Heiko Schmaljohann
email: heiko.schmaljohann@ifv-vogelwarte.de

Philipp J J Becker

Hakan Karaardic

Beat Felix Liechti

Celia Grande

B Naef-Daenzer

Nocturnal exploratory flights, departure time and, direction in a migratory songbird

Keywords: Northern Wheatear, exploratory flight, departure time, departure direction, radio telemetry

Stopover studies have concentrated so far mostly on mechanisms regulating the temporal organisation on the day-to-day level. Taking advantage of the small and isolated island of Helgoland in the North Sea, we investigated the stopover and departure behaviour of a nocturnal migrant by using radio telemetry. Special attention was paid particularly to nocturnal behaviour, their departure times within the night, and departure directions. Here we show that Northern Wheatears, Oenanthe oenanthe, performed regularly nocturnal exploratory flights on nights before and on departure night, which might be a common behaviour of nocturnal migrants to evaluate meteorological conditions aloft prior to departure. We proposed that migrants being prepared for an endurance flight would depart early in the night within a short time window, whereas individuals departing with low fuel load would be less prone to take off early. Our data, however, could not support this hypothesis. In respect of the migratory direction there was a significant correlation between departure direction and departure fuel load. Northern Wheatears with high departure fuel loads headed more towards north than lean migrants, which departed mostly towards the nearest coastline, i.e. east to south. Thus, birds with high fuel loads showed their seasonally appropriate migratory direction irrespectively of the ecological barrier ahead, whereas lean birds avoided this direction. To our knowledge, this is the first study that investigates the relationship of fuel load and departure direction in a free-flying songbird.

Introduction

While migrating between breeding and wintering areas, birds spend far more of their time and energy at stopover sites than they do flying along their routes [START_REF] Hedenström | Optimum Fuel Loads in Migratory Birds: Distinguishing Between Time and Energy Minimization[END_REF][START_REF] Wikelski | Costs of migration in freeflying songbirds[END_REF]. Stopover ecology has taken over a major role in building up an understanding of how bird migration is organized. The optimal migration strategies from [START_REF] Alerstam | Optimal bird migration: the relative importance of time, energy, and safety[END_REF] predict how birds should organize their stopover in respect of time, energy, and predation [START_REF] Houston | Models of optimal avian migration: state, time and predation[END_REF], which was confirmed by several studies (Hedenström 2007). It is now well understood, why migrants decide setting off from a stopover site during the season [START_REF] Jenni | Behavioural and Physiological Reactions to Environmental Variation in Bird Migration: a review[END_REF][START_REF] Schaub | Departure of migrating European robins, Erithacus rubecula, from a stopover site in relation to wind and rain[END_REF][START_REF] Tsvey | Influence of body condition and weather on departures of first-year European robins, Erithacus rubecula, form an autumn migratory stopover site[END_REF][START_REF] Schaub | Fuel stores, fuel accumulation, and the decision to depart from a migration stopover site[END_REF]Goymann et al. 2010) and what influences migrants' general flight directions [START_REF] Liechti | The relevance of wind for optimal migration theory[END_REF][START_REF] Liechti | Birds: blowin' by the wind?[END_REF]). Only little is, however, known about the nocturnal behaviour on the ground prior departure and why birds depart at a certain night-time and in a certain direction from a stopover site.

To investigate these issues, we conducted a radio telemetry study on an island in the German North Sea. Helgoland offers favourable conditions to investigate nocturnal behaviour of migrants, because (1) owing to its small size (1 km 2 ) presence/absence can be precisely determined, (2) birds having left the island can be treated as having continued their migration, because next stopover sites are at least 50 km away, and (3) birds performing local nocturnal flights will not be misidentified as departing birds, because detection range of radio transmitters is larger than the study area itself. As the study species, we chose the Northern Wheatear, Oenanthe oenanthe, a trans-Sahara migrant, which assembles frequently in two subspecies on Helgoland during migration [START_REF] Dierschke | Stopover of Northern Wheatears Oenanthe oenanthe at Helgoland: where do the migratory routes of Scandinavian and Nearctic birds join and split?[END_REF].

These birds of the nominate subspecies, O. o. oenanthe (oenanthe wheatears hereafter), breed in Scandinavia, whereas the subspecies O. o. leucorhoa (leucorhoa wheatears hereafter) breeds on Iceland, Greenland, and Eastern Canada. Individuals stopping over are, hence, from the same species but face different migratory goals during spring migration.

Migrants having decided not to set off at a certain night might seek for a sheltered site and rest to recover from sleep deprivation of the previous migratory flight [START_REF] Schwilch | Do migratory birds need a nap after a long non-stop flight?[END_REF]. Thus, these birds are supposed to rest during the night, whereas birds preparing for departure might show a higher nocturnal activity prior their departure, which might include screening the horizon for compass calibration [START_REF] Cochran | Migrating Songbirds Recalibrate Their Magnetic Compass Daily from Twilight Cues[END_REF][START_REF] Mouritsen | Migratory birds use head scans to detect the direction of the earth's magnetic field[END_REF]), exploring weather conditions, and possibly performing short nocturnal flights to test meteorological conditions aloft [START_REF] Liechti | Birds: blowin' by the wind?[END_REF]. We hypothesize that nocturnally migrating songbirds show a higher night-time activity on departure night than on nights before.

Prior to departure, migrants need to calibrate their compasses for orientation. The appropriate cues are best available during the twilight period, hence nocturnally migrating songbirds are supposed to depart after this period [START_REF] Moore | Sunset and the orientation behaviour of migrating birds[END_REF]Muheim et al. 2006a). This coincides well with the majority of departure events (Bruderer & Liechti 1995;[START_REF] Liechti | Nocturnal spring migration on Mallorca -schedules of departure and passage[END_REF][START_REF] Bruderer | Bird migration across the Mediterranean[END_REF][START_REF] Gauthreaux | Bird migration in the region of the Gulf of Mexico[END_REF][START_REF] Bolshakov | Spring nocturnal migration of Reed Warblers Acrocephalus scirpaceus: departure, landing and body condition[END_REF]Bulyuk & Tsevy 2006;[START_REF] Schmaljohann | Songbird migration across the Saharathe non-stop hypothesis rejected![END_REF].

However, such radar and mist-net data do not provide spatial and temporal information about departure events of a priori specifically selected individuals. Radio telemetry studies do so [START_REF] Cooke | Biotelemetry: a mechanistic approach to ecology[END_REF]) and revealed that individual departure events may occur much later in the night than the exodus of migrants after sunset might suggest [START_REF] Cochran | Migratory flights of Hylocichla thrushes in spring: a radiotelemetry study[END_REF]Åkesson et al. 1996a;[START_REF] Moore | Time of departure by Summer Tanagers (Piranga rubra) from a stopover site following spring trans-gulf migration[END_REF][START_REF] Åkesson | Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk[END_REF][START_REF] Bolshakov | Initiation of nocturnal flight in two species of long-distance migrants (Ficedula hypoleuca and Acrocephalus schoenobaenus) in spring: a telemetry study[END_REF][START_REF] Bolshakov | Time of nocturnal departures in European robins, Erithacus rubecula, in relation to celestial cues, season, stopover duration and fat score[END_REF]), see also [START_REF] Bulyuk | Timing of nocturnal autumn migratory departures in juvenile European Robins (Erithacus rubecula) and endogenous and external factors[END_REF].

Factors influencing the day-to-day departure decision are also supposed to control for migrants' departure time within the night. These decisions are, generally, ruled by an interplay of environmental and endogenous factors [START_REF] Jenni | Behavioural and Physiological Reactions to Environmental Variation in Bird Migration: a review[END_REF] with meteorological conditions having a paramount effect on flight [START_REF] Liechti | Birds: blowin' by the wind?[END_REF]) and orientation performance (Åkesson et al. 2001), while other environmental factors such as predation danger [START_REF] Fransson | Migratory fueling in blackcaps (Sylvia atricapilla) under perceived risk of predation[END_REF]Schmaljohann & Dierschke 2005) and competition [START_REF] Rappole | Relationships between behaviour, physiology and weather in avian transients at a migration site[END_REF][START_REF] Dierschke | Differential timing of spring migration in Northern Wheatears Oenanthe oenanthe: hurried males or weak females?[END_REF] seem to play a less important role. Fuel deposition rate and stopover duration determine departure fuel load, which in turn defines the potential flight range. Combining the potential flight range with the direction provides the potential flight vector. Migrants with high fuel loads can reach their next migratory goal by a direct flight and can keep their seasonal appropriate migratory direction, whereas leaner birds may need to fly towards the nearest available stopover site. The latter may, hence, be forced to select a flight direction differing from the seasonal appropriate one, if stopover sites are not in the same direction as their overall migratory goal. The length of the potential flight vector might initiate the departure decision within the night: Intended long flights need to be initiated soon after sunset, whereas migrants with short flight vectors do not need to depart early to accomplish their flight during the dark. We hypothesize that migrant's nocturnal departure time is driven by its potential flight vector.

Migrant's potential flight vector plays also an important role for the departure direction, when birds face a particularly inhospitable range of their journey. Long potential flight vectors allow crossing any obstacle by a non-stop flight and keeping the seasonal appropriate migratory direction, whereas short flight vectors drive birds to a detour avoiding the barrier [START_REF] Alerstam | Detours in Bird Migration[END_REF][START_REF] Sandberg | Stored fat and the Migratory Orientation of Birds[END_REF]. Unfavourable weather conditions [START_REF] Richardson | Northeastward reverse migration of birds over Nova Scotia, Canada, in Autumn[END_REF] and the inability to orientate sufficiently well [START_REF] Komenda-Zehnder | Is reverse migration a common feature of nocturnal bird migration? -an analysis of radar data from Israel[END_REF][START_REF] Zehnder | Observation of free-flying nocturnal migrants at Falsterbo: occurrence of reverse flight directions in autumn[END_REF] might deviate birds' flight direction from the expected one as well. Departure direction can, therefore, only be explained in the light of bird's potential flight vector [START_REF] Alerstam | Reoriented bird migration in coastal areas: dispersal to suitable testing grounds?[END_REF][START_REF] Lindström | The adaptive significance of reoriented migration of chaffinch Fringilla coelebs and bramblings F. montifringilla during autumn in southern Sweden[END_REF][START_REF] Sandberg | Interaction of body condition and magnetic orientation in autumn migrating Robins, Erithacus rubecula[END_REF]Åkesson et al. 1996b;[START_REF] Moore | Time of departure by Summer Tanagers (Piranga rubra) from a stopover site following spring trans-gulf migration[END_REF][START_REF] Sandberg | Migratory orientation of red-eyed vireos, Vireo olivaceus, in relation to energetic condition and ecological context[END_REF][START_REF] Sandberg | Stored fat and the Migratory Orientation of Birds[END_REF][START_REF] Deutschlander | Fuel reserves affect migratory orientation of thrushes and sparrows both before and after crossing an ecological barrier near their breeding grounds[END_REF]. We hypothesize that migrants with long potential flight vectors will adjust their departure direction more towards their migratory goal than migrants with short potential flight vectors. In respect of the subspecies, we hypothesize that oenanthe wheatears with long potential flight vectors depart towards north to northeast and leucorhoa birds towards northwest [START_REF] Dierschke | Stopover of Northern Wheatears Oenanthe oenanthe at Helgoland: where do the migratory routes of Scandinavian and Nearctic birds join and split?[END_REF], whereas lean birds of either subspecies might depart in any direction directly towards the mainland.

Methods

Study site

The study was conducted on the German island of Helgoland in the North Sea (54°11′N, 07°55′E); the distance to the nearest coast is about 50 km. Helgoland consists of two islands: the main island (1 km 2 , Helgoland hereafter), on which the field study was carried out, and the small Düne island (0.7 km 2 , Düne hereafter), which lies 800 m to the east of Helgoland. The term "island" refers to both.

Study species

During spring season 2008 138 Northern Wheatears were trapped with spring traps baited with mealworms, Tenebrio molitor, throughout the daylight period from 6th April to 23rd May 2008.

Within 5 min until release, Northern Wheatears were aged and sexed by plumage colouration according to [START_REF] Svensson | Identification guide to European passerines[END_REF], measured (maximum wing length to the nearest 0.5 mm after [START_REF] Svensson | Identification guide to European passerines[END_REF]) and weighed to the nearest 0.2 g with a Pesola balance. Fat score was estimated on a nineclass [START_REF] Kaiser | A new multi-category classification of subcutaneous fat deposits of songbirds[END_REF]) and size of the breast muscle on a four-class scale [START_REF] Bairlein | European-African Songbird Migration Network[END_REF]. Each bird was marked with an individual combination of one aluminium and four split colour-rings. Wing length was used to identify the subspecies: males and females with wing lengths exceeding 102 mm and 97 mm, respectively, were treated as belonging to the Iceland/Greenland subspecies O. o. leucorhoa, whereas wing lengths below 99 mm (males) and 96 mm (females) indicated nominate birds [START_REF] Svensson | Identification guide to European passerines[END_REF].

Birds with wing length from the range of overlap were not assigned to a subspecies.

Departure fuel load

We compared fuel loads rather than body masses owing to the large size differences [range of wing length in this study: 92.5 -110. departure fuel load i = (departure body mass ilean body mass i ) / lean body mass i .

(2)

We obtained birds' departure body mass by either remote weighing or modelling. For remote weighing bowls supplied with mealworms ad libitum were placed on electric balances so that body mass development of individually ringed Northern Wheatears could be surveyed; for further information about this technique see [START_REF] Schmaljohann | Optimal bird migration and predation risk: a field experiment with Northern Wheatears Oenanthe oenanthe[END_REF]. Birds' body mass from the evening of departure (later than 7 pm) was defined as departure body mass. This procedure was successful for four birds. Departure body masses of the others were modelled. Based on field data from our northern wheatear research group on Helgoland, we estimated the 24 h' (day + night) body mass increase to be 0.9 g (Delingat et al. 2006[START_REF] Delingat | Diurnal patterns of body mass change during stopover in a migrating songbird[END_REF].

Flight range calculation

The potential flight range of a bird depends on its departure fuel load, which corresponds to the available energy reserves. According to [START_REF] Delingat | Obligatory barrier crossing and adaptive fuel management in migratory birds: the case of the Atlantic crossing in Northern Wheatears (Oenanthe oenanthe)[END_REF], flight range Y [km] can be estimated as:

Y i = 100 U ln(1+ departure fuel load i ), (3)
with U as the average airspeed of a northern wheatear, which is 46 km/h (13 m/s) after [START_REF] Bruderer | Flight characteristics of birds: I. radar measurements of speeds[END_REF].

Direct flights to the breeding grounds lead leucorhoa wheatears towards northwest for 1700 km to Iceland or 2400 km to Greenland. No resting and refuelling sites are available en route for at least 700 km (Scotland). We divided leucorhoa wheatears into two groups: 1) with an estimated flight range exceeding 800 km (corresponding to departure fuel loads ≥ 0.19) and 2) with shorter flight ranges. Leucorhoa wheatears of the latter group had insufficient fuel to cross the North Sea without any wind assistance. The breeding areas of oenanthe wheatears are to the north and east and hence, the next potential stopover sites are reachable with rather low fuel loads [START_REF] Dierschke | Stopover of Northern Wheatears Oenanthe oenanthe at Helgoland: where do the migratory routes of Scandinavian and Nearctic birds join and split?[END_REF][START_REF] Delingat | Obligatory barrier crossing and adaptive fuel management in migratory birds: the case of the Atlantic crossing in Northern Wheatears (Oenanthe oenanthe)[END_REF].

Radio transmitters

Radio transmitters were constructed by the Swiss Ornithological Institute in cooperation with the University of Applied Sciences in Burgdorf, Switzerland (Naef-Daenzer 1993;[START_REF] Naef-Daenzer | Miniaturization (0.2 g) and evaluation of attachment techniques of telemetry transmitters[END_REF]. They were attached to 26 Northern Wheatears using a Rappole-type harness made from 0.5 mm elastic cord [START_REF] Rappole | New harness design for attachment of radio transmitters to small passerines[END_REF]; length of leg-loops were adjusted individually to birds according to [START_REF] Naef-Daenzer | An allometric function to fit leg-loop harnesses to terrestrial birds[END_REF]. The elastic cord degraded when exposed to air, and radio transmitters fell eventually off. Although radio-tagging birds is an interference, no adverse effects were found, when transmitters had been fitted properly [START_REF] Naef-Daenzer | A test for effects of radio-tagging on survival and movements of small birds[END_REF], and increase in flight costs seemed to be low [START_REF] Irvine | Cost of Carrying Radio Transmitters: a Test with Racing Pigeons Columba Livia[END_REF]. Radio transmitters, including battery and harness, weighed 0.8 g. Because lowest body mass of the Northern Wheatears was 21 g (mean ± SD: 23.7 ± 3 g) in the present study, mass of radio transmitters represents at maximum 4% (mean: 3.4%) of birds body mass. Thus, the relative load was lower than 5%, which is suggested to be the upper permissible load limit [START_REF] Cochran | (ed) Wildlife management techniques manual[END_REF][START_REF] Caccamise | An aerodynamic basis for selecting transmitter loads in birds[END_REF].

Detection properties

Batteries' lifetime was about 30 days. Detection range was tested by placing a radio transmitter on a ferry leaving the island and locating its position every few seconds by means of a GPS device. We tracked the radio transmitter with Yagi 3EL2 antennas (Vårgårda, Sweden) in combination with hand-held receivers, YAESU FT-290RII, from Helgoland. The detection range of the radio transmitters depends on the orientation of transmitter's antenna, when pointing towards Helgoland detection range was 8 km, when turned by 45° 10 km and when being perpendicular towards Helgoland 15 km, respectively. We tracked departing birds for (mean ± SD) 15.3 ± 8.5 min (n = 26).

Because Northern Wheatear's airspeed is about 13 m/s [START_REF] Bruderer | Flight characteristics of birds: I. radar measurements of speeds[END_REF], detection range was approximately 12 km, which coincided well with the predicted detection range from the ferry experiment.

Telemetry routine

The 26 radio-tagged Northern Wheatears were tracked during all nights until their departure.

For convenient use each antenna was attached to a 2 m high wooden pole, where a compass was aligned with the antenna axis. Location of the bird was from where the strongest signal came. To test our accuracy of detection, we determined blindfolded the location of a fixed radio-transmitter in a distance of about 750 m (mean ± SD, deviation: 3 ± 5°, n = 49). We estimated our recordings of departing Northern Wheatears to be correct to the nearest 3° (n = 4), 5° (n = 19), and 10° (n = 3) owing to our field experience. We searched the whole island shortly after each individual departure event and during the following day, but departed birds never recurred.

Nocturnal behaviour

We present here a first attempt to quantify bird's nocturnal behaviour on the ground. We assigned for each individual the different fluctuations in the received signal strength to different activity classes. This assignment was based on visual observations during daylight and the twilight period, when we could link fluctuations in the signal strength to defined activity classes.

Unknown: Bird was located, but small fluctuations in the signal strength could not be recorded due to general tracking problems, strong wind or more than one bird to be tracked.

Motionless:

The signal strength was constant, hence, birds did not move.

Little activity: Fluctuations in signal strength were present but weak. We defined this category as small changes in bird's location, i.e. turning around or moving slightly for less than a metre.

High activity: Fluctuations in signal strength were strong, but birds did not fly. We defined such changes as high activity including movements for several meters. We observed that Northern Wheatears were searching for food during daytime and the twilight periods producing as strong fluctuations in the signal strength as we recorded during night-times. This suggests that Northern Wheatears had fed during the night, which is supported by feeding until the onset of darkness [START_REF] Palmgren | On the diurnal rhythm of activity and rest in birds[END_REF][START_REF] Ramenofsky | Migratory Behaviour: New Insights from Captive Studies[END_REF] or even later [START_REF] Morton | Diurnal feeding patterns in white-crowned sparrows Zonotrichia leucophrys gambelii[END_REF].

Exploratory flights: Northern Wheatears performed nocturnal flights for several minutes and landed afterwards at the same or a different site.

To compare the nocturnal behaviour, we present here data of the first four activity classes as relative proportions of the whole observation period per bird and corresponding night. Thus, the different activity values sum up to 100% for the different groups (see Figs 1 and2), but might be based on differently long observation periods per bird and night depending on departure time and end of observation. For some birds and nights no such data could be recorded.

Weather data

We had two sources of meteorological data. 445, 445 -1145, 1145 -2375, and 2375 -4000 m, respectively). Because departure time was in all cases closest to the 0 a.m. data of NOAA, we considered only these data for our analyses. NOAA wind data correlated significantly with wind speed (r = 0.41, n = 61, P = 0.0001), and wind direction (r C-C = 0.523, n = 61, P = 0.0002 [START_REF] Jammalamadaka | Topics in Circular Statistics[END_REF])) data from German Weather Service, so that we had confidence in considering NOAA data from higher altitudes for our analyses.

We used NOAA data to estimate the possible available tailwind component at the four different pressure levels per day around midnight. We defined the overall migratory goal for oenanthe wheatears as 80° (mean departure direction in this study, see Fig. 1) and for leucorhoa wheatears as 315° (direction towards Iceland and Greenland, see Fig. 2). Individual tailwind component was calculated as:

tailwind component i = cos (wind direction imigratory goal i ) x wind speed i .

(4)

Predation danger

We defined predation danger here according to [START_REF] Lank | Death and danger at migratory stopovers: problems with "predation risk[END_REF], as the probability of becoming a prey if no antipredator behaviour was shown. Predation danger was assessed by daily counts as the abundance of raptors posing a potential threat to Northern Wheatears: Eurasian sparrowhawk, Accipiter nisus, common kestrel, Falco tinnunculus, merlin, F. columbarius, Eurasian hobby, F. subbuteo, and peregrine falcon F. peregrinus.

Northern wheatear abundance

Daily counts of Northern Wheatears on Helgoland provided information about species' abundance indicating intraspecific competition.

Position of Sun

We used a freely available internet calculator for astronomical data We analysed different variables possibly influencing sun's elevation at departure by Spearman's rank correlations. Although sun's position below the horizon is more crucial for departure decisions having a more important effect on orientation cues than simply the time elapsed after sunset, we conducted the same analyses also with departure time after sunset as the response variable to compare our results with other studies. Duration from sunset to certain twilight periods changes in course of the season, i.e., at a certain time after sunset sun's position varies with time of season (see Fig. 3).

Results

Here we present data from 26 radio-tagged Northern Wheatears, including 17 oenanthe (all 2nd year males) and 9 leucorhoa (five 2nd year, two adult males, and two 2nd year females) birds.

Stopover duration after radio-tagging varied between 1 and 6 days (mean ± SD: 1.9 ± 1.3 days, n = 26); 15 birds stayed on Helgoland only for one day. Stopover duration did not differ between subspecies (Wilcoxon signed-ranks test: T = 76, n oenanthe = 17, n leucorhoa = 9, P = 1).

Nocturnal behaviour

Of the 26 radio-tagged birds, we could identify the nocturnal behaviour of 23 birds during departure night, of ten during the night before, of five during two nights before departure and of four on earlier nights, respectively. Nocturnal activity did not differ between the two nights before departure and departure night (see Fig. 4) and was generally rather low. Some birds showed only sometimes high activity on the ground. Although leucorhoa wheatears had a higher temporal proportion of little activity than oenanthe wheatears during departure night (Fig. 5), this difference was not statistically significant (ANOVA: F 83,1 = 0.0122, P = 0.912) and oenanthe wheatears' behaviour was often categorized as unknown, which might disguise little activity.

Exploratory flights

During The ratios of birds performing and not performing exploratory flights per night did not differ significantly between the nights before departure and departure night (all 2-sample tests for equality of proportions: P > 0.05). Although birds staying on the island could have perform their exploratory flights at lower sun's elevation than birds departing, sun's elevation of the first exploratory flights did not differ between these two (Wilcoxon signed-ranks test: T = 42, n departure night = 10, n nights before departure = 6, P = 0.22). Northern Wheatears performing exploratory flights on the night before departure did not seem to have been prevented from setting off by unfavourable meteorological conditions, because they did not differ between their departure and previous night (tested for cloud cover, wind speed, wind direction, and rain duration, all Wilcoxon signed-ranks tests: P > 0.15).

Sun's elevation at departure and departure time i. departure in relation to sun's elevation and time after sunset

All Northern Wheatears departed well after the end of civil twilight, when the sun was at least 6° below the horizon; mean sun's elevation ± SD at departure was -14.8 ± 3.4° (n = 26; median = -14.1°). Significantly more Northern Wheatears departed after the Nautical Twilight Period (sun's elevation -12°) compared to the Nautical Twilight Period and before (χ 2 = 13, Df = 1, n after NTP = 20, n NTP and before = 6, P = 0.0003; Fig. 3). There was no significant difference between the two subspecies in sun's elevation (mean ± SD) at departure (oenanthe -14 ± 3.0°, median = -13°; leucorhoa -16 ± Mean ± SD departure time after sunset was 183 ± 66 min (n = 26, median = 176 min).

ii. influence of environmental factors on sun's elevation at departure

Sun's elevation at departure of Northern Wheatears did neither correlate with wind direction (r C-L = 0.15, n = 26, F 2,23 ≈ 0.57, P > 0.05) nor wind speed at ground level (r S = 0.05, n = 26, P = 0.793). There was also no significant correlation between sun's elevation and either tailwind component at the four pressure levels (all r S : P > 0.15). Cloud cover varied in general very little, but was slightly less at departure (mean ± SD, 0.7 ± 1.0) than around sunset (mean ± SD, 2.1 ± 2.0;

Wilcoxon signed-ranked test: T= 177, n = 26, P = 0.002). The statistical difference in cloud cover at departure and around sunset has only little biological significance, because cloud cover at sunset was in comparision to overcast situations very low. Cloud height did not have an influence on suns's elevation at departure (r S = 0.40, n = 26, P = 0.40). Precipitation was very low; it rained on three days during the whole study period and only two birds departed during nights when light drizzle occurred.

We, consequently, omitted precipitation from our analysis. Time of season did not influence departure time after sunset (r S = -0.1876, n = 26, P = 0.3587); here we could not test sun's elevation at departure because of its apparent correlation with season. As in the abiotic factors, predation pressure (r S = -0.15, n = 26, P = 0.478) and competition did not have an effect on the sun's elevation at departure ( r S = -0.15, n = 26, P = 0.472)). Instead of sun's elevation at departure we calculated these correlations also with departure time after sunset, but none was significant.

iii. influence of endogenous factors on sun's elevation at departure

There was no correlation of sun's elevation at departure with stopover duration (r S = -0. 2.47, adj-R 2 = 0.227, n = 26, P = 0.068). We calculated these correlations and model also with departure time after sunset, but none proofed to be significant either.

Departure direction

Oenanthe wheatears departed in directions between northwest and southeast (Fig. 1).

Distribution of departure direction was unimodal (Rayleigh's test: z = 0.69, n = 17, P < 0.001) with a mean direction of 80° (r = 0.69, n = 17). In contrast, leucorhoa wheatears did not show a preferred departure direction (Rayleigh's test: z = 0.08, n = 9, P = 0.94; Fig. 2). There was no effect of season on the departure direction for both subspecies separately and combined (for all three correlations r C-L < 0.10, F < 1.5, P > 0.50).

i. influence of weather on departure direction

Discussion

Departure behaviour

Migrants did not show a higher activity on the ground during departure night than on previous nights (Fig. 4). Birds are either generally resistant to sleep deprivation during migration [START_REF] Rattenborg | Migratory Sleeplessness in the White-Crowned Sparrow (Zonotrichia leucophrus gambelii)[END_REF], or might recover from the overall lack of sleep by some diurnal naps [START_REF] Fuchs | Daytime naps in night-migrating birds: behavioural adaptation to seasonal sleep deprivation in the Swainson's thrush, Catharus ustulatus[END_REF].

Some Northern Wheatears showed a high activity on the ground even after sunset suggesting feeding behaviour, as shown and implied by other studies [START_REF] Palmgren | On the diurnal rhythm of activity and rest in birds[END_REF][START_REF] Morton | Diurnal feeding patterns in white-crowned sparrows Zonotrichia leucophrys gambelii[END_REF][START_REF] Ramenofsky | Migratory Behaviour: New Insights from Captive Studies[END_REF].

About one half of the birds performed exploratory flights on departure nights and before. It seems unlikely that these flights could had been initiated by disturbance, because flights lasted up to several minutes and some birds returned to the initial site, which seems unlikely in case of disturbance.

One might assume that early exploratory flights should indicate early departures from the stopover site and hence indicate a temporal relation between both of them. However, the time of the first exploratory flight until departure did not correlate with sun's elevation at departure (Fig. 6). The reason for this remains unclear. Although some of the exploratory flights shown on Helgoland might be aborted migratory flights of birds being reluctant to continue flying when facing the large sea crossing, we suppose that the exploratory flights are a general behaviour of nocturnal migrants. They might serve (1) to test orientation abilities [START_REF] Mukhin | Nocturnal life of young songbirds well before migration[END_REF], (2) to familiarize themselves with the environment for upcoming departure, here e.g. detecting the distance to the nearest coast, (3) to examine the meteorological conditions aloft for potential departure, or (4) might be caused by strong migratory restlessness ("Zugunruhe"; Gwinner 1990). Similar but longer lasting exploratory flights were detected in young Reed Warblers, Acrocephalus scirpaceus. These were interpreted as premigratory flights to develop stellar compass and for gathering information of geographical cues [START_REF] Mukhin | Nocturnal life of young songbirds well before migration[END_REF]. We suggest that exploratory flights are a common behaviour of nocturnally migrating songbirds to evaluate meteorological conditions aloft, as suggested by [START_REF] Liechti | Birds: blowin' by the wind?[END_REF]. [START_REF] Schaub | Departure of migrating European robins, Erithacus rubecula, from a stopover site in relation to wind and rain[END_REF] even pointed out that a crucial point for understanding departure decisions of migrants is to discover, how migrants predict wind conditions aloft.

Departure time

Northern Wheatears did not depart within a fixed time window after sunset but significantly more departed after the Nautical Twilight Period than before (Fig. 3), which are both in line with other radio telemetry studies [START_REF] Cochran | Migratory flights of Hylocichla thrushes in spring: a radiotelemetry study[END_REF]Åkesson et al. 1996a;[START_REF] Moore | Time of departure by Summer Tanagers (Piranga rubra) from a stopover site following spring trans-gulf migration[END_REF][START_REF] Åkesson | Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk[END_REF][START_REF] Bolshakov | Initiation of nocturnal flight in two species of long-distance migrants (Ficedula hypoleuca and Acrocephalus schoenobaenus) in spring: a telemetry study[END_REF][START_REF] Cochran | Migrating Songbirds Recalibrate Their Magnetic Compass Daily from Twilight Cues[END_REF][START_REF] Bolshakov | Time of nocturnal departures in European robins, Erithacus rubecula, in relation to celestial cues, season, stopover duration and fat score[END_REF] or mist-netting studies [START_REF] Bolshakov | Spring nocturnal migration of Reed Warblers Acrocephalus scirpaceus: departure, landing and body condition[END_REF][START_REF] Bulyuk | Timing of nocturnal autumn migratory departures in juvenile European Robins (Erithacus rubecula) and endogenous and external factors[END_REF]. The skylight polarization pattern, used to calibrate the compass systems [START_REF] Cochran | Migrating Songbirds Recalibrate Their Magnetic Compass Daily from Twilight Cues[END_REF]Muheim et al. 2006b[START_REF] Muheim | Magnetic compass of migratory Savannah sparrows is calibrated by skylight polarization at sunrise and sunset[END_REF], is available before and during but not after the Nautical Twilight Period. Birds departing after this period might have explored the polarization pattern until its disappearance. Even under overcast situations the skylight polarization pattern is available [START_REF] Hegedüs | Polarization patterns of thick clouds: overcast skies have distribution of the angle of polarization similar to that of clear skies[END_REF], so that the availability of stars or in turn the cloud cover is not likely to influence the schedule of nocturnal departure, as in our and other study species: Summer Tanagers, Piranga rubra, Reed Warblers, and European Robins, Erithacus rubecula [START_REF] Moore | Time of departure by Summer Tanagers (Piranga rubra) from a stopover site following spring trans-gulf migration[END_REF][START_REF] Åkesson | Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk[END_REF][START_REF] Bulyuk | Timing of nocturnal autumn migratory departures in juvenile European Robins (Erithacus rubecula) and endogenous and external factors[END_REF][START_REF] Bolshakov | Time of nocturnal departures in European robins, Erithacus rubecula, in relation to celestial cues, season, stopover duration and fat score[END_REF]). However, Hylochicla thrushes from North America departed later at night with an increasing cloud cover [START_REF] Cochran | Migratory flights of Hylocichla thrushes in spring: a radiotelemetry study[END_REF].

In contrast to the high variation in departure times after sunset found in individually radiotagged songbirds [START_REF] Moore | Time of departure by Summer Tanagers (Piranga rubra) from a stopover site following spring trans-gulf migration[END_REF][START_REF] Åkesson | Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk[END_REF][START_REF] Bolshakov | Time of nocturnal departures in European robins, Erithacus rubecula, in relation to celestial cues, season, stopover duration and fat score[END_REF]; this study) and the two hours lasting exodus of nocturnal migrants detected by radar [START_REF] Liechti | Nocturnal spring migration on Mallorca -schedules of departure and passage[END_REF][START_REF] Bruderer | Bird migration across the Mediterranean[END_REF], [START_REF] Cochran | Orientation and other migratory behaviours of a Swainson's Thrush followed for 1500 km[END_REF] found a very restricted time period (range 9-13 min after evening civil twilight) for one migratory Swainson's Thrush, Catharus ustulata, being radio tracked for seven successive nights. In accordance with this result is a consistency in the initiation of flight activity of captive Redstarts, Phoenicurus phoenicurus, in consecutive nights demonstrating that the internal clock might govern the timing of migratory behaviour [START_REF] Coppack | Circadian flight schedules in night-migrating birds caught on migration[END_REF], though contrary results

were shown by [START_REF] Palmgren | On the diurnal rhythm of activity and rest in birds[END_REF]. These studies indicate that individual migrants might have a preferred departure time within the night. Because we could track our study birds only once, we could not assess whether individuals departed at the same time in consecutive nights. An autonomous circadian clock might purport the preferred time window for departure, environmental and migrant's body condition will, however, most likely modulate the exact time of the departure within the genetically predetermined time window [START_REF] Åkesson | Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk[END_REF][START_REF] Bulyuk | Timing of nocturnal autumn migratory departures in juvenile European Robins (Erithacus rubecula) and endogenous and external factors[END_REF]. The relation between the environment and the endogenous rhythms might be highly complex, which might disguise clear effects of single meteorological conditions on departure time within the night (Bulyuk & Tsevy 2006).

Departure direction

In contrast to other studies [START_REF] Åkesson | Wind selectivity of migratory flight departures in birds[END_REF], Åkesson et al. 2002[START_REF] Schaub | Departure of migrating European robins, Erithacus rubecula, from a stopover site in relation to wind and rain[END_REF][START_REF] Liechti | Birds: blowin' by the wind?[END_REF]), wind direction did not have an influence on the departure direction of Northern Wheatears. Because experienced tailwind (mean: 1 m/s; 1 st quantile: -3.0 m/s, 3 rd quantile: 4.7 m/s) and general wind speed (seasonal mean ± SD: 6.6 ± 2.6 m/s, n = 34) were much lower than bird's potential airspeed of 13 m/s [START_REF] Bruderer | Flight characteristics of birds: I. radar measurements of speeds[END_REF], and because only strong winds (headwinds > 5 m/s) were assumed to influence departure direction (Åkesson et al. 2002) and departure decision [START_REF] Erni | Wind and rain govern the intensity of nocturnal bird migration in central Europea log-linear regression analysis[END_REF][START_REF] Schaub | Departure of migrating European robins, Erithacus rubecula, from a stopover site in relation to wind and rain[END_REF]), both were not influenced as strongly by wind in this study than as in others.

The mean departure direction of oenanthe wheatears (80°, r = 0.69, n = 17; Fig. 1) in this study differed significantly from the more northwards orientated direction determined by [START_REF] Dierschke | Stopover of Northern Wheatears Oenanthe oenanthe at Helgoland: where do the migratory routes of Scandinavian and Nearctic birds join and split?[END_REF] during a release experiment on Helgoland (10°, r = 0.63, n = 17; Watson's Two-Sample Test of Homogeneity: test statistic = 0.256, P < 0.05). Birds from [START_REF] Dierschke | Stopover of Northern Wheatears Oenanthe oenanthe at Helgoland: where do the migratory routes of Scandinavian and Nearctic birds join and split?[END_REF] were kept in captivity with food ad libitum for several days and were released after sunset with a light stick attached to their tail feathers allowing tracking them for about 1 km by means of binoculars. Owing to the more northbound departure direction, we would expect higher departure fuel loads and with that longer flight ranges in their oenanthe wheatears than in our birds. However, the contrary was the case (mean departure fuel load ± SD: [START_REF] Dierschke | Stopover of Northern Wheatears Oenanthe oenanthe at Helgoland: where do the migratory routes of Scandinavian and Nearctic birds join and split?[END_REF], 0.07 ± 0.09, n = 17; this study, 0.14 ± 0.08; Wilcoxon signed-ranks test: W = 81, n oenanthe = 17, P = 0.0287). Besides body condition, birds' origin might explain deviant departure directions in the two studies. Only few wheatears of British origin were probably in our study, because mean wind direction was 67° (Rayleigh's test: z = 0.42, n = 863, P < 0.0001, 24 measurements per day on ground) and westerly wind prevailed only on two days. In [START_REF] Dierschke | Stopover of Northern Wheatears Oenanthe oenanthe at Helgoland: where do the migratory routes of Scandinavian and Nearctic birds join and split?[END_REF] study, wind came from westerly directions during one third of the study period, so that a higher proportion of British wheatears might have been drifted towards Helgoland than in our study. Thus, more birds could have departed towards northwest in their than in our study to compensate for the drift.

Departure direction of oenanthe and leucorhoa wheatears correlated significantly with their departure fuel load. Migrants with a high departure fuel load headed further northwards than lean birds, which generally headed towards the nearest coastline. Similar results were found in orientation experiments of thrushes and larks also during spring migration [START_REF] Sandberg | Migratory orientation of red-eyed vireos, Vireo olivaceus, in relation to energetic condition and ecological context[END_REF][START_REF] Sandberg | Stored fat and the Migratory Orientation of Birds[END_REF][START_REF] Deutschlander | Fuel reserves affect migratory orientation of thrushes and sparrows both before and after crossing an ecological barrier near their breeding grounds[END_REF]. There, only birds with high fuel loads showed their seasonally appropriate migratory direction in orientation cages in front of an ecological barrier, whereas lean birds avoided this direction. A similar, though owing to the small sample size not statistically verifiable phenomenon was shown by our leucorhoa wheatears. The three leucorhoa wheatears that departed towards northwest carried sufficient fuel loads for a successful flight of at least 700 km to cross the North Sea without any wind assistance (mean ± SD: 1544 ± 883 km, n = 3). Also the two oenanthe wheatears departing towards northwest or north had such high fuel loads (Fig. 7). Other leucorhoa wheatears had generally lower fuel loads and departed in any other direction. This tendency is supported by the release experiment from [START_REF] Dierschke | Stopover of Northern Wheatears Oenanthe oenanthe at Helgoland: where do the migratory routes of Scandinavian and Nearctic birds join and split?[END_REF]. Their leucorhoa wheatears had relatively high mean departure fuel loads corresponding to mean ± SD potential flight ranges of 967 ± 641 km (n = 8). If considering only those birds with a flight range exceeding 700 km, distribution of departure direction was unimodal towards northwest (298°; Rayleigh test: z = 0.70, n = 6, P = 0.0474). Thus, there are hints that leucorhoa wheatears only cross the North Sea towards northwest, if they possess high departure fuel loads, whereas lean migrants are more reluctant to proceed their migration across ecological barriers [START_REF] Alerstam | Reoriented bird migration in coastal areas: dispersal to suitable testing grounds?[END_REF][START_REF] Alerstam | Detours in Bird Migration[END_REF][START_REF] Sandberg | Migratory orientation of red-eyed vireos, Vireo olivaceus, in relation to energetic condition and ecological context[END_REF][START_REF] Sandberg | Stored fat and the Migratory Orientation of Birds[END_REF][START_REF] Deutschlander | Fuel reserves affect migratory orientation of thrushes and sparrows both before and after crossing an ecological barrier near their breeding grounds[END_REF]. Deviations from the seasonal appropriated migratory direction were known from field observations [START_REF] Koch | Vogelzug unter Einfluss von Leitlinie und Windrichtung[END_REF][START_REF] Haartman L Von | Umschlagende Zugrichtung beim Buchfinken, Fingilla c. coelebs L., im Herbst[END_REF][START_REF] Haartman L Von | Beobachtungen über umschlagende Zugrichtung der Bachstelze, Motacilla alba L., im Herbst[END_REF][START_REF] Rabøl | Reversed migration as the cause of westward vagrancy by four Phylloscopus warblers[END_REF][START_REF] Alerstam | Reoriented bird migration in coastal areas: dispersal to suitable testing grounds?[END_REF][START_REF] Richardson | Northeastward reverse migration of birds over Nova Scotia, Canada, in Autumn[END_REF], ring recovery analyses (Åkesson et al. 1996b), orientation cage experiments [START_REF] Sandberg | Migratory orientation of red-eyed vireos, Vireo olivaceus, in relation to energetic condition and ecological context[END_REF][START_REF] Sandberg | Orientation of nocturnally migrating Swainson's thrush at dawn and dusk: Importance of energetic condition and geomagnetic cues[END_REF][START_REF] Sandberg | Stored fat and the Migratory Orientation of Birds[END_REF][START_REF] Deutschlander | Fuel reserves affect migratory orientation of thrushes and sparrows both before and after crossing an ecological barrier near their breeding grounds[END_REF], radar [START_REF] Richardson | Northeastward reverse migration of birds over Nova Scotia, Canada, in Autumn[END_REF]Liechti & Bruderer 1995;[START_REF] Komenda-Zehnder | Is reverse migration a common feature of nocturnal bird migration? -an analysis of radar data from Israel[END_REF], and infra-red studies [START_REF] Zehnder | Observation of free-flying nocturnal migrants at Falsterbo: occurrence of reverse flight directions in autumn[END_REF]. Migrants might have left the stopover sites because of strong competition [START_REF] Rappole | Relationships between behaviour, physiology and weather in avian transients at a migration site[END_REF][START_REF] Moore | Evidence of food-based competition among passerine migrants during stopover[END_REF], high predation danger (Lindstöm 1990;[START_REF] Moore | Resumption of feeding under risk of predation[END_REF][START_REF] Fransson | Migratory fueling in blackcaps (Sylvia atricapilla) under perceived risk of predation[END_REF], and/or low quality stopover sites [START_REF] Ktitorov | The importance of landscape context for songbirds on migration: body mass gain is related to habitat cover[END_REF]. Orientation away from any inhospitable habitat seems to be an adaptive behaviour for lean birds [START_REF] Alerstam | Reoriented bird migration in coastal areas: dispersal to suitable testing grounds?[END_REF][START_REF] Lindström | The adaptive significance of reoriented migration of chaffinch Fringilla coelebs and bramblings F. montifringilla during autumn in southern Sweden[END_REF][START_REF] Sandberg | Migratory orientation of red-eyed vireos, Vireo olivaceus, in relation to energetic condition and ecological context[END_REF][START_REF] Sandberg | Orientation of nocturnally migrating Swainson's thrush at dawn and dusk: Importance of energetic condition and geomagnetic cues[END_REF][START_REF] Sandberg | Stored fat and the Migratory Orientation of Birds[END_REF][START_REF] Deutschlander | Fuel reserves affect migratory orientation of thrushes and sparrows both before and after crossing an ecological barrier near their breeding grounds[END_REF]) and might reduce the apparent high mortality rate on migration [START_REF] Sillett | Variation in survivorship of a migratory songbird throughout its annual cycle[END_REF]. Although unfavourable weather [START_REF] Richardson | Timing of bird migration in relation to weather: updated review[END_REF]) and errors in orientation [START_REF] Komenda-Zehnder | Is reverse migration a common feature of nocturnal bird migration? -an analysis of radar data from Israel[END_REF][START_REF] Zehnder | Observation of free-flying nocturnal migrants at Falsterbo: occurrence of reverse flight directions in autumn[END_REF] will lead to reverse migration as well, most reverse migration is an individual-based and intended reaction up on the actual stopover situation.

Coming from their Sahelian wintering areas leucorhoa wheatears have to eventually switch their migratory direction from north to northwest or even west to reach their breeding areas on Iceland, Greenland or Eastern Canada during spring migration. Such a switch in the migratory direction is likely to be genetically determined [START_REF] Gwinner | Endogenously controlled changes in migratory direction of the garden warbler, Sylvia borin[END_REF][START_REF] Helbig | Migratory orientation of blackcaps (Sylvia atricapilla): population-specific shifts of direction during the autumn migration[END_REF]), but in leucorhoa wheatears such a change in the migratory direction is only of use, if birds have stored sufficient fuel load and selected favourable meteorological conditions to successfully cross the Atlantic. Migrants are, therefore, supposed to refuel intensively before crossing this marine obstacle as many songbird migrants do before crossing the Sahara [START_REF] Odum | Lipid levels in birds preparing to cross the Sahara[END_REF]. We hypothesize that the trigger to actually perform the genetically determined migratory shift towards northwest might be the interplay of migrant's high fuel load and favourable meteorological conditions for the Atlantic crossing.

grateful for valuable comments on this manuscript to Franz Bairlein, Lukas Jenni, and two anonymous 568 referees.

569

Figure 1. Nocturnal activity on the ground two and one days before departure and departure night.

Given are means and standard error of percentage, for further information see Methods. "dep day" stands for "departure day" (solid triangle), "dep day -1" for "one day before departure day" (solid circle) and "dep day -2" for "two days before departure day" (solid square), respectively. Values per specific day sum up to 1.

  5 mm]. Lean body mass resulted from a linear regression of body mass from 220 Northern Wheatears with fat score < 2 (only traces of fat visible) and muscle score < 2 (low in breast muscle) caught on Helgoland in the years 1998-2002 and 2008: lean body mass i = 0.29 x wing length i -6.85 (1) (F 1,218 = 95.07, adj-R 2 = 0.30, n = 220, P > 0.0001). Departure fuel load was calculated for each individual as:

(

  http://www.sunearthtools.com/dp/tools/pos_sun.php ) to determine sun's elevation at the time of the exploratory flights and departures. Sun's elevation defines the different twilight periods: The Civil Twilight starts at sunset, when sun's elevation is at 0° and ends when the top edge of the sun is 6° below the horizon. The Nautical and Astronomical Twilight periods end at sun's elevation of -12° and -18°, respectively (Fig.3; Rozenberg 1966).StatisticsStatistics were calculated using the statistical software package R (R Development Core Team 2009) including packages "circular", "boot", and "bootstrap". Uniformity of directions was tested with the Rayleigh's Test of Uniformity[START_REF] Batschelet | Circular statistics in biology[END_REF][START_REF] Jammalamadaka | Topics in Circular Statistics[END_REF]. Circularcircular and circular-linear correlations were calculated following[START_REF] Jammalamadaka | Topics in Circular Statistics[END_REF]. The p-value for a circular-linear correlation was approximated by bootstrapping. Out of each circular and linear variable a certain number of values (here 26 equalling sample size of the radiotagged Northern Wheatears) were sampled randomly with replacement and their circular-linear correlation coefficient estimated. This was bootstrapped 2000 times in all cases. The number of such bootstraps with a correlation coefficient larger than the one of the original data set divided by the total number of bootstraps gives a robust estimate of the corresponding p-value[START_REF] Crawley | Daily stopovers as optimal migration strategy in a long-distance migrating passerine: the Northern Wheatear[END_REF].

  departure night ten out of 23 Northern Wheatears performed single exploratory flights, of which two birds showed twice such flight behaviour. The exploratory flights lasted between 1 and 9 min (mean ± SD: 2.5 ± 2.5 min, n = 12) and occurred from 48 to 314 min after sunset. Sun's elevation during the exploratory flights varied between 0.23° and -18.90°. In four cases birds landed in the same area, from where they had initiated their exploratory flights. They landed eight times at different sites on the island. Although we could estimate neither flight altitude nor range during the exploratory flights, three birds covered at least 1 km during such flights, because they landed on the Düne before resuming migration. Actual departure after the first exploratory flight occurred when the sun was 8.7 ± 4.6° (mean ± SD, range = 0.6 -14.3 °, n = 10) further below the horizont or after 67 ± 36 min (mean ± SD, n = 10, range = 15 -120 min; compare Fig.6). The three Northern Wheatears that had rested on the Düne before departure set off from there after 60, 79, and 110 min, respectively. The time of the first exploratory flight until departure did not correlate significantly with sun's elevation at departure (Spearman rank correlation: r S = 0.36, n = 10, P = 0.313; Fig.6).Northern Wheatears performed exploratory flights also on nights before departure: Sun's elevation was -14.3 ± -6.1° (mean ± SD, range = -0.88 --22.4 °, n = 11) and time after sunset was 173 ± 71 min (mean ± SD, range = 55 -168 min, n = 11). During one night before departure four out of ten birds performed exploratory flights: One did three(-14.4°, -17.2°, -18.1° and 159, 213, 258 min after sunset), two birds two flights(-0.9°, -15.5° and 55, 187 min; -20.1°, -22.4° and 213, 268 min after sunset, respectively) and one bird only one flight (-10.7° and 122 min after sunset). Only one out of five (-17.3° and 225 min after sunset) and one out of three (-7.2°, -13.3° and 73, 133 min after sunset) birds showed exploratory flights during two and three nights before departure, respectively.

  4.0°, median = -16°, Wilcoxon signed-ranks test: T = 102, n oenanthe = 17, n leucorhoa = 9, P = 0.1815) and departure time after sunset (mean ± SD, oenanthe 172 ± 56 min, median = 163 min; leucorhoa 203 ± 81 min, median = 208 min, Wilcoxon signed-ranks test: T = 57, n oenanthe = 17, n leucorhoa = 9, P = 0.3057).

  037, n = 26, P = 0.856), departure fuel load (r S = -0.093, n =26, P = 0.652), and deviation of the departure direction from the final migratory goal (r C-L = 0.033, n = 26, F 2,23 ≈ 0.79, P ≈ 0.693). We tested the effect of the flight vector on sun's elevation by combining departure fuel load, flight direction and their interactions in a linear model. To accommodate the departure direction in the linear model, we included the corresponding sine and cosine values instead of the angular values. This model did not explain a significant proportion of the variability in the departure time of Northern Wheatears (F 5,20 =

  Wind direction did not correlate with departure direction of Northern Wheatears; this was neither the case for German Weather Service, NOAA data, nor in respect of the subspecies (r C-C : all P > 0.05). Departure direction did not correlate with available tailwind component at the four different pressure levels (for all four correlations r C-L < 0.25, n = 26, F 2,23 < 1.4, P > 0.05), though all correlation coefficients were positive. The lacking effect of tailwind component on departure direction might be explained by low values of tailwind: at 1000 mb pressure level tailwind component was (mean ± SD) 1.23 ± 4.48 m/s [1 st quantile: -2.8 m/s, 3 rd quantile: 4.7 m/s], at 925 mb 0.45 ± 4.35 m/s [-2.1 m/s, 2.9 m/s], at 850 mb 0.11 ± 3.64 m/s [-2.0 m/s, 2.3 m/s], and at 700 mb 0.05 ± 4.31 m/s [-3.0 m/s, 3.5 m/s] (n = 26), respectively.ii. effect of departure fuel load on departure direction Departure fuel load did not differ significantly between oenanthe (mean ± SD: 0.14 ± 0.08, n = 17) and leucorhoa wheatears (mean ± SD: 0.22 ± 0.23, n = 9; Wilcoxon signed-ranks test: T = 71, P = 0.787). Departure direction of all Northern Wheatears correlated significantly with departure fuel load (r C-L = 0.59, n = 26, F 2,23 ≈ 8.4, bootstrap: n = 2000, P ≈ 0.021; Fig.7). This means the higher the departure fuel load, the stronger was the northwards component in the departure direction. Such circular-linear correlations were not significant on the subspecies-level (for both correlations: r C-L < 0.37, F < 3.48, P > 0.23). Leucorhoa wheatears having departed towards northwest carried high fat loads enabling flight bouts of more than 700 km (800, 1312, and 2521 km). Birds departing in any other direction had in general lower fuel loads and hence, lower flight ranges. Only one of those had a potential flight range of about 1380 km but flew southwards (162°); mean flight range ± SD of the remaining five birds was 331 ± 251 km (n = 5; Fig.2).

Figure 2 .

 2 Figure 2. Nocturnal activity on the ground on the day of departure of oenanthe (solid circles) and

Figure 3 .

 3 Figure 3. Departure directions of 17 Northern Wheatears of the oenanthe subspecies. Their mean

Figure 4 .

 4 Figure 4. Departure directions of nine Northern Wheatears of the leucorhoa subspecies, which did not

Figure 5 .

 5 Figure 5. Time of departure of radio-tagged Northern Wheatears in relation to elevation of the sun on

Figure 6 .

 6 Figure 6. Sun's elevation against time of first exploratory flight until dearture. There is no apparent

Figure 7 .

 7 Figure 7. Departure direction against departure fuel load. For clarity reasons y-axis does not start with

  From the Helgoland station of the German

	Weather Service we obtained cloud cover [x/8], cloud height [max 2500 m], wind speed [m/s] and
	direction [10°], amount [mm], and duration [min] of precipitation at 2 m above sea level on an hourly
	basis. From the National Oceanic and Atmospheric Administration (NOAA, Boulder, Colorado, USA)
	we downloaded NCEP Reanalysis wind speed and direction data
	(http://www.cdc.noaa.gov/cdc/data.ncep.reanalysis.derived.html). We used data from four different
	pressure levels (1000, 925, 850, and 700 mbar) representing four altitude intervals (ground level -

Acknowledgement

We are extremely grateful to Franz Bairlein who helped and supported this project in various ways. Volker Dierschke kindly provided some additional data about the release experiments on Helgoland. Continuous field work was only possible because of important support from Ommo Hüppop, Freimut Schramm, Felix Jachmann, and several volunteers of the Institute of Avian Research "Vogelwarte Helgoland". Meteorological data were kindly supplied by Deutscher Wetterdienst and the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from their Web site at http://www.cdc.noaa.gov/. Northern Wheatears were caught, ringed, and radio-tagged under a licence from the Ministry for Agriculture, the Environment and Rural Areas, Schleswig-Holstein, Germany.

We also wish to thank Ulric Lund very much for his advice about circular statistics. We are very