred to George Kennedy of Montreal and the team's record improved over the next seasons. The team won its first Stanley Cup championship in the 1915–16 season. In 1917, with four other NHA teams, the Canadiens formed the NHL, and they won their first NHL Stanley Cup during the 1923–24 season, led by Howie Morenz. The team moved from the Mount Royal Arena to the Montreal Forum for the 1926–27 season.

The club began the 1930s decade successfully, with Stanley Cup wins in 1930 and 1931. The Canadiens and its cross-town rivals the Montreal Maroons, who stopped playing after the 1937–38 NHL season, declined both on the ice and economically during the Great Depression. Losses grew to the point where the team owners considered selling the team to interests in Cleveland, Ohio, though local investors were ultimately found to finance the Canadiens. The Maroons suspended operations, and several of their players moved to the Canadiens. Led by the "Punch Line" of Maurice "Rocket" Richard, Toe Blake and Elmer Lach in the 1940s, the Canadiens enjoyed success again atop the NHL. From 1953 to 1960, the franchise won six Stanley Cups, including a record five straight from 1956 to 1960, with a new set of stars coming to prominence: Jean Beliveau, Dickie Moore, Doug Harvey, Bernie "Boom Boom" Geoffrion, Jacques Plante and Richard's younger brother, Henri.

The Canadiens added ten more championships in 15 seasons from 1965 to 1979, with another dynastic run of four-straight Cups from 1976 to 1979. In the 1976–77 season, the Canadiens set three still-standing team records – fewest losses (8) in an 80-game season, the longest home unbeaten streak (34), and best goal differential (+216) – and one record that lasted until the 2022–23 Boston Bruins beat it, for the most points (132) in an 80-game season. The next season, 1977–78, the team had a 28-game unbeaten streak, the second-longest in NHL history. The next generation of stars included Guy Lafleur, Yvan Cournoyer, Ken Dryden, Pete Mahovlich, Jacques Lemaire, Pierre Larouche, Steve Shutt, Bob Gainey, Serge Savard, Guy Lapointe and Larry Robinson. Scotty Bowman, who would later set a record for most NHL victories by a coach, was the team's head coach for its last five Stanley Cup victories in the 1970s.

The Canadiens won Stanley Cups in 1986, led by rookie star goaltender Patrick Roy, and in 1993, continuing their streak of winning at least one championship in every decade from the 1910s to the 1990s (this streak came to an end in the 2000s). In 1996, the Habs moved from the Montreal Forum, their home during 70 seasons and 22 Stanley Cups, to Molson Centre (now called Bell Centre).

Following Roy's departure in 1995, the Canadiens fell into an extended stretch of mediocrity, missing the playoffs in four of their next ten seasons and failing to advance past the second round of the playoffs until 2010. By the late 1990s, with both an ailing team and monetary losses exacerbated by a record-low value of the Canadian dollar, Montreal fans feared their team would end up relocated to the United States. Team owner Molson Brewery sold control of the franchise and the Molson Centre to American businessman George N. Gillett Jr. in 2001, with the right of first refusal for any future sale by Gillett and a condition that the NHL Board of Governors must unanimously approve any attempt to move to a new city. Led by club president Pierre Boivin, the Canadiens returned to being a lucrative enterprise, earning additional revenues from broadcasting and arena events. In 2009, Gillett sold the franchise to a consortium led by the Molson family which included The Woodbridge Company, BCE/Bell, the Fonds de solidarité FTQ, Michael Andlauer, Luc Bertrand and the National Bank Financial Group for $575 million, more than double the $275 million he spent on the purchase eight years prior.

During the 2008–09 season, the Canadiens celebrated their 100th anniversary with various events, including hosting both the 2009 NHL All-Star Game, and the 2009 NHL Entry Draft. The Canadiens became the first team in NHL history to reach 3,000 victories with their 5–2 victory over the Florida Panthers on December 29, 2008. For the 2020–21 season, the league moved the Canadiens along with the other six teams from Canada to the North Division. Due to the COVID-19 pandemic, the Canadiens only played against teams in the division in the regular season to avoid travel restrictions between the United States and Canada. All teams in the division played without fans to begin the season. The Canadiens advanced through the 2021 Stanley Cup playoffs, beating the Toronto Maple Leafs in the first round of the playoffs 4–3, overcoming a 3–1 Maple Leafs lead in the series. The Canadiens then swept the Winnipeg Jets in the second round, advancing to the Stanley Cup semifinals. The Canadiens defeated the Vegas Golden Knights in the semifinals, clinching an overtime victory in Game 6 of the series, and reaching their first Stanley Cup Finals in 28 years, whilst also being the first Canadian team to reach the Finals since the Vancouver Canucks in 2011. Montreal lost the Finals to the Tampa Bay Lightning, 4 games to 1.

In 2021–22, the Canadiens were unable to replicate their success from the prior season, ultimately finishing last in the league for the first time since the 1939–40 season and the first time in the NHL's expansion era, in what was one of the worst seasons in the team's history. In the process they set team records for most regulation losses (49), most goals against (319), fewest wins (22), and fewest points (55), while their .335 point percentage was the team's third-worst ever, after only 1925–26 (.319) and 1939–40 (.260). As a result, team owner Geoff Molson authorized a "rebuild" of the roster over an extended period, a first in the modern history of the franchise. The Canadiens finished fifth-last in the subsequent 2022–23 season.

Team identity
The Canadiens organization operates in both English and French. For many years, public address announcements and press releases have been given in both languages, and the team website and social media outlets are in both languages as well. At home games, the first stanza of O Canada is sung in French, and the chorus is sung in English.

Crest and sweater design
One of sport's oldest and most recognizable logos, the classic 'C' and 'H' of the Montreal Canadiens was first used together in the 1917–18 season, when the club changed its name to "Club de hockey Canadien" from "Club athlétique Canadien", before evolving to its current form in 1952–53. The "H" stands for "hockey", not "Habitants," a popular misconception. According to NHL.com, the first man to refer to the team as "the Habs" was American Tex Rickard, owner of the Madison Square Garden, in 1924. Rickard apparently told a reporter that the "H" on the Canadiens' sweaters was for "Habitants". In French, the "Habitants" nickname dates back to at least 1914, when it was printed in Le Devoir to report a 9–3 win over Toronto on the ninth of February.

The team's colours since 1911 are blue, white and red. The home sweater is predominantly red in colour. There are four blue and white stripes, one across each arm, one across the chest and the other across the waistline. The main road sweater is mainly white with a red and blue stripe across the waist, red at the end of both arm sleeves red shoulder yokes. The basic design has been in use since 1914 and took its current form in 1925, generally evolving as materials changed. Because of the team's lengthy history and significance in Quebec, the sweater has been referred to as 'La Sainte-Flanelle' (the holy flannel sweater).

Since 2015, the Canadiens' home red sweater is the only uniform in the league to feature the French language version of the NHL shield logo (LNH) on the neck collar, in acknowledgment of Montreal's French Canadian heritage. The road white sweater retains the English NHL shield logo.

The Canadiens used multiple designs prior to adopting the aforementioned design in 1914. The original shirt of the 1909–10 season was blue with a white C. The second season had a red shirt featuring a green maple leaf with the C logo, and green pants. Lastly, the season before adopting the current look the Canadiens wore a "barber pole" design jersey with red, white and blue stripes, and the logo being a white maple leaf reading "CAC", "Club athlétique Canadien". All three designs were worn during the 2009–10 season as part of the Canadiens centenary.

In the 2020–21 season, the Canadiens unveiled a "Reverse Retro" alternate uniform in collaboration with Adidas. The uniform was essentially the same as their regular red uniform, but with blue as the primary colour and red as the stripe colour. A second "Reverse Retro" uniform was released in the 2022–23 season, again using the same template but with red relegated to the logo only and featuring a light blue base with white/dark blue/white stripes.

The Canadiens' colours are a readily identifiable aspect of French Canadian culture. In the short story "The Hockey Sweater", Roch Carrier described the influence of the Canadiens and their jersey within rural Quebec communities during the 1940s. The story was later made into an animated short, The Sweater, narrated by Carrier. A passage from the short story appears on the 2002 issue of the Canadian five-dollar bill.

Motto
"Nos bras meurtris vous tendent le flambeau, à vous toujours de le porter bien haut." "To you from failing hands we throw the torch. Be yours to hold it high."

The motto is from the poem "In Flanders Fields" by John McCrae, which was written in 1915, the year before the Canadiens won their first Stanley Cup championship. The motto appears on the wall of the Canadiens' dressing room as well as on the inside collar of the new Adidas 2017–18 jerseys.

Mascot
Beginning in the 2004–05 NHL season, the Canadiens adopted Youppi! as their official mascot, the first costumed mascot in their long history. Youppi was the longtime mascot for the Montreal Expos baseball team but was dropped from the franchise when they moved to Washington, D.C. in 2004–05 and became the Washington Nationals. With the switch, Youppi became the first mascot in professional sports to switch leagues. He is also the first mascot in professional sports to get ejected from a game. In June 2020, Youppi became the first mascot from a Canadian-based club to be honoured in The Mascot Hall of Fame. Youppi's induction in the Mascot Hall of Fame was decided by a long voting process, which included the public vote.

In 2022, the Canadiens introduced an "unofficial official mascot" for its Reverse Retro series of games that season.

Rivalries
The Canadiens have developed strong rivalries with two fellow Original Six franchises, with whom they frequently shared divisions and competed in postseason play. The oldest is with the Toronto Maple Leafs, who first faced the Canadiens as the Toronto Arenas in 1917. The teams met 16 times in the playoffs, including five Stanley Cup Finals. Featuring the two largest cities in Canada and two of the largest fanbases in the league, the rivalry is sometimes dramatized as being emblematic of Canada's English and French linguistic divide. From 1938 to 1970, they were the only two Canadian teams in the league.

The team's other Original Six rival are the Boston Bruins, who since their NHL debut in 1924 have played the Canadiens more than any other team in both regular season play and the playoffs combined. The teams have played 34 playoff series, seven of which were in the finals.

The Canadiens also had an intraprovincial rivalry with the Quebec Nordiques during their existence from 1979 to 1995, nicknamed the "Battle of Quebec".

Broadcasting
Montreal Canadiens games are broadcast locally in both the French and English languages. CHMP 98.5 is the Canadiens' French-language radio flagship. As of the 2017–18 season, the team's regional television in both languages, and its English-language radio rights, are held by Bell Media. CKGM, TSN Radio 690, is the English-language radio flagship; it acquired the rights under a seven-year deal which began in the 2011–12 season. In June 2017, Bell Media reached a five-year extension.

Regional television rights in French are held by Réseau des sports (RDS) under a 12-year deal that began in the 2014–15 NHL season. A sister to the English-language network TSN, RDS was the only French-language sports channel in Canada until the 2011 launch of TVA Sports, and was also the previous national French rightsholder of the NHL; as a result, the Canadiens forwent a separate regional contract, and allowed all of its games to be televised nationally in French as part of RDS's overall NHL rights.

With TVA Sports becoming the national French rightsholder in the 2014–15 season through a sub-licensing agreement with Sportsnet, RDS subsequently announced a 12-year deal to maintain regional rights to Canadiens games not shown on TVA Sports. As a result, games on RDS are blacked out outside the Canadiens' home market of Quebec, Atlantic Canada and parts of Eastern Ontario shared with the Ottawa Senators. At least 22 Canadiens games per season (primarily through its Saturday night La super soirée LNH), including all playoff games, are televised nationally by TVA Sports.

TSN2 assumed the English-language regional television rights in the 2017–18 season, with John Bartlett on play-by-play, and Dave Poulin, Mike Johnson and Craig Button on colour commentary. All other games, including all playoff games, are televised nationally by Sportsnet or CBC. Bartlett returned to Sportsnet over the 2018 off-season, and was succeeded by Bryan Mudryk.

English-language regional rights were previously held by Sportsnet East (with CJNT City Montreal as an overflow channel), under a 3-year deal that expired after the 2016–17 season; the games were called by Bartlett and Jason York. Prior to this deal, TSN held the rights from 2010 through 2014; the games were broadcast on a part-time channel with Dave Randorf on play-by-play.

Season-by-season record
''This is a list of the last five seasons completed by the Canadiens. For the full season-by-season history, see List of Montreal Canadiens seasons.''

Note: GP = Games played, W = Wins, L = Losses, T = Ties, OTL = Overtime Losses, Pts = Points, GF = Goals for, GA = Goals against

Retired numbers
The Canadiens have retired 15 numbers in honour of 18 players, the most of any team in the NHL. All honourees were born in Canada and were members of at least two Stanley Cup winning Canadiens teams. Howie Morenz was the first honouree, on November 2, 1937. The NHL retired Wayne Gretzky's No. 99 for all its member teams at the 2000 NHL All-Star Game.

Hall of Fame
The Montreal Canadiens have an affiliation with a number of inductees to the Hockey Hall of Fame. Thirty-seven of these players are from three separate notable dynasties: 12 from 1955 to 1960, 11 from 1964 to 1969, and 13 from 1975 to 1979. Howie Morenz and Georges Vezina were the first Canadiens given the honour in 1945, while Pierre Turgeon was the most recently inducted, in 2023. Along with players, a number of inductees from the builders category are affiliated with the club. The first inductee was vice president William Northey in 1945. The most recent inductee was Pat Burns in 2014.

In addition to players and builders, five broadcasters for the Montreal Canadiens have been awarded the Foster Hewitt Memorial Award from the Hockey Hall of Fame. The first two recipients of the award were Danny Gallivan and Rene Lecavalier in 1984. The other three award recipients are Doug Smith (1985), Dick Irvin Jr. (1988), and Gilles Tremblay (2002).

Players • Marty Barry

• Jean Beliveau

• Toe Blake

• Emile Bouchard

• Harry Cameron

• Guy Carbonneau

• Chris Chelios

• Sprague Cleghorn

• Yvan Cournoyer

• Gord Drillon

• Ken Dryden

• Dick Duff

• Bill Durnan

• Tony Esposito

• Bob Gainey

• Herb Gardiner

• Bernard Geoffrion

• Doug Gilmour

• George Hainsworth

• Joe Hall

• Doug Harvey

• Tom Johnson

• Aurele Joliat

• Elmer Lach

• Guy Lafleur

• Newsy Lalonde

• Rod Langway

• Jacques Laperriere

• Guy Lapointe

• Jack Laviolette

• Jacques Lemaire

• Frank Mahovlich

• Joe Malone

• Sylvio Mantha

• Dickie Moore

• Howie Morenz

• Reg Noble

• Buddy O'Connor

• Bert Olmstead

• Didier Pitre

• Jacques Plante

• Ken Reardon

• Mark Recchi

• Henri Richard

• Maurice Richard

• Larry Robinson

• Patrick Roy

• Denis Savard

• Serge Savard

• Steve Shutt

• Babe Siebert

• Tommy Smith

• Pierre Turgeon

• Rogatien Vachon

• Georges Vezina

• Shea Weber

• Gump Worsley

• Roy Worters

Builders • Scotty Bowman

• Pat Burns

• Joe Cattarinich

• Leo Dandurand

• Tommy Gorman

• Dick Irvin

• Hartland Molson

• William Northey

• Ambrose O'Brien

• Sam Pollock

• Donat Raymond

• Frank Selke

Team captains

 * Jack Laviolette, 1909–1910, 1911–1912
 * Newsy Lalonde, 1910–1911, 1912–1913, 1916–1922
 * Jimmy Gardner, 1913–1915
 * Howard McNamara, 1915–1916
 * Sprague Cleghorn, 1922–1925
 * Billy Coutu, 1925–1926
 * Sylvio Mantha, 1926–1932, 1933–1936
 * George Hainsworth, 1932–1933
 * Albert "Babe" Siebert, 1936–1939
 * Walter Buswell, 1939–1940
 * Toe Blake, 1940–1948
 * Bill Durnan, 1948 (January–April)
 * Emile Bouchard, 1948–1956
 * Maurice Richard, 1956–1960
 * Doug Harvey, 1960–1961
 * Jean Beliveau, 1961–1971
 * Henri Richard, 1971–1975
 * Yvan Cournoyer, 1975–1979
 * Serge Savard, 1979–1981
 * Bob Gainey, 1981–1989
 * Guy Carbonneau and Chris Chelios, 1989–1990 (co-captains)
 * Guy Carbonneau, 1990–1994
 * Kirk Muller, 1994–1995
 * Mike Keane, 1995 (April–December)
 * Pierre Turgeon, 1995–1996
 * Vincent Damphousse, 1996–1999
 * Saku Koivu, 1999–2009
 * Brian Gionta, 2010–2014
 * Max Pacioretty, 2015–2018
 * Shea Weber, 2018–2022
 * Nick Suzuki, 2022–present

Head coaches

 * Joe Cattarinich and Jack Laviolette, 1909–1910
 * Adolphe Lecours, 1911
 * Napoleon Dorval, 1911–1913
 * Jimmy Gardner, 1913–1915
 * Newsy Lalonde, 1915–1921, 1932–1934
 * Leo Dandurand, 1921–1926
 * Cecil Hart, 1926–1932, 1936–1938
 * Newsy Lalonde and Leo Dandurand, 1934–1935
 * Sylvio Mantha, 1935–1936
 * Cecil Hart and Jules Dugal, 1938–1939
 * Albert "Babe" Siebert, 1939
 * Alfred "Pit" Lepine, 1939–1940
 * Dick Irvin, 1940–55
 * Hector "Toe" Blake, 1955–1968
 * Claude Ruel, 1968–1970, 1979–1981
 * Al MacNeil, 1970–1971
 * Scotty Bowman, 1971–1979
 * Bernie Geoffrion, 1979
 * Bob Berry, 1981–1984
 * Jacques Lemaire, 1984–1985
 * Jean Perron, 1985–1988
 * Pat Burns, 1988–1992
 * Jacques Demers, 1992–1995
 * Mario Tremblay, 1995–1997
 * Alain Vigneault, 1997–2000
 * Michel Therrien, 2000–2003, 2012–2017
 * Claude Julien, 2003–2006, 2017–2021
 * Bob Gainey, 2006 (January–May) 2009 (March–June) (interim)
 * Guy Carbonneau, 2006–2009
 * Jacques Martin, 2009–2011
 * Randy Cunneyworth, 2011–2012 (interim)
 * Dominique Ducharme, 2021–2022
 * Martin St. Louis, 2022–present

Source:

First-round draft picks

 * 1963: Garry Monahan (1st overall)
 * 1964: Claude Chagnon (6th overall)
 * 1965: Pierre Bouchard (5th overall)
 * 1966: Phil Myre (5th overall)
 * 1967: Elgin McCann (8th overall)
 * 1968: Michel Plasse (1st overall), Roger Belisle (2nd overall), and Jim Pritchard (3rd overall)
 * 1969: Rejean Houle (1st overall), and Marc Tardif (2nd overall)
 * 1970: Ray Martynuik (5th overall), and Chuck Lefley (6th overall)
 * 1971: Guy Lafleur (1st overall), Chuck Arnason (7th overall), and Murray Wilson (11th overall)
 * 1972: Steve Shutt (4th overall), Michel Larocque (6th overall), Dave Gardner (8th overall), and John Van Boxmeer (14th overall)
 * 1973: Bob Gainey (8th overall)
 * 1974: Cam Connor (5th overall), Doug Risebrough (7th overall), Rick Chartraw (10th overall), Mario Tremblay (12th overall), and Gord McTavish (15th overall)
 * 1975: Robin Sadler (9th overall), and Pierre Mondou (15th overall)
 * 1976: Peter Lee (12th overall), Rod Schutt (13th overall), and Bruce Baker (18th overall)
 * 1977: Mark Napier (10th overall), and Norm Dupont (18th overall)
 * 1978: Danny Geoffrion (8th overall), and Dave Hunter (17th overall)
 * 1980: Doug Wickenheiser (1st overall)
 * 1981: Mark Hunter (7th overall), Gilbert Delorme (18th overall), and Jan Ingman (19th overall)
 * 1982: Alain Heroux (19th overall)
 * 1983: Alfie Turcotte (17th overall)
 * 1984: Petr Svoboda (5th overall), and Shayne Corson (8th overall)
 * 1985: Jose Charbonneau (12th overall), and Tom Chorske (16th overall)
 * 1986: Mark Pederson (15th overall)
 * 1987: Andrew Cassels (17th overall)
 * 1988: Eric Charron (20th overall)
 * 1989: Lindsay Vallis (13th overall)
 * 1990: Turner Stevenson (12th overall)
 * 1991: Brent Bilodeau (17th overall)
 * 1992: David Wilkie (20th overall)
 * 1993: Saku Koivu (21st overall)
 * 1994: Brad Brown (18th overall)
 * 1995: Terry Ryan (8th overall)
 * 1996: Matt Higgins (18th overall)
 * 1997: Jason Ward (11th overall)
 * 1998: Eric Chouinard (16th overall)
 * 2000: Ron Hainsey (13th overall) and Marcel Hossa (16th overall)
 * 2001: Mike Komisarek (7th overall), and Alexander Perezhogin (25th overall)
 * 2002: Chris Higgins (14th overall)
 * 2003: Andrei Kostitsyn (10th overall)
 * 2004: Kyle Chipchura (18th overall)
 * 2005: Carey Price (5th overall)
 * 2006: David Fischer (20th overall)
 * 2007: Ryan McDonagh (12th overall), and Max Pacioretty (22nd overall)
 * 2009: Louis Leblanc (18th overall)
 * 2010: Jarred Tinordi (22nd overall)
 * 2011: Nathan Beaulieu (17th overall)
 * 2012: Alex Galchenyuk (3rd overall)
 * 2013: Michael McCarron (25th overall)
 * 2014: Nikita Scherbak (26th overall)
 * 2015: Noah Juulsen (26th overall)
 * 2016: Mikhail Sergachev (9th overall)
 * 2017: Ryan Poehling (25th overall)
 * 2018: Jesperi Kotkaniemi (3rd overall)
 * 2019: Cole Caufield (15th overall)
 * 2020: Kaiden Guhle (16th overall)
 * 2021: Logan Mailloux (31st overall)
 * 2022: Juraj Slafkovsky (1st overall), and Filip Mesar (26th overall)
 * 2023: David Reinbacher (5th overall)
 * 2024: Ivan Demidov (5th), and Michael Hage (21st overall)

Franchise scoring leaders
These are the top-ten-point-scorers in franchise history. Figures are updated after each completed NHL regular season. Note: Pos = Position; GP = Games Played; G = Goals; A = Assists; Pts = Points; P/G = Points per game
 * – current Canadiens player

Sources: ,

Records – skaters

 * Career

 * Most seasons: 20, Henri Richard
 * Most games: 1,256, Henri Richard
 * Most goals: 544, Maurice Richard
 * Most assists: 728, Guy Lafleur
 * Most points: 1,246, Guy Lafleur
 * Most penalty minutes: 2,248, Chris Nilan
 * Most consecutive games played: 560, Doug Jarvis

 * Season

 * Most goals in a season: 60, Steve Shutt (1976–77); Guy Lafleur (1977–78)
 * Most powerplay goals in a season: 20, Yvan Cournoyer (1966–67)
 * Most powerplay goals in a season, defenceman: 19, Sheldon Souray (2006–07)*
 * Most assists in a season: 82, Pete Mahovlich (1974–75)
 * Most points in a season: 136, Guy Lafleur (1976–77)
 * Most penalty minutes in a season: 358, Chris Nilan (1984–85)
 * Most points in a season, defenceman: 85, Larry Robinson (1976–77)
 * Most points in a season, rookie: 71, Mats Naslund (1982–83); Kjell Dahlin (1985–86)
 * Most goals in a season, defenceman: 28, Guy Lapointe (1974–75)

* Indicates a league record.

Source:

Records – goaltenders

 * Career

 * Most games played: 707, Carey Price
 * Most seasons: 16, Georges Vezina
 * Most shutouts: 75, George Hainsworth
 * Most wins: 360, Carey Price

 * Season

 * Most games in a season: 72, Carey Price (2010–11)
 * Most wins in a season: 44, Carey Price (2014–15)
 * Most shutouts in a season: 22, George Hainsworth (1928–29)*

* Indicates a league record.

Source:

title: Montreal Canadiens


text:
Ornithology

Ornithology is a branch of zoology that concerns the study of birds. Several aspects of ornithology differ from related disciplines, due partly to the high visibility and the aesthetic appeal of birds. It has also been an area with a large contribution made by amateurs in terms of time, resources, and financial support. Studies on birds have helped develop key concepts in biology including evolution, behaviour and ecology such as the definition of species, the process of speciation, instinct, learning, ecological niches, guilds, island biogeography, phylogeography, and conservation.

While early ornithology was principally concerned with descriptions and distributions of species, ornithologists today seek answers to very specific questions, often using birds as models to test hypotheses or predictions based on theories. Most modern biological theories apply across life forms, and the number of scientists who identify themselves as "ornithologists" has therefore declined. A wide range of tools and techniques are used in ornithology, both inside the laboratory and out in the field, and innovations are constantly made. Most biologists who recognise themselves as "ornithologists" study specific biology research areas, such as anatomy, physiology, taxonomy, ecology, or behaviour.

Definition and etymology
The word "ornithology" comes from the late 16th-century Latin ornithologia meaning "bird science" from the Greek ὄρνις ornis ("bird") and λόγος logos ("theory, science, thought").

History
The history of ornithology largely reflects the trends in the history of biology, as well as many other scientific disciplines, including ecology, anatomy, physiology, paleontology, and more recently, molecular biology. Trends include the move from mere descriptions to the identification of patterns, thus towards elucidating the processes that produce these patterns.

Early knowledge and study
Humans have had an observational relationship with birds since prehistory, with some stone-age drawings being amongst the oldest indications of an interest in birds. Birds were perhaps important as food sources, and bones of as many as 80 species have been found in excavations of early Stone Age settlements. Waterbird and seabird remains have also been found in shell mounds on the island of Oronsay off the coast of Scotland. Cultures around the world have rich vocabularies related to birds. Traditional bird names are often based on detailed knowledge of the behaviour, with many names being onomatopoeic, and still in use. Traditional knowledge may also involve the use of birds in folk medicine and knowledge of these practices are passed on through oral traditions (see ethno-ornithology). Hunting of wild birds as well as their domestication would have required considerable knowledge of their habits. Poultry farming and falconry were practised from early times in many parts of the world. Artificial incubation of poultry was practised in China around 246 BC and around at least 400 BC in Egypt. The Egyptians also made use of birds in their hieroglyphic scripts, many of which, though stylized, are still identifiable to species. Early written records provide valuable information on the past distributions of species. For instance, Xenophon records the abundance of the ostrich in Assyria (Anabasis, i. 5); this subspecies from Asia Minor is extinct and all extant ostrich races are today restricted to Africa. Other old writings such as the Vedas (1500–800 BC) demonstrate the careful observation of avian life histories and include the earliest reference to the habit of brood parasitism by the Asian koel (Eudynamys scolopaceus). Like writing, the early art of China, Japan, Persia, and India also demonstrate knowledge, with examples of scientifically accurate bird illustrations.

Aristotle in 350 BC in his History of animals noted the habit of bird migration, moulting, egg laying, and lifespans, as well as compiling a list of 170 different bird species. However, he also introduced and propagated several myths, such as the idea that swallows hibernated in winter, although he noted that cranes migrated from the steppes of Scythia to the marshes at the headwaters of the Nile. The idea of swallow hibernation became so well established that even as late as in 1878, Elliott Coues could list as many as 182 contemporary publications dealing with the hibernation of swallows and little published evidence to contradict the theory. Similar misconceptions existed regarding the breeding of barnacle geese. Their nests had not been seen, and they were believed to grow by transformations of goose barnacles, an idea that became prevalent from around the 11th century and noted by Bishop Giraldus Cambrensis (Gerald of Wales) in Topographia Hiberniae (1187). Around 77 AD, Pliny the Elder described birds, among other creatures, in his Historia Naturalis.

The earliest record of falconry comes from the reign of Sargon II (722–705 BC) in Assyria. Falconry is thought to have made its entry to Europe only after AD 400, brought in from the east after invasions by the Huns and Alans. Starting from the eighth century, numerous Arabic works on the subject and general ornithology were written, as well as translations of the works of ancient writers from Greek and Syriac. In the 12th and 13th centuries, crusades and conquest had subjugated Islamic territories in southern Italy, central Spain, and the Levant under European rule, and for the first time translations into Latin of the great works of Arabic and Greek scholars were made with the help of Jewish and Muslim scholars, especially in Toledo, which had fallen into Christian hands in 1085 and whose libraries had escaped destruction. Michael Scotus from Scotland made a Latin translation of Aristotle's work on animals from Arabic here around 1215, which was disseminated widely and was the first time in a millennium that this foundational text on zoology became available to Europeans. Falconry was popular in the Norman court in Sicily, and a number of works on the subject were written in Palermo. Emperor Frederick II of Hohenstaufen (1194–1250) learned about an falconry during his youth in Sicily and later built up a menagerie and sponsored translations of Arabic texts, among which the popular Arabic work known as the Liber Moaminus by an unknown author which was translated into Latin by Theodore of Antioch from Syria in 1240-1241 as the De Scientia Venandi per Aves, and also Michael Scotus (who had removed to Palermo) translated Ibn Sīnā's Kitāb al-Ḥayawān of 1027 for the Emperor, a commentary and scientific update of Aristotle's work which was part of Ibn Sīnā's massive Kitāb al-Šifāʾ. Frederick II eventually wrote his own treatise on falconry, the De arte venandi cum avibus, in which he related his ornithological observations and the results of the hunts and experiments his court enjoyed performing.

Several early German and French scholars compiled old works and conducted new research on birds. These included Guillaume Rondelet, who described his observations in the Mediterranean, and Pierre Belon, who described the fish and birds that he had seen in France and the Levant. Belon's Book of Birds (1555) is a folio volume with descriptions of some 200 species. His comparison of the skeleton of humans and birds is considered as a landmark in comparative anatomy. Volcher Coiter (1534–1576), a Dutch anatomist, made detailed studies of the internal structures of birds and produced a classification of birds, De Differentiis Avium (around 1572), that was based on structure and habits. Konrad Gesner wrote the Vogelbuch and Icones avium omnium around 1557. Like Gesner, Ulisse Aldrovandi, an encyclopedic naturalist, began a 14-volume natural history with three volumes on birds, entitled ornithologiae hoc est de avibus historiae libri XII, which was published from 1599 to 1603. Aldrovandi showed great interest in plants and animals, and his work included 3000 drawings of fruits, flowers, plants, and animals, published in 363 volumes. His Ornithology alone covers 2000 pages and included such aspects as the chicken and poultry techniques. He used a number of traits including behaviour, particularly bathing and dusting, to classify bird groups.

William Turner's Historia Avium (History of Birds), published at Cologne in 1544, was an early ornithological work from England. He noted the commonness of kites in English cities where they snatched food out of the hands of children. He included folk beliefs such as those of anglers. Anglers believed that the osprey emptied their fishponds and would kill them, mixing the flesh of the osprey into their fish bait. Turner's work reflected the violent times in which he lived, and stands in contrast to later works such as Gilbert White's 1789 The Natural History and Antiquities of Selborne that were written in a tranquil era.

In the 17th century, Francis Willughby (1635–1672) and John Ray (1627–1705) created the first major system of bird classification that was based on function and morphology rather than on form or behaviour. Willughby's Ornithologiae libri tres (1676) completed by John Ray is sometimes considered to mark the beginning of scientific ornithology. Ray also worked on Ornithologia, which was published posthumously in 1713 as Synopsis methodica avium et piscium. The earliest list of British birds, Pinax Rerum Naturalium Britannicarum, was written by Christopher Merrett in 1667, but authors such as John Ray considered it of little value. Ray did, however, value the expertise of the naturalist Sir Thomas Browne (1605–82), who not only answered his queries on ornithological identification and nomenclature, but also those of Willoughby and Merrett in letter correspondence. Browne himself in his lifetime kept an eagle, owl, cormorant, bittern, and ostrich, penned a tract on falconry, and introduced the words "incubation" and "oviparous" into the English language. Towards the late 18th century, Mathurin Jacques Brisson (1723–1806) and Comte de Buffon (1707–1788) began new works on birds. Brisson produced a six-volume work Ornithologie in 1760 and Buffon's included nine volumes (volumes 16–24) on birds Histoire naturelle des oiseaux (1770–1785) in his work on science  Histoire naturelle générale et particulière (1749–1804). Jacob Temminck sponsored François Le Vaillant [1753–1824] to collect bird specimens in Southern Africa and Le Vaillant's six-volume Histoire naturelle des oiseaux d'Afrique (1796–1808) included many non-African birds. His other bird books produced in collaboration with the artist Barraband are considered among the most valuable illustrated guides ever produced. Louis Pierre Vieillot (1748–1831) spent 10 years studying North American birds and wrote the Histoire naturelle des oiseaux de l'Amerique septentrionale (1807–1808?). Vieillot pioneered in the use of life histories and habits in classification. Alexander Wilson composed a nine-volume work, American Ornithology, published 1808-1814, which is the first such record of North American birds, significantly antedating Audubon. In the early 19th century, Lewis and Clark studied and identified many birds in the western United States. John James Audubon, born in 1785, observed and painted birds in France and later in the Ohio and Mississippi valleys. From 1827 to 1838, Audubon published The Birds of America, which was engraved by Robert Havell Sr. and his son Robert Havell Jr. Containing 435 engravings, it is often regarded as the greatest ornithological work in history.

Scientific studies
The emergence of ornithology as a scientific discipline began in the 18th century, when Mark Catesby published his two-volume Natural History of Carolina, Florida, and the Bahama Islands, a landmark work which included 220 hand-painted engravings and was the basis for many of the species Carl Linnaeus described in the 1758 Systema Naturae. Linnaeus' work revolutionised bird taxonomy by assigning every species a binomial name, categorising them into different genera. However, ornithology did not emerge as a specialised science until the Victorian era—with the popularization of natural history, and the collection of natural objects such as bird eggs and skins. This specialization led to the formation in Britain of the British Ornithologists' Union in 1858. In 1859, the members founded its journal The Ibis. The sudden spurt in ornithology was also due in part to colonialism. At 100 years later, in 1959, R. E. Moreau noted that ornithology in this period was preoccupied with the geographical distributions of various species of birds. "No doubt the preoccupation with widely extended geographical ornithology, was fostered by the immensity of the areas over which British rule or influence stretched during the 19th century and for some time afterwards."

- Moreau

The bird collectors of the Victorian era observed the variations in bird forms and habits across geographic regions, noting local specialization and variation in widespread species. The collections of museums and private collectors grew with contributions from various parts of the world. The naming of species with binomials and the organization of birds into groups based on their similarities became the main work of museum specialists. The variations in widespread birds across geographical regions caused the introduction of trinomial names. The search for patterns in the variations of birds was attempted by many. Friedrich Wilhelm Joseph Schelling (1775–1854), his student Johann Baptist von Spix (1781–1826), and several others believed that a hidden and innate mathematical order existed in the forms of birds. They believed that a "natural" classification was available and superior to "artificial" ones. A particularly popular idea was the Quinarian system popularised by Nicholas Aylward Vigors (1785–1840), William Sharp Macleay (1792–1865), William Swainson, and others. The idea was that nature followed a "rule of five" with five groups nested hierarchically. Some had attempted a rule of four, but Johann Jakob Kaup (1803–1873) insisted that the number five was special, noting that other natural entities such as the senses also came in fives. He followed this idea and demonstrated his view of the order within the crow family. Where he failed to find five genera, he left a blank insisting that a new genus would be found to fill these gaps. These ideas were replaced by more complex "maps" of affinities in works by Hugh Edwin Strickland and Alfred Russel Wallace. A major advance was made by Max Fürbringer in 1888, who established a comprehensive phylogeny of birds based on anatomy, morphology, distribution, and biology. This was developed further by Hans Gadow and others.

The Galapagos finches were especially influential in the development of Charles Darwin's theory of evolution. His contemporary Alfred Russel Wallace also noted these variations and the geographical separations between different forms leading to the study of biogeography. Wallace was influenced by the work of Philip Lutley Sclater on the distribution patterns of birds. For Darwin, the problem was how species arose from a common ancestor, but he did not attempt to find rules for delineation of species. The species problem was tackled by the ornithologist Ernst Mayr, who was able to demonstrate that geographical isolation and the accumulation of genetic differences led to the splitting of species.

Early ornithologists were preoccupied with matters of species identification. Only systematics counted as true science and field studies were considered inferior through much of the 19th century. In 1901, Robert Ridgway wrote in the introduction to The Birds of North and Middle America that: "There are two essentially different kinds of ornithology: systematic or scientific, and popular. The former deals with the structure and classification of birds, their synonymies, and technical descriptions. The latter treats of their habits, songs, nesting, and other facts pertaining to their life histories."

This early idea that the study of living birds was merely recreation held sway until ecological theories became the predominant focus of ornithological studies. The study of birds in their habitats was particularly advanced in Germany with bird ringing stations established as early as 1903. By the 1920s, the Journal für Ornithologie included many papers on the behaviour, ecology, anatomy, and physiology, many written by Erwin Stresemann. Stresemann changed the editorial policy of the journal, leading both to a unification of field and laboratory studies and a shift of research from museums to universities. Ornithology in the United States continued to be dominated by museum studies of morphological variations, species identities, and geographic distributions, until it was influenced by Stresemann's student Ernst Mayr. In Britain, some of the earliest ornithological works that used the word ecology appeared in 1915. The Ibis, however, resisted the introduction of these new methods of study, and no paper on ecology appeared until 1943. The work of David Lack on population ecology was pioneering. Newer quantitative approaches were introduced for the study of ecology and behaviour, and this was not readily accepted. For instance, Claud Ticehurst wrote: "Sometimes it seems that elaborate plans and statistics are made to prove what is commonplace knowledge to the mere collector, such as that hunting parties often travel more or less in circles."

- Ticehurst

David Lack's studies on population ecology sought to find the processes involved in the regulation of population based on the evolution of optimal clutch sizes. He concluded that population was regulated primarily by density-dependent controls, and also suggested that natural selection produces life-history traits that maximize the fitness of individuals. Others, such as Wynne-Edwards, interpreted population regulation as a mechanism that aided the "species" rather than individuals. This led to widespread and sometimes bitter debate on what constituted the "unit of selection". Lack also pioneered the use of many new tools for ornithological research, including the idea of using radar to study bird migration.

Birds were also widely used in studies of the niche hypothesis and Georgii Gause's competitive exclusion principle. Work on resource partitioning and the structuring of bird communities through competition were made by Robert MacArthur. Patterns of biodiversity also became a topic of interest. Work on the relationship of the number of species to area and its application in the study of island biogeography was pioneered by E. O. Wilson and Robert MacArthur. These studies led to the development of the discipline of landscape ecology. John Hurrell Crook studied the behaviour of weaverbirds and demonstrated the links between ecological conditions, behaviour, and social systems. Principles from economics were introduced to the study of biology by Jerram L. Brown in his work on explaining territorial behaviour. This led to more studies of behaviour that made use of cost-benefit analyses. The rising interest in sociobiology also led to a spurt of bird studies in this area.

The study of imprinting behaviour in ducks and geese by Konrad Lorenz and the studies of instinct in herring gulls by Nicolaas Tinbergen led to the establishment of the field of ethology. The study of learning became an area of interest and the study of bird songs has been a model for studies in neuroethology. The study of hormones and physiology in the control of behaviour has also been aided by bird models. These have helped in finding the proximate causes of circadian and seasonal cycles. Studies on migration have attempted to answer questions on the evolution of migration, orientation, and navigation.

The growth of genetics and the rise of molecular biology led to the application of the gene-centered view of evolution to explain avian phenomena. Studies on kinship and altruism, such as helpers, became of particular interest. The idea of inclusive fitness was used to interpret observations on behaviour and life history, and birds were widely used models for testing hypotheses based on theories postulated by W. D. Hamilton and others.

The new tools of molecular biology changed the study of bird systematics, which changed from being based on phenotype to the underlying genotype. The use of techniques such as DNA-DNA hybridization to study evolutionary relationships was pioneered by Charles Sibley and Jon Edward Ahlquist, resulting in what is called the Sibley-Ahlquist taxonomy. These early techniques have been replaced by newer ones based on mitochondrial DNA sequences and molecular phylogenetics approaches that make use of computational procedures for sequence alignment, construction of phylogenetic trees, and calibration of molecular clocks to infer evolutionary relationships. Molecular techniques are also widely used in studies of avian population biology and ecology.

Rise to popularity
The use of field glasses or telescopes for bird observation began in the 1820s and 1830s, with pioneers such as J. Dovaston (who also pioneered in the use of bird feeders), but instruction manuals did not begin to insist on the use of optical aids such as "a first-class telescope" or "field glass" until the 1880s.

The rise of field guides for the identification of birds was another major innovation. The early guides such as Thomas Bewick's two-volume guide and William Yarrell's three-volume guide were cumbersome, and mainly focused on identifying specimens in the hand. The earliest of the new generation of field guides was prepared by Florence Merriam, sister of Clinton Hart Merriam, the mammalogist. This was published in 1887 in a series Hints to Audubon Workers: Fifty Birds and How to Know Them in Grinnell's Audubon Magazine. These were followed by new field guides, from the pioneering illustrated handbooks of Frank Chapman to the classic Field Guide to the Birds by Roger Tory Peterson in 1934, to Birds of the West Indies published in 1936 by Dr. James Bond - the same who inspired the amateur ornithologist Ian Fleming in naming his famous literary spy.

The interest in birdwatching grew in popularity in many parts of the world, and the possibility for amateurs to contribute to biological studies was soon realized. As early as 1916, Julian Huxley wrote a two-part article in The Auk, noting the tensions between amateurs and professionals, and suggested the possibility that the "vast army of bird lovers and bird watchers could begin providing the data scientists needed to address the fundamental problems of biology." The amateur ornithologist Harold F. Mayfield noted that the field was also funded by non-professionals. He noted that in 1975, 12% of the papers in American ornithology journals were written by persons who were not employed in biology related work.

Organizations were started in many countries, and these grew rapidly in membership, most notable among them being the Royal Society for the Protection of Birds (RSPB) in Britain and the Audubon Society in the US, which started in 1885. Both these organizations were started with the primary objective of conservation. The RSPB, born in 1889, grew from a small Croydon-based group of women, including Eliza Phillips, Etta Lemon, Catherine Hall and Hannah Poland. Calling themselves the "Fur, Fin, and Feather Folk", the group met regularly and took a pledge "to refrain from wearing the feathers of any birds not killed for the purpose of food, the ostrich only exempted." The organization did not allow men as members initially, avenging a policy of the British Ornithologists' Union to keep out women. Unlike the RSPB, which was primarily conservation oriented, the British Trust for Ornithology was started in 1933 with the aim of advancing ornithological research. Members were often involved in collaborative ornithological projects. These projects have resulted in atlases which detail the distribution of bird species across Britain. In Canada, citizen scientist Elsie Cassels studied migratory birds and was involved in establishing Gaetz Lakes bird sanctuary. In the United States, the Breeding Bird Surveys, conducted by the United States Geological Survey, have also produced atlases with information on breeding densities and changes in the density and distribution over time. Other volunteer collaborative ornithology projects were subsequently established in other parts of the world.

Techniques
The tools and techniques of ornithology are varied, and new inventions and approaches are quickly incorporated. The techniques may be broadly dealt under the categories of those that are applicable to specimens and those that are used in the field, but the classification is rough and many analysis techniques are usable both in the laboratory and field or may require a combination of field and laboratory techniques.

Collections
The earliest approaches to modern bird study involved the collection of eggs, a practice known as oology. While collecting became a pastime for many amateurs, the labels associated with these early egg collections made them unreliable for the serious study of bird breeding. To preserve eggs, a tiny hole was made and the contents extracted. This technique became standard with the invention of the blow drill around 1830. Egg collection is no longer popular; however, historic museum collections have been of value in determining the effects of pesticides such as DDT on physiology. Museum bird collections continue to act as a resource for taxonomic studies. The use of bird skins to document species has been a standard part of systematic ornithology. Bird skins are prepared by retaining the key bones of the wings, legs, and skull along with the skin and feathers. In the past, they were treated with arsenic to prevent fungal and insect (mostly dermestid) attack. Arsenic, being toxic, was replaced by less-toxic borax. Amateur and professional collectors became familiar with these skinning techniques and started sending in their skins to museums, some of them from distant locations. This led to the formation of huge collections of bird skins in museums in Europe and North America. Many private collections were also formed. These became references for comparison of species, and the ornithologists at these museums were able to compare species from different locations, often places that they themselves never visited. Morphometrics of these skins, particularly the lengths of the tarsus, bill, tail, and wing became important in the descriptions of bird species. These skin collections have been used in more recent times for studies on molecular phylogenetics by the extraction of ancient DNA. The importance of type specimens in the description of species make skin collections a vital resource for systematic ornithology. However, with the rise of molecular techniques, establishing the taxonomic status of new discoveries, such as the Bulo Burti boubou (Laniarius liberatus, no longer a valid species) and the Bugun liocichla (Liocichla bugunorum), using blood, DNA and feather samples as the holotype material, has now become possible.

Other methods of preservation include the storage of specimens in spirit. Such wet specimens have special value in physiological and anatomical study, apart from providing better quality of DNA for molecular studies. Freeze drying of specimens is another technique that has the advantage of preserving stomach contents and anatomy, although it tends to shrink, making it less reliable for morphometrics.

In the field
The study of birds in the field was helped enormously by improvements in optics. Photography made it possible to document birds in the field with great accuracy. High-power spotting scopes today allow observers to detect minute morphological differences that were earlier possible only by examination of the specimen "in the hand". The capture and marking of birds enable detailed studies of life history. Techniques for capturing birds are varied and include the use of bird liming for perching birds, mist nets for woodland birds, cannon netting for open-area flocking birds, the bal-chatri trap for raptors, decoys and funnel traps for water birds. The bird in the hand may be examined and measurements can be made, including standard lengths and weights. Feather moult and skull ossification provide indications of age and health. Sex can be determined by examination of anatomy in some sexually nondimorphic species. Blood samples may be drawn to determine hormonal conditions in studies of physiology, identify DNA markers for studying genetics and kinship in studies of breeding biology and phylogeography. Blood may also be used to identify pathogens and arthropod-borne viruses. Ectoparasites may be collected for studies of coevolution and zoonoses. In many cryptic species, measurements (such as the relative lengths of wing feathers in warblers) are vital in establishing identity. Captured birds are often marked for future recognition. Rings or bands provide long-lasting identification, but require capture for the information on them to be read. Field-identifiable marks such as coloured bands, wing tags, or dyes enable short-term studies where individual identification is required. Mark and recapture techniques make demographic studies possible. Ringing has traditionally been used in the study of migration. In recent times, satellite transmitters provide the ability to track migrating birds in near-real time.

Techniques for estimating population density include point counts, transects, and territory mapping. Observations are made in the field using carefully designed protocols and the data may be analysed to estimate bird diversity, relative abundance, or absolute population densities. These methods may be used repeatedly over large timespans to monitor changes in the environment. Camera traps have been found to be a useful tool for the detection and documentation of elusive species, nest predators and in the quantitative analysis of frugivory, seed dispersal and behaviour.

In the laboratory
Many aspects of bird biology are difficult to study in the field. These include the study of behavioural and physiological changes that require a long duration of access to the bird. Nondestructive samples of blood or feathers taken during field studies may be studied in the laboratory. For instance, the variation in the ratios of stable hydrogen isotopes across latitudes makes establishing the origins of migrant birds possible using mass spectrometric analysis of feather samples. These techniques can be used in combination with other techniques such as ringing.

The first attenuated vaccine developed by Louis Pasteur, for fowl cholera, was tested on poultry in 1878. Anti-malarials were tested on birds which harbour avian-malarias. Poultry continues to be used as a model for many studies in non-mammalian immunology.

Studies in bird behaviour include the use of tamed and trained birds in captivity. Studies on bird intelligence and song learning have been largely laboratory-based. Field researchers may make use of a wide range of techniques such as the use of dummy owls to elicit mobbing behaviour, and dummy males or the use of call playback to elicit territorial behaviour and thereby to establish the boundaries of bird territories. Studies of bird migration including aspects of navigation, orientation, and physiology are often studied using captive birds in special cages that record their activities. The Emlen funnel, for instance, makes use of a cage with an inkpad at the centre and a conical floor where the ink marks can be counted to identify the direction in which the bird attempts to fly. The funnel can have a transparent top and visible cues such as the direction of sunlight may be controlled using mirrors or the positions of the stars simulated in a planetarium.

The entire genome of the domestic fowl (Gallus gallus) was sequenced in 2004, and was followed in 2008 by the genome of the zebra finch (Taeniopygia guttata). Such whole-genome sequencing projects allow for studies on evolutionary processes involved in speciation. Associations between the expression of genes and behaviour may be studied using candidate genes. Variations in the exploratory behaviour of great tits (Parus major) have been found to be linked with a gene orthologous to the human gene DRD4 (Dopamine receptor D4) which is known to be associated with novelty-seeking behaviour. The role of gene expression in developmental differences and morphological variations have been studied in Darwin's finches. The difference in the expression of Bmp4 have been shown to be associated with changes in the growth and shape of the beak.

The chicken has long been a model organism for studying vertebrate developmental biology. As the embryo is readily accessible, its development can be easily followed (unlike mice). This also allows the use of electroporation for studying the effect of adding or silencing a gene. Other tools for perturbing their genetic makeup are chicken embryonic stem cells and viral vectors.

Collaborative studies
With the widespread interest in birds, use of a large number of people to work on collaborative ornithological projects that cover large geographic scales has been possible. These citizen science projects include nationwide projects such as the Christmas Bird Count, Backyard Bird Count, the North American Breeding Bird Survey, the Canadian EPOQ or regional projects such as the Asian Waterfowl Census and Spring Alive in Europe. These projects help to identify distributions of birds, their population densities and changes over time, arrival and departure dates of migration, breeding seasonality, and even population genetics. The results of many of these projects are published as bird atlases. Studies of migration using bird ringing or colour marking often involve the cooperation of people and organizations in different countries.

Applications
Wild birds impact many human activities, while domesticated birds are important sources of eggs, meat, feathers, and other products. Applied and economic ornithology aim to reduce the ill effects of problem birds and enhance gains from beneficial species. The role of some species of birds as pests has been well known, particularly in agriculture. Granivorous birds such as the queleas in Africa are among the most numerous birds in the world, and foraging flocks can cause devastation. Many insectivorous birds are also noted as beneficial in agriculture. Many early studies on the benefits or damages caused by birds in fields were made by analysis of stomach contents and observation of feeding behaviour. Modern studies aimed to manage birds in agriculture make use of a wide range of principles from ecology. Intensive aquaculture has brought humans in conflict with fish-eating birds such as cormorants.

Large flocks of pigeons and starlings in cities are often considered as a nuisance, and techniques to reduce their populations or their impacts are constantly innovated. Birds are also of medical importance, and their role as carriers of human diseases such as Japanese encephalitis, West Nile virus, and influenza H5N1 have been widely recognized. Bird strikes and the damage they cause in aviation are of particularly great importance, due to the fatal consequences and the level of economic losses caused. The airline industry incurs worldwide damages of an estimated US$1.2 billion each year.

Many species of birds have been driven to extinction by human activities. Being conspicuous elements of the ecosystem, they have been considered as indicators of ecological health. They have also helped in gathering support for habitat conservation. Bird conservation requires specialized knowledge in aspects of biology and ecology, and may require the use of very location-specific approaches. Ornithologists contribute to conservation biology by studying the ecology of birds in the wild and identifying the key threats and ways of enhancing the survival of species. Critically endangered species such as the California condor have had to be captured and bred in captivity. Such ex situ conservation measures may be followed by reintroduction of the species into the wild.

Additional sources

 * (Reprinted from the 1884 Encyclopædia Britannica)
 * (Reprinted from the 1884 Encyclopædia Britannica)
 * (Reprinted from the 1884 Encyclopædia Britannica)
 * (Reprinted from the 1884 Encyclopædia Britannica)

title: Ornithology