Rima Tfaily Souayed

Dominique Gaiti
email: dominique.gaiti@utt.fr

Guy Pujolle
email: guy.pujolle@lip6.fr

A COMBINATION OF SIGNALLING PROTOCOLS FOR FUTURE INTERNET GENERATION

Keywords: Signalling protocols, NSIS protocol, AAA protocols, SIP protocol, and policy-based management protocols

The signalling in IP networks, particularly signalling the quality of service (QoS), has been a hot topic for many years and a lot of work has been done on end-to-end QoS signalling. The most important work remains the Resource ReServation Protocol (RSVP), developed by the Internet Engineering Task Force (IETF). RSVP, however, suffers from scalability problem in core networks. Other protocols face inter-working difficulties among domains in which different QoS solutions are deployed. Given the above, a new IETF work group (WG) termed Next Steps in Signalling (NSIS) WG was launched and is supposed to work on the requirements, architecture and protocols signalling purposes in future Internet generation. In this paper, we study some existing protocols and their infrastructures and how they can interoperate with a simplified solution for signalling in IP networks. We also propose an architecture upon which these protocols are to function.

Introduction

Future Internet generation have the challenge of satisfying both end-users and network-operators. The challenge consists of providing the QoS at a high security level, while controlling and managing the mobility in a network in addition to offering an Authentication, Authorization, and Accounting (AAA) services as well as a robust and dynamic control over networks. Providing services such as QoS necessitates some signalling to describe the required service level and then reserve the corresponding resources by using the deployed resource reservation mechanisms in a network [Bru 02]. The most known QoS signalling protocol is the Resource ReServation Protocol (RSVP) [Bra 97], which serves as a starting point to determine the requirements for the simplified solution of signalling which inherited its name from "Next Steps in Signalling" (NSIS) work group (WG). We have to note, however, that a new signalling protocol should use existing QoS technologies whenever it is possible [Bru 02] [Fre 02].

Security is a very important issue in networks especially when it concerns reserving scarce resources and charging for the service(s). Users' requests for higher quality of service than that negotiated in their Service Level Agreements (SLA s) must be rejected and malicious users' capability of modifying accounting information should be rendered impossible. Evidently, users should be charged according to what they have requested and consumed in terms of QoS and resources. So, there is a need to interoperate the new signalling protocol with AAA protocols such as Diameter and Radius [Fre 02].

To offer quality of service and AAA services, it is mandatory for network administrators to regulate who has access to a given set of resources and services under certain conditions [Ren 01]. Nowadays, networks are larger and their devices are more and more complex. Therefore, deployment of the services mentioned above is critically dependent on the use of a policy-based infrastructure that allows Internet service providers (ISPs) and consequently their administrators to regulate their network rather than configuring individual devices [Raj 99]. A policy infrastructure allows ISP intentions to be translated into differential packet treatment of network packet flows. Two well known protocols, Simple Network Management Protocol (SNMP) and Common Open Policy Service (COPS), are used for this purpose.

In this paper, first we explain briefly the policy based networking and the AAA infrastructures including the AAA services and the general NSIS framework proposed by the NSIS WG. Second, we offer a review as well as analysis of protocols mentioned above to choose the most appropriate protocols that interoperate each other with the new signalling protocol to be designed. Third, we provide some assumptions concerning this signalling protocol and propose an architecture upon which these protocols are to function. Finally, we present a conclusion of this paper and state some related perspectives.

Policy-based networking infrastructure

Nowadays, network-operators control their networks via policies. What are these policies? And which network is called policy-based? Policies denote the unified regulations of access to network resources and services based on administrative criteria [Raj 99]. These regulations might be expressed and implemented at different levels. The network view (figure 1) is composed of different nodal views that correspond to policy objectives at various nodes. The nodal view, in turn, is composed of policy rules that are considered as atomic instructions through which various nodes are controlled. So, a policy-based networking (figure 2) is any network managed via a common set of policy definitions. Its

Authentication Authorization Accounting (AAA) infrastructure and services

The deployment of resource reservation mechanisms forces network operators to use an AAA system in order to verify users' claimed identity, determine their access rights before granting network access for them, collect information on their resource usage, calculate the corresponding price and provide fine-grained set of details for billing [Laa 00]. The AAA system is also needed to avoid fraudulent use of QoS and to manage service level agreements (SLAs) among operators. This system is also based on policies to manage AAA services but it has some problems 2 that have led us to adopt a generic AAA architecture [Ren 01] that is compatible with that of policy based described above.

NSIS Framework structure

1 Policy Enforcement Point 2 Explaining these problems is outside this paper scope NSIS protocol, being designed, has two independent layers, an application signalling layer and a common transport signalling layer. Its WG has determined an overall framework structure [Fre 02], where we could identify three entities (figure 3): NSIS Initiator (NI), NSIS Forwarder (NF) and NSIS Responder (NR). NI entity is responsible for initiating a signalling request to reserve network resources. This signalling may be triggered by a user application, a network management entity, or other protocols. It is intercepted by NF, which-in turn-interacts with the local resource management function (RMF), and then forwards it to the next NSIS node. Reaching the NR, the signalling might interact with applications located in that node before signalling is terminated.

Existing protocols analysis

RSVP [Bra 97] serves as a signalling protocol for applications requesting resource reservation. It is used in the Integrated Services (IntServ) approach which is a per-flow based QoS framework. RSVP negotiates an end-to-end reservation between the sender and the receiver and adopts a receiver-initiated reservation style designed for multicast environments and accommodates heterogeneous receiver service needs. RSVP is a robust protocol and allows IntServ to deliver fine-grained QoS guarantees. However, it suffers from two problems [Zho 00]. First, the fine granularity of micro -flows implies a significant routing load. Moreover, the management and maintenance of a large number of states in nodes increase flow setup time and message overhead. Second, whenever the number of flows is huge, particularly in backbone networks, routers become unable to manage these flows and to offer a separate queue for each one. This management of micro flows constitutes a considerable obstacle in term of scalability and cannot be tolerated in core networks.

RSVP includes much more functionality and complexity than it is required in some IP networks. In fact, the QoS problem, in such networks, is significantly simpler to solve. The trade-off between performance and functionality is one of the key issues here. In addition, the QoS sensitive applications do not use RSVP multicast capabilities [De M 02]. For these reasons, RSVP usage, in our framework, is limited to local access networks.

Session Initiation Protocol (SIP) is an application level control protocol [Ros 02

] developed in IETF. It is used for setting up, modifying and terminating multimedia sessions and for making Internet telephony calls between two or more parties. It runs over both transport protocols: the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). Studies on using Stream Control Transfer Protocol (SCTP) as the transport protocol for SIP have emerged. SIP follows the client-server paradigm. SIP system has two types of network components: user agents and network servers. User agents are end systems that initiate and accept sessions on behalf of users, while network servers provide call routing, service location and supplementary services such as redirection.

SIP is widely used by backbone providers due to its simplicity. However, it supports neither QoS services nor mobility services [Pan 98]. Hence, SIP needs to inter-operate with other protocols that support these services and exchange accounting information about user resource usage.

Radius was designed in the mid nineties to provide AAA services for dial up point to point protocol (PPP) and terminal access servers [Rig 97]. It carries authentication and authorization information between a Network Access Server (NAS), which desires to authenticate its links, and an Authentication Server. The NAS, which is a client of Radius, sends an access request to a given Radius server which would authenticate the user and return information to its client about delivering services for the Dial-in-user.

As routers and Network Access Servers have increased in complexity and density, Radius has become less useful in such large networks. The need arises for a more powerful protocol that effectively handles the problems encountered in Radius. Diameter [Cal 02] is proposed by the AAA WG as an alternative to radius. Although Diameter does not use the same Protocol Data Unit (PDU) as Radius, it could sufficiently borrow from it in order to guarantee a backward compatibility. The idea is to create a Base Protocol that could be extended to allow new access methods. Diameter consists of two parts [Cal 02]: a Base Protocol and different extensions and applications. The Base Protocol is intended to provide an AAA framework for mobile hosts, NASs and roaming operations to be used by all Diameter applications. Diameter application part defines application specific functions and data units each of which is specified separately. This protocol has so far three applications [Cal 01]: NASREQ, Mobile IP and Cryptographic Message Syntax (CMS) security application which is closely connected to the Base Protocol in order to provide security to all applications. Diameter Base Protocol runs on both transport protocols: TCP and SCTP (Stream Control Transmission Protocol). SCTP is more enhanced due to the connection oriented relationship that exists among Diameter peers. It is capable of categorizing several independent streams to a single SCTP association instead of keeping all streams as independent TCP connections. Diameter connections are secured by IP Security (IPSec) and/or Transport Layer Security (TLS). It is a peer-to -peer protocol; any Diameter node could initiate a request and Diameter messages are routed according to the Network Access Identifier (NAI) of a particular user.

Diameter Diameter server could send unsolicited requests and appears suitable for exchanging policies among domains. Its disadvantage is that the transport protocol over which it runs, i.e., SCTP, is not so far widely used.

COPS [Dur 00

] is a request/response protocol between a PEP (policy enforcement point) and a PDP (policy decision point) for exchanging policy information and conveying decisions made by PDP (figure 6). In this client/server model, PEP sends requests to its PDP which, in turn, sends decisions back to the PEP. Upon receiving PEP requests, the policy server stores request states until they are explicitly deleted by the PEP whenever they are no longer applicable [Dur 00]. This storage allows PDP to generate, at any time, asynchronous decisions for a currently installed request state. COPS runs over TCP to assure reliable exchange of messages. It is designed to be scalable and extensible. It could support diverse PEP specific information without requiring modifications to the Base Protocol. Like Diameter, COPS relies on CMS objects to provide security and does not require any key to be distributed offline. CMS supports "Key exchanged capability" to exchange keys. COPS provides message level security for authentication, replay protection and message integrity [Eks 00] [Mit 01]. It could also reuse existing protocols such as IPSEC or TLS for security in order to authenticate and secure the channel between the PEP and the PDP.

COPS [Dur 00] is used in the market only as a policy based management protocol. It is considered acceptable as an AAA protocol according to the requirements defined in the AAA WG [Eks 00] [Mit 01]. Therefore, additions are made [Ren 01] to extend it from the client/server model to a proxy -based model supporting AAA. In our framework, we use COPS for Provisioning (COPS-PR) [Cha 01]. In addition, an extension of COPS with new client type is to be defined to assure the communication between the policy server, which is acting as NF, and the RMF entity.

SNMP [Cas 90

] is a standardized protocol, designed by the IETF and used in network management. It allows the communication of management information among agents located in the network elements and a network management station, i.e., a network control center. The management information is stored in management information Base (MIB). SNMP is basically a connectionless protocol that runs over (UDP) but, now, it also runs over TCP. SNMP latest version[Cas 99] [Nat 00], SNMPV3, supports new SNMP message formats and provides more security mechanisms for messages as well as access control and remote configuration of SNMP parameters. It is also characterized by its modularity. It adopts some software engineering techniques that allow architecture scalability.

Lately, the IETF "Configuration Management with SNMP (SNMPConf)" working group studied whether SNMP could be used for policy-based configuration management [Mac 02]. Its goal was to provide a SNMP framework for both policy-based configuration management and monitoring of network. This work group believed that the SNMP framework could provide all the necessary capabilities required for configuration and monitoring. The main advantage of this approach is that traditional configuration methods could be used in combination with more powerful policybased configuration operations. Using objects with the same name space for configuration as well as for monitoring would facilitate error detection and recovery processes. Despite the efforts that were done to overcome SNMP disadvantages, we could not choose SNMP as a protocol for policy based management networks for many reasons. First, a MIB must still be used with SNMP over UDP along with SNMP over TCP [Mcc 99] [San 99]. As a result, the disadvantage of using a UDP would remain. Second, SNMP was successfully deployed because of its simplicity. The efforts to increase its functionalities increased also its degree of complexity. So, its stability might have been altered. COPS is QoS dedicated. Yet, it is open for new functionalities. As a result, none of these protocols is capable by itself of providing a secure connection, managing the QoS and the mobility and offering AAA services. There is a need to enhance some protocol functionality, make possible their interoperability and suggest or design, if necessary, a new protocol capable of doing certain functions that other protocols are not capable of.

Solution: architecture and interoperability among protocols

As we explained before, our objective is to unify existing incompatible signalling protocols or at least to make possible their coexistence. At the beginning, we preferred having multiple signalling protocols; each tailored to perform well its specific task instead of having a single complex protocol to perform all tasks. Yet, it is also important to avoid extremities because splitting up tasks too much complicates the interaction among the different signalling protocols. Below, we propose an architecture upon which these protocols are to function and state its assumptions.

Some assumptions and mode of operations

Our work is mainly an end-to-edge. The new signalling protocol is initiated by an end host (respectively edge node) and is terminated by an edge node (respectively end host) of a domain. Initiation and termination by core nodes are also included. We concentrate on an out-of-bound signalling in core network, i.e., the core network configuration split between signalling and data paths. In this case, signalling messages are routed through nodes which might not be in the data path. So, in end-to-edge approach, an edge node might be a proxy located on an off-path. Nothing prohibits using both in-bound and out-of-bound approaches in core networks.

In the new signalling, a sender-initiated signalling approach is used. In this approach, the sender of the data flow initiates and maintains the resource reservation used for that flow. Therefore, acknowledgements and notifications could be securely delivered to the sending node. Unlike RSVP, nodes do not have to maintain backward routing states. In a sender-initiated approach, a mobile node can initiate a reservation of its incoming flows as soon as it moves to another roaming sub-network. Otherwise, i.e., in a receiver-initiated signalling approach, a mobile node has to inform the receiver about its handover procedure for allowing it to initiate a reservation.

The new signalling protocol should be used to set up both type of resource reservation, unidirectional and bidirectional. Unidirectional resource reservation is required for media stream with no feedback, while bidirectional one is required for voice-call flow. The latter is more advantageous for mobile nodes compared to the unidirectional. In case two communicated mobile nodes change their access points, the signalling procedure, after handover, might be more efficient [Fre 02]. Using Multicast, as in RSVP, would violate the simplicity objective of the signalling protocol. As a result, this signalling would support unicast only. Other assumptions such as "Addressing" will be determined in the near future as we progress in our work. In fact, signalling messages could be addressed directly to the destination or to a closer node by neighboring signalling aware entity [Fre 02].

Framework components

Figure 4 presents a simplified "next network generation". It is composed of an access network, which makes its own decisions concerning the QoS provisioning and a single core domain, which our work is concentrated in. In figure 4, we distinguish some important entities: an access network edge node, core network edge nodes, a QoS policy server, a Security server/AAA server, a SIP proxy, a RMF entity and a Monitoring Function entity. The nodes implement the signalling protocol with different degree of complexity depending on the location of the node i.e., edge node, interior node, etc.

In the next generation of core networks, each domain is expected to have at least two policy servers acting as a QoS policy server and a Diameter server/ AAA server. Having one policy server that acts as a QOS policy server and as an AAA server at the same time could be a possibility. Other servers would be alternative servers. These policy servers are also NFs (Network Forwarders) 1 . Each node, i.e., router, in turn, should act simultaneously as PEP and as a Diameter client, whereas SIP proxy and other proxies should implement the signalling protocol and be diameter clients.

At boot-up time, servers and clients inform each other about the features they support. Each client establishes a connection with its server in order to communicate their capabilities such as the interface types supported. The protocol used between a PDP and its PEPs is COPS, while Diameter is used between an AAA server and its clients. We suggest studying the possibility of running COPS over Stream Control Transmission Protocol (SCTP). In case this is feasible, a node may establish only one connection with the policy server(s). Both types of policies, AAA policies and QoS policies are stored in one (or more) policy repository. Independence is a key feature among these policies. Upon user request performed by service equipment in a NAS, an AAA server requests the corresponding policies from its PR and takes the policy decision of providing the requested service.

Interaction among the signalling protocol and servers (SIP and policy servers)

SIP system does not support user policies and accounting and billing features. It needs to interact with a policy server in order to exchange with it some policy and accounting information. In this case, a SIP server acts as a Diameter client. When a SIP server is being notified to set up a call for a user, it first initiates a Diameter request command to its AAA policy server with all the information about the user. The AAA server, in turn, checks the request against the admission control policy database and returns a Diameter response message. If the response is approved, the caller sends a SIP INVITE message to the called user. After session establishment, the caller must send a notification to the server to start the accounting process. By receiving a termination notification call from the user or a SIP BYE message from the called user, the caller informs its server to stop the accounting.

SIP, as it is used by backbone operators, is not capable of requesting QoS. By implementing the signalling protocol, SIP can request a service quality. When a SIP server receives a user inquiry for establishing a multimedia session, it formulates a request to the QoS policy server. This request may be conveyed via the signalling protocol (see figure 5). As SIP interacts with Diameter, the QoS policy server checks SIP request against its admission control policy database and sends back a response message. In order to minimize session setup time, a SIP server must contact the two policy servers simultaneously (figures 4 , 5). These policy servers should negotiate before generating a coherent response(s ). In case a QoS request for a given user is rejected, the AAA server should be notified. The AAA server should also be notified about the agreed QoS level for a given user. The two policy servers could communicate via signalling protocol messages.

Sign

Signalling protocol interaction with the Resource Management Function

The signalling protocol serves to convey signalling information. It is not involved in reserving resources. Yet, it interacts with resource reservation mechanisms. The RMF entity is charged with resource provisioning according to user SLSs, providing feedback information to the NF about resource availability, topology, configuration, etc. It could, via monitoring capabilities, detect and notify the NF about a SLS violation. The RMF entity helps admission control entity of the QoS policy server to take a decision about accepting or rejecting a given user request at a given time. This admission is per-flow. It may be also per-class, i.e., per-class of service. The NF-RMF interface should allow the negotiation of dynamic SLS parameters. Such negotiation will lead to reconfigure some nodes. The negotiation protocol used for this interface will most probably be an extension of COPS. Finally, standardizing the future Internet generation protocol implies also the standardization of both the SLS parameters and the NF-RMF interface.

QoS technology in core networks

One of the main objectives is to use the existing QoS technologies whenever possible. As DiffServ is being implemented by most operators, we suggest using it in the future core networks. It is important to keep in mind that DiffServ needs, like other existed IP solutions [De M 02], some improvements. We believe that DiffServ needs a simple signalling protocol and it might benefit from the one being designed.

Conclusion and perspectives

In this paper, we analyzed the existing signalling protocols (such as RSVP) and policy-based management protocols (such as COPS and Diameter). We chose the most appropriate protocols with which the signalling protocol needs to interoperate for maintaining simplicity, scalability, and speed. We investigated thoroughly one part of the solution to signalling information. The solution is composed of protocols for core networks: COPS for provisioning the QoS, Diameter for accounting, SNMP for monitoring, RSVP for local access networks, SIP (placed at edges of access networks) for multimedia session and a protocol for signalling QoS and non QoS information. We also proposed an architecture upon which the above protocols are to function. Such architecture allows a separation between resource provisioning as well as management on one hand and resource reservation signalling as well as admission control on the other hand. The above framework along with its protocols will provide a certain level of security for connections as well as delivering AAA services in addition to providing a relatively good quality of service and dynamic control of networks.

However, for our design to fully function, the operators need to adopt and deploy it. To help doing so, standardisation of the signalled QoS parameters and the interface between the Resource Management Function and the Network Forwarder should be accomplished. Once these requirements are achieved, signalling and providing end-toend QoS would be easier. Upon accomplishing the signalling protocol structure, an optimization work will follow in order to maintain simplicity, scalability, and fast signalling in addition to avoiding function redundancy especially in core networks. We will try to extend and enhance COPS functionalities due to its extensibility feature and to study the interaction between resource signalling and mobility protocols, during path updating.

Figure 1 :

 1 Figure 1: A Conceptual Hierarchy of Policies Figure 2: A policy-based Networking management components are: a policy server(s) also called Policy Decision Point (PDP) that makes policy decisions, network nodes (PEP 1 s) that enforce policy actions, a policy repository (PR) where policies are stored and retrieved to help PDP making its decision, and a policy management tool that enables the administrator to enter the intended policies [Raj 99].

Figure 3 :

 3 Figure 3: Basic NSIS Framework

  In summary, RSVP has a scalability problem [Zho 00]. Radius [Mit 01] and Diameter [Cal 02] [Cal 01] are exclusively security oriented. SIP is simple but presents adaptability difficulties in managing quality of service and mobility [Pan 98]. SNMP is successful in monitoring, but it lacks high level configuration management [McC 99].

Figure 4 :

 4 Figure 4: Next generation signalling protocol framework

  is proposed by the AAA WG as the next generation AAA protocol [Ren 01]. It is still under development and testing, but some codes are downloadable from the Internet. Unlike Radius, Diameter is used for larger networks because of its capacity to handle 2^32 requests simultaneously [Eks 00] [Mit 01]. Since it supports backward capabilities with Radius, Diameter agents are capable of acting as Radius gateways, i.e., sending Radius messages through the network without the need for any conversion. Unlike radius, Diameter does not require a shared secret among communicated peers. Its AVPs are protected by using Cryptographic Message syntax. Many mechanisms and algorithms are supported with CMS application, such as Hashing, Content Encryption, etc. Diameter is integrated with mobile IP to allow roaming for wireless network and it is designed to provide AAA framework for mobile-IP application, NAS REQuirements (NASREQ) application and ROAMing Operations (ROAMOPS) [Cal 02]. Moreover,


Alice Cibois
email: alice.cibois@ville-ge.ch

Jean-Claude Thibault

Nicholas R Friedman

Kevin E Omland

Laure Desutter- Grandcolas

Tony Robillard

Eric Pasquet

Reed warblers in the Marquesas Islands: song divergence and plumage convergence of two distinct lineages

Keywords: Acrocephalidae, plumage colouration, convergent evolution, vocalization, island evolution, microsatellites, mtDNA, Oceania

The Pacific Ocean is the site of the most important diversification of insular reed warblers (Acrocephalus). In the Marquesas Islands (Eastern Polynesia), reed warblers belong to two distinct lineages that have reached the archipelago independently. We used mitochondrial sequences and microsatellite markers and found evidence of gene flow between the two lineages. One island in particular, Nuku Hiva, seems to act as the main contact zone between the two taxa. We then used reflectance spectrometry to evaluate the variation of carotenoid colouration among all populations.

The Marquesan reed warblers exhibit convergent evolution in carotenoid-based plumage colouration, but variation among islands was important. Having two different phylogenetic origins, the two lineages acquired their yellow colour independently and there is stochastic variation in plumage among islands. Finally, we analysed the songs of reed warblers from the main islands. The multifactorial analysis showed that vocalizations of the Marquesan reed warblers differ slightly between species, significant differences between islands leading to different dialects. Vocal characteristics may act as the main pre-zygotic barriers when birds from different lineages come into contact. This study provides another example of convergence in plumage evolution in insular bird species. Furthermore, it highlights the importance of integrative analyses, using both genetic and phenotypical characters, for the study of the evolutionary history of the Pacific Ocean avifauna.

Introduction

The Acrocephalidae represent a morphologically conserved family within the Old World warblers [START_REF] Fregin | Multi-locus phylogeny of the family Acrocephalidae (Aves: Passeriformes) -The traditional taxonomy overthrown[END_REF][START_REF] Alström | A review of the recent advances in the systematics of the avian superfamily Sylvioidea[END_REF]. Most species have nondescript plumage, with dull brownish or greyish upperparts and whitish to bright yellow underparts, a few species showing supercilium, crown stripe, or streaks [START_REF] Kennerley | Reed and Bush Warblers[END_REF]. Having 6 genera and 61 species (Gill and Donsker 2018), they reach their highest species richness in western central Asia and Africa. Reed warblers forage primarily on insects and in the temperate regions most reed warblers are migratory.

Around one-third of the species are found on oceanic islands, often very far away from continental land masses, where they have become sedentary endemics. On most of these tropical or subtropical islands, acrocephalids occupy completely different habitats from those occupied by their continental relatives. Although wetlands are used when available, most species are found in woodland, dry bush or scrub [START_REF] Leisler | The Reed Warblers: Diversity in a Uniform Bird Family[END_REF][START_REF] Bell | From wetlands to islands: morphological variation, plumage and song in Pacific island Acrocephalus warblers[END_REF].

3

Having 20 endemic species, the Pacific Ocean represents the most important diversification of insular acrocephalids, all in the genus Acrocephalus. They have a disjunct distribution, as they are found discontinuously from Micronesia to the Pitcairn group and Hawaii (only small islands with shrubland), but are absent from the large and species-rich islands in southern Melanesia and central Polynesia (for instance Vanuatu, Fiji, Samoa). The Pacific reed warblers are able to thrive on very small and isolated islands, where resources are sparse, an ability that is consistent with part of the supertramp definition [START_REF] Diamond | Colonization of exploded volcanic islands by birds: The supertramp strategy[END_REF]. They are often the only land bird on low-lying atolls. However, they also have notably long lifespans and are territorial, living in pairs or trios [START_REF] Mosher | Ecology of the endangered Nightingale Reed-Warbler (Acrocephalus luscinia) on Saipan, Micronesia[END_REF][START_REF] Thibault | Birds of Eastern Polynesia. A biogeographic Atlas[END_REF].

Molecular phylogenies provide evidence for a complex pattern of colonization of the Pacific by reed warblers, in which most lineages are much younger than the ages of the islands they occupy [START_REF] Fregin | Multi-locus phylogeny of the family Acrocephalidae (Aves: Passeriformes) -The traditional taxonomy overthrown[END_REF][START_REF] Cibois | Charting the course of reed-warblers across the Pacific islands[END_REF]. The Pacific reed warblers are divided into two main clades (supplemental Figure S1). The first includes all species from Micronesia (except Guam), Australia (colonized from an insular ancestor, thus a case of reverse colonization), and two Polynesian taxa from the Line Islands and the southern Marquesas. The second lineage includes all remaining Eastern Polynesian taxa and the Hawaiian species. The taxa endemic to three archipelagos (Mariana, Marquesas and Society Islands) are polyphyletic, suggesting several independent colonization events [START_REF] Cibois | Uniform phenotype conceals double colonization by reed-warblers of a remote Pacific archipelago[END_REF][START_REF] Cibois | Systematics of the extinct reed-warblers of the Society islands, Eastern Polynesia[END_REF][START_REF] Saitoh | The complex systematics of Acrocephalus of the Mariana Islands, western Pacific[END_REF]. Overall, the colonization of reed-warblers in the Pacific has been very rapid within the last two million years and it did not follow a regular, stepping-stone-like pattern, as distribution solely might have suggested. [START_REF] Leisler | Evolution of island warblers: beyond bills and masses[END_REF] analysed the morphological variation among insular and continental species and found general evidence of convergent evolution of stronger legs, shorter rictal bristles and broader and rounder wings on islands compared to continental taxa. Their results contrast with those of [START_REF] Komdeur | Why Seychelles Warblers fail to recolonize nearby islands: unwilling or unable to fly there[END_REF], who found no flight differences between the Seychelles Warbler (Acrocephalus sechellensis) and three species that are migratory but do not migrate long-distances.

They concluded that the Seychelles Warbler may have a behavioural reluctance to cross open seawater rather than a morphological impediment. It is likely that the conclusions of these two different studies are both true within the large insular reed warbler radiation: the original colonizers must have good capacities to sustain over-water dispersal, even if those were facilitated by rare events like hurricanes. But during this oceanic radiation, the reed warblers have also developed behavioural adaptations to their new local environment: often a wider prey spectrum, a more terrestrial niche, or a different social system, and this has led to many idiosyncrasies that often differ between archipelagos. Thus, global rules, as proposed when studying the evolution of insular birds, often do not fit the evolution of reed warblers in the Pacific.

The most puzzling example of complex colonization pattern is found in the Marquesas Islands, which have two distinct lineages, one in the north and another in the south [START_REF] Cibois | Uniform phenotype conceals double colonization by reed-warblers of a remote Pacific archipelago[END_REF]). The Northern Marquesan Reed Warbler (Acrocephalus percernis) is closely related to Polynesian species from the Tuamotu and Gambier Islands, whereas the Southern Marquesan Reed Warbler (Acrocephalus mendanae) is closely related to Micronesian taxa (from Nauru and Pagan). Despite their distinct origin, both reached the Marquesas at roughly the same time, around 0.6 million years ago (Mya) according to a mitochondrial molecular clock. The two taxa are cryptic in the sense that they were considered a single species on the basis of plumage similarities -both are relatively large birds, with bright yellow underparts, olive upperparts [START_REF] Kennerley | Reed and Bush Warblers[END_REF]. Furthermore, that this archipelago is so remote led [START_REF] Murphy | Birds collected during the Whitney South Sea Expedition[END_REF] and all subsequent ornithologists to assume they would have been monophyletic.

In addition to the molecular phylogeny, [START_REF] Cibois | Uniform phenotype conceals double colonization by reed-warblers of a remote Pacific archipelago[END_REF] studied the morphology of these two species using multivariate analyses. Overall there is a large overlap in morphometrics, although on average the southern taxon is larger than the northern one. The analyses also suggested a higher influence of resource availability than of phylogeny on the morphology of the Marquesan reed warblers: birds on dry and smaller islands tend to be smaller than birds on large islands, irrespective of their phylogenetic origin. These first genetic and morphological studies, however, left unresolved several issues regarding the evolution of these two taxa. First, the molecular phylogeny was based solely on mitochondrial genes and a limited number of individuals (26, covering all subspecies and 2 to 5 individuals per island). Our first objectives were then to increase the sampling and to include a combination of mitochondrial and nuclear markers. As trials with nuclear intron sequences revealed no variation between these two recently diverged groups, we chose hypervariable microsatellite markers that have been previously used to decipher the phylogeography of insular populations of reed warblers [START_REF] Richardson | Fifty Seychelles warblers (Acrocephalus sechellensis) microsatellite loci polymorphic in Sylviidae species and their cross-species amplification in other passerine birds[END_REF][START_REF] Cibois | Influence of Quaternary sea-level variations on a land bird endemic to Pacific atolls[END_REF]. Second, while previous ornithologists have noted plumage in these groups as being similar, qualitative comparisons performed by human observers can in some cases mislead [START_REF] Eaton | Avian visual perspective on plumage coloration confirms rarity of sexually monochromatic North American passerines[END_REF]. To investigate the colouration of the two Marquesas reed warblers in a more objective way, we used reflectance spectrometry to evaluate the variation of carotenoid colouration among all populations. Finally, we measured and compared vocalizations, which are known as major prezygotic barriers in many closely related passerine species, in particular among species with similar plumages, like in Acrocephalus [START_REF] Lemaire | Mixed song, interspecific competition and hybridisation in the reed and marsh warblers (Acrocephalus scirpaceus and palustris)[END_REF] or Phylloscopus [START_REF] Martens | Intraspecific differentiation of Sino-Himalayan bushdwelling Phylloscopus leaf warblers, with description of two new taxa[END_REF]. Marquesan reed warblers are known to produce a combination of whistles and rasping sounds that includes many repetitions of particular motifs [START_REF] Kennerley | Reed and Bush Warblers[END_REF]. Subtle differences between the different subspecies have been noted, but not all islands were sampled for sounds and individual variation was not assessed [START_REF] Macpherson | More birds of Polynesia[END_REF].

Thus, our first goal in this study is to test whether gene flow occurs between the two taxa, and additionally within each of the island groups. We then evaluate within this system two main mechanisms known to play a role in bird species recognition, based on visual and vocal characters.

Globally insular birds are expected to have colour that is less elaborate than their continental counterparts [START_REF] Doutrelant | Worldwide patterns of bird colouration on islands[END_REF], though in the Passeriformes there are many exceptions to this rule. The Marquesan reed warblers have yellow underparts that are brighter than most continental Acrocephalus [START_REF] Bell | From wetlands to islands: morphological variation, plumage and song in Pacific island Acrocephalus warblers[END_REF]. This may be explained by convergent evolution, which occurs frequently in insular species [START_REF] Warren | Tracking island colonization history and phenotypic shifts in Indian Ocean bulbuls (Hypsipetes: Pycnonotidae)[END_REF][START_REF] Jones | Plumage convergence and evolutionary history of the Island Thrush in the Philippines[END_REF]. This convergence might have been favoured by environmental factors over the signal divergence that is expected to be less important in allopatric species than in sympatric taxa [START_REF] Martin | Rapid sympatry explains greater color pattern divergence in high latitude birds[END_REF]. Regarding vocalization, if gene flow between islands is low, dialects are expected to develop over time by cultural drift or adaptation [START_REF] Potvin | The relative roles of cultural drift and acoustic adaptation in shaping syllable repertoires of island bird populations change with time since colonization[END_REF]. But because the two Marquesan reed warblers have diverged relatively recently, we might expect to identify differences in vocalization that could play a role in species recognition, particularly if gene flow occurs between the two taxa.

Methods

Genetics

From 2000 to 2010, we mist-netted and sampled 135 reed-warblers in the nine Marquesas islands (supplemental Figure S1). We used playback of recorded songs to lure birds into nets. Because reedwarbler males are strongly territorial, most birds captured were males (recognized as singing individuals). We avoided recapture by marking the forehead of captured birds with biodegradable paint and the locations of all singing individuals were geo-referenced. Additionally, we used toe pads from 11 specimens collected by the Whitney South Sea Expedition in the American Museum of Natural History (AMNH), in New York (for a summary of this expedition, see [START_REF] Thibault | Birds of Eastern Polynesia. A biogeographic Atlas[END_REF]). These toe pads were sampled for a previous study [START_REF] Cibois | Uniform phenotype conceals double colonization by reed-warblers of a remote Pacific archipelago[END_REF].

Genomic DNA was extracted, amplified and sequenced using standard protocols, as in [START_REF] Cibois | Uniform phenotype conceals double colonization by reed-warblers of a remote Pacific archipelago[END_REF]. We amplified and sequenced a portion of the mitochondrial cytochrome b gene with the primers 43L/17H (879 bp) [START_REF] Edwards | Mitochondrial resolution of a deep branch in the genealogical tree for perching birds[END_REF][START_REF] Cibois | Uniform phenotype conceals double colonization by reed-warblers of a remote Pacific archipelago[END_REF]. Contigs were assembled using Sequencher (Genecodes, Ann Arbor, Michigan). All sequences were deposited in GenBank (accession numbers MK517292-MK517409; Table S1). For the mitochondrial data, number of haplotypes, nucleotide diversity and mismatch distributions were calculated using DnaSP 6.12.01 [START_REF] Rozas | DnaSP 6: DNA Sequence Polymorphism Analysis of Large Data Sets[END_REF]. Haplotype networks were constructed using Network 5.0.0.3 [START_REF] Bandelt | Median-joining networks for inferring intraspecific phylogenies[END_REF]. Signals of population expansion were tested using Harpending's raggedness index r (Schneider and Excoffier 1999) using Arlequin 3.1 [START_REF] Excoffier | Arlequin ver. 3.1: An integrated software package for population genetics data analysis[END_REF]. We also used Fu's Fs and Tajima's D to test for selective neutrality and population equilibrium [START_REF] Tajima | Statistical method for testing the neutral mutation hypothesis by DNA polymorphism[END_REF]) using DnaSP. The ΘST values between pairs of population were calculated in Arlequin (sequential Bonferroni corrections were applied when appropriate). For the calculation of the pairwise permutations, the individuals from Hatutaa were included into the Eiao population, and the individuals sampled in Mohotani were included into the Hiva Oa group.

We genotyped six microsatellite loci originally developed for the Seychelles Warbler: Ase12, Ase13, Ase18, Ase34, Ase51, Ase55 [START_REF] Richardson | Fifty Seychelles warblers (Acrocephalus sechellensis) microsatellite loci polymorphic in Sylviidae species and their cross-species amplification in other passerine birds[END_REF]. Microsatellite fragments were amplified using a touch-down PCR as described in the original publication. Genotyping was conducted on a 310 Genetic Analyzer (Applied Biosystem) using multiplex PCR (Qiagen). Results were visualized using Genescan Analysis 3.1.2 (Applied Biosystems). The genetic diversity was quantified for allelic richness, observed heterozygosity and expected heterozygosity. These statistics were estimated using Arlequin, Fstat 2.9.3.2 [START_REF] Goudet | FSTAT (Version 1.2): A Computer Program to Calculate F-Statistics[END_REF] and MSA [START_REF] Dieringer | Microsatellite analyser (MSA): a platform independent analysis toll for large microsatellite data sets[END_REF]. Deviations from Hardy-Weinberg genotype frequency equilibrium were tested with Arlequin (10 000 permutations). The FST values between pairs of populations were calculated in Arlequin (sequential Bonferroni corrections were applied when appropriate). For the calculation of the pairwise permutations, the individuals from Hatutaa were included into the Eiao population, and the individuals sampled in Mohotani were included into the Hiva Oa group.

Bayesian assignment of individuals to clusters without using prior information of the sampling locations was conducted using Structure 2.3.1 [START_REF] Pritchard | Inference of population structure using multilocus genotype data[END_REF] and the microsatellite data. The inferred number of clusters, K, was tested from K=1 to K=10. Each run was pursued for 1 000 000 iterations, with a burnin period of 100 000. A correlated allele frequency and admixture model was used. Convergence was assessed with ten independent runs. The inferred number of clusters K was estimated using the value of K with the highest probability: posterior probabilities were computed as indicated in [START_REF] Pritchard | Inference of population structure using multilocus genotype data[END_REF], and an additional ad hoc statistic (∆K) was also estimated, following the simulation study of [START_REF] Evanno | Detecting the number of clusters of individuals using the software STRUCTURE: a simulation study[END_REF]. We estimated the number of immigrants per generation using Migrate 3.6 [START_REF] Beerli | How to use migrate or why are markov chain monte carlo programs difficult to use? In ´Population Genetics for Animal Conservation[END_REF], which follows a MCMC-based maximum-likelihood approach based on an expansion of the coalescence model (from present to the most recent common ancestor). The runs consisted of 10 short chains (sampling 10 000 trees) and three long chains (sampling 100 000 trees) with a burn in period of 10 000 trees. All runs were repeated five times to verify consistency of results. To avoid applying a unique mutation rate to the microsatellite loci, we inferred the number of immigrants per generation Ni by multiplying the mutation-scaled effective population size Θ to the mutation-scaled effective immigration rate M.

Finally, for both data sets, we tested the hypothesis of Isolation by distance (IBD) among island groups using a Mantel test between the genetic distances (estimated as ΘST/(1-ΘST) for mtDNA and Fst /(1-Fst) for microsatellites) and the logarithm of geographic distances [START_REF] Rousset | Genetic differentiation and estimation of gene flow from F-statistics under isolation-by-distance[END_REF].

Plumage colouration

We assessed the carotenoid-based colouration of Marquesan reed warblers using reflectance spectrometry, a method used to quantify colour independently of the spectral sensitivity of the birds [START_REF] Andersson | Quantifying Colors[END_REF]. We followed the techniques used in previous studies of plumage colouration of several groups of Passeriformes [START_REF] Hofmann | Using spectral data to reconstruct evolutionary changes in coloration: carotenoid color evolution in New World orioles[END_REF][START_REF] Kiere | Using color to define species boundaries: Quantitative analysis in the Orchard Oriole complex supports the recognition of two species[END_REF]). Plumage colour was measured using an Ocean Optics USB 2000 spectrometer, with a full spectrum Xenon light source (Ocean Optics PX-2). Measurements were calibrated by a dark and a Spectralon diffuse white standard (Labsphere, North Sutton, New Hampshire). We worked on specimens collected during the Whitney South Sea Expedition and held at the American Museum Natural History collection. We measured 9 to 15 museum skins per island (all males, not molting, in fresh plumage). We focused on the bright yellow underparts of the birds; we measured three regions, throat, breast, and lower belly, which correspond to the visible gradation from more intense on the throat to less intense on the lower belly. The plumage regions followed the topography provided by [START_REF] Andersson | Quantifying Colors[END_REF].

We took three non-overlapping measurements of each region, perpendicular to the plumage surface, and we calculated the mean of the three measures. Initially we also measured the colour of the nape. However, as preliminary analyses showed that the colour of this area was too patchy and gave a large standard deviation between hue values, this region was removed from further analyses.

The evaluation of the colour space was based on three characters: hue, saturation and brightness.

Hue was estimated by the reflectance midpoint within the visible spectral range (400-700 nm, not including the UV peak between 300 and 400 nm), which is a commonly used measure for carotenoidbased plumage [START_REF] Friedman | Convergent gains of red carotenoid-based coloration in the New World blackbirds[END_REF]. Saturation, or chroma, corresponds to our perception of the richness of a hue. In the Marquesan reed warblers, the most pronounced colour is yellow (500-600 nm) and so we measured the saturation of this colour as the reflectance from 500-600 nm divided by the overall reflectance (300-700 nm) and multiplied by 100 (i.e., yellow chroma, the percentage of the feather's reflectance curve in the yellow part of the spectrum; Montgomerie 2006). Brightness was measured as the mean reflectance over the spectral range (300-700 nm). We compared Marquesan reed warblers with another bright yellow species, the Tahiti Reed Warbler (Acrocephalus caffer). We tested the significance of variances between taxa and populations using Multivariate analysis of variance (MANOVA and nested MANOVA, using the Pillai test for significance; Pillai 1955).

We used linear discriminant analysis (LDA), first to infer which of the colour variables we measured were the most discriminant between populations. Second, we compared two discriminant functions:

(1) habitat richness in terms of vegetation diversity, in which the islands are classified either as "rich" with more than 100 indigenous plants or "poor" with less than 100 plant species (due to its proximity to the large Hiva Oa, Tahuata is classified as "rich" as well), and (2) phylogeny, in which the islands are classified according to the two clades, Northern or Southern. All analyses were conducted using the packages ggplot2, Mass, and devtools in R (R-Core-Team 2017), with the data standardized with the scale function (zero mean and unit variance).

Vocalization

Between 1987 and 2012, we recorded males of each Marquesan population, using a Sony ECM 929LT condenser microphone and a Sony WM D6C tape recorder. Tapes were digitized at the Muséum national d'Histoire naturelle, Paris (MNHN, ISYEB) and sounds were deposited in the Sound Library of the MNHN (Accession numbers MNHN-SO-2019-32 to MNHN-SO-2019-59). For the analysis, we selected 4 males from each population (except Hatutaa and Mohotani). We used both short and long songs, but not the alarm calls. Sonograms were generated and a song analysis was performed using Avisoft-SASLab Pro 5.2.07 (Avisoft Bioacoustics). For each individual, up to 5 sequences of 4 seconds continuous vocalization were selected. In our study, we separate two different components of the songs, hereafter called syllables: the "whistle" (shown as horizontal or curved lines in sonograms, see Figure 3) and the "rasping" (shown as multiple blurring lines in sonograms, see Figure 3). In each sequence, we measured the number of syllables, the type of syllable (whistle or rasping), and the number of time the whistle or rasping was repeated. For each syllable we measured duration, start frequency, end frequency, peak frequency, maximum frequency, and minimum frequency (examples for these measures can be seen on this webpage https://www.avisoft.com/tutorial_measure3.htm).

Mean values of these parameters over all sequences were computed for each individual. We tested the significance of variances between taxa and populations using Multivariate analysis of variance (MANOVA and nested MANOVA, using the Pillai test for significance), and linear discriminant analysis to infer which of the variables were the most discriminant between populations. As in the analysis of plumage colouration, we also compared two discriminant functions, "habitat richness" and "phylogeny" (see above).

Results

Genetics

28 haplotypes were found in the mitochondrial sequences of over 143 individuals, global haplotype diversity (Hd) and nucleotide diversity (Pi) being 0.92 and 0.34, respectively. The lowest genetic diversity was found in Ua Pou (Hd 0.43 and Pi 0.01) and the highest in Nuku Hiva (Hd 0.72 and Pi 0.11). We compared haplotype diversity and island area and we found no significant relationship (R 2 = 0.2379, p = 0.2669, supplemental Figure S2). The haplotypes were divided into two main groups, separated by 6 base pairs that corresponded to the two taxa. We found 15 haplotypes for A. percernis (Hd 084, nucleotide diversity Pi 0.09, Figure 1a) and 13 haplotypes for A. mendanae (Hd 085, nucleotide diversity Pi 0.09, Figure 1b). A single individual from Nuku Hiva (A. percernis), Nuku20, had a haplotype from the Southern group that was otherwise found only in individuals from Ua Pou. This individual was identified as male in the field, and its sex was later confirmed with the P2/P8 set of primers used for PCR-based sexing in birds [START_REF] Griffiths | A DNA test to sex most birds[END_REF]. Mismatch distributions for each taxon were multimodal (Figure 1), indicating historically stable populations, Fu's Fs (South 1.34, North -0.41) and Tajima's D (South 1.14, North 0.40) being non-significant. Harpending's raggedness index was significant for South (r=0.097, P<0.05) and close to significant for North (r=0.074, P<0.10), indicating a poor fit of the observed and expected mismatch distributions. Global ΘST was high (0.69; 95% confidence interval 0.59-0.79), and pairwise ΘST indicated significant genetic structure between all pairs of islands, except between two pairs of close islands (Hiva Oa-Mohotani/Tahuata, Eiao-Hatutaa/Nuku Hiva). The Mantel test was significant for South (p=0.0389) but not for North (p=0.1696), suggesting that some isolation by distance might be prevalent for the mitochondrial haplotypes only in the Southern part of the archipelago (the most isolated islands, Ua Pou and Fatu Iva, also have the most divergent sequences).

The allelic richness of the six microsatellite loci varied from 1.04 to 3.36 (Table S2). Deviation from HWE (α = 0.05) was found in two cases (Nuku Hiva for locus 1, Hiva Oa for Locus 3), but null allele frequency estimated by FreeNa for these loci and populations were less than or equal to 0.2, suggesting that null alleles were uncommon to rare in our dataset. Pairwise FST values averaged over the six loci were high (0.28; 95% confidence interval 0.22-0.36), and these values indicated significant genetic structure between all pairs of islands except two different pairs of islands (Fatu Iva/Ua Pou and Hiva Oa-Mohotani /Tahuata). The Mantel test between genetic and geographic distances was not significant for either of the two island groups (p=0.5028 and 0.4956 for South and North, respectively). Bayesian clustering analysis revealed K=4 as the best estimate for the number of clusters among Marquesan reed-warblers (Figure 2). The first cluster (in blue) corresponds mainly to individuals from three northern islands (Eiao, Hatutaa, Nuku Hiva), the second cluster (in green) corresponds to individuals from Ua Huka, the third (yellow) and fourth (red) correspond to a mixture of individuals from the five southern islands (Fatu Iva, Hiva Oa, Mohotani, Tahuata and Ua Pou). The two first clusters correspond mainly to A. percernis, the two others to A. mendanae.

Admixture was lowest in Ua Huka (100 % of individuals in cluster 2) and highest in Ua Pou (33% of the individuals could not be attributed to a single cluster). Several individuals were classified in a cluster that does not correspond to their taxon: six individuals from Nuku Hiva (A. percernis) were classified in the yellow or red "Southern" clusters (but not Nuku20, the male with a mitochondrial haplotype from Ua Pou) and five individuals from Fatu Iva and Mohotani (A. mendanae) were classified in the green "Northern" cluster. Migrate results suggested gene flow between the two groups with a migration rate larger from the Southern islands group (m=6.64) than on the opposite direction (m=2.44).

Plumage colouration

Colour measurements (means and standard deviations) are provided on Table S3. Values of hue, chroma and brightness correspond to the general gradation observed on the underparts of all individuals, from a more saturated yellow on the throat to less bright and less saturated yellow on the lower belly. Overall the Tahiti Reed Warbler A. caffer is less yellow than the Marquesan reed warblers: A. caffer and A. percernis differed significantly in the degree of saturation of yellow in the breast (MANOVA's Pillai test P<0.001) and in the throat (P<0.01); A. caffer and A. mendanae also differed in the degree of saturation of yellow in the breast (P<0.001) and in the throat (P<0.01).

Finally, they differed in the value of hue in the throat (P<0.01) and lower belly (P<0.01).

Within the Marquesan reed warblers, the colour measurements differed significantly when the two species are compared and all variables considered (P<0.05, Table 1), the only significant difference being the hue of the throat (P<0.01). However, the nested MANOVA revealed that the variation among populations within taxa was more important than the variance between taxa, only six variables having significant differences, including the hue of the throat (Table 1). It is likely that the significance of this variable between taxa was due to the large variation observed among populations. This assumption was confirmed by the results of the LDA, in which the confidence ellipses of the two taxa overlapped almost completely in the projection of the two first axes (supplemental Figure S3).

When the two taxa are analysed separately, the LDA coefficients revealed that in A. mendanae, the saturation and the hue of the throat were the most discriminant factors in the first axis, and the saturation and hue of the lower belly were the most discriminant factors for the second axis (supplemental Figure S4). In this taxon, the population from Ua Pou was the most divergent, having intense yellow in all its underparts (highest values of saturation, see supplemental Table S3), whereas the birds from Fatu Iva exhibited the highest values of hue (throat, breast and lower belly).

In A. percernis, the hue of the breast and of the lower belly were the strongly explanatory factors in the first axis, variables from the breast being important in the second axis (supplemental Figure S5).

The population on Hatutaa, the smallest and driest island of the archipelago, was the most divergent in the LDA, having a light brownish yellow (high values for hue and low values for brightness, see supplemental Table S3). Results of the LDA for the two different predictive functions were significant (P<0.05), but both functions misclassified almost one third of the individuals (27.19% individuals misclassified with "Habitat richness", and 25.43% with "Phylogeny"). This suggests that none of these predictive functions described accurately the variation observed in the plumage colouration in the Marquesan reed warblers.

Vocalizations

Vocalization characteristics of the Marquesan reed warblers are given in supplemental Table S4. The Marquesan reed warblers provide complex and diverse vocalizations, having 13 to 21 different syllables within a 4 second interval. The repetition of similar motifs was found in all individuals, although in different proportions. On average, individuals from Ua Pou used the highest number of repetitions (3.6 repetitions during 4 seconds recording, see an example in supplemental Figure S6), whereas the lowest numbers of repetitions (less than 1 during 4 seconds recording, on average) were found for two individuals from Ua Huka and one bird from Tahuata. The ratio of whistle and rasping syllables varied from 70% of whistle on Tahuata to 66% of rasping on Ua Huka. The vocalization measurements differed significantly when the two species were compared and all variables considered (P<0.001, Table 2), significant difference being only for two variables, number of repetitions and proportion of whistles (both with P<0.01). The variation among populations within taxa is also significant when all variables are considered (P<0.01), and four variables were also significant individually (number of syllables, number of repetitions, proportion of whistles, and duration).

Overall, the two taxa differ in their vocalizations and there was little overlap in the LDA space, but the variation between islands was substantial (Figure 3). The LDA coefficients revealed that the proportion of whistles and the start frequency were the most discriminant factors in the first axis, and the number of repetitions and the end frequency were the most discriminant factors for the second axis. Results of the LDA for the two different predictive functions were significant (P<0.05), and both functions classified correctly almost all individuals, "Phylogeny" (3.6% of misclassified individuals) being a better predictive function than "Habitat Richness" (7.1% misclassified individuals). This suggests that despite the differences in song characteristics observed between islands, the vocalizations of Marquesan reed warblers have kept patterns that correspond to each of the two lineages.

Discussion

Gene flow between lineages

Gene flow among avian species is not a rare phenomenon, particularly in passerines [START_REF] Ottenburghs | The Avian Hybrids Project: gathering the scientific literature on avian hybridization[END_REF]. This is even more apparent when studying closely related species, or groups of recently diverged species, as gene flow often occurs during speciation [START_REF] Nosil | Speciation with gene flow could be common[END_REF][START_REF] Mallet | How reticulated are species?[END_REF]. For instance, [START_REF] Reifová | Patterns of gene flow and selection across multiple species of Acrocephalus warblers: footprints of parallel selection on the Z chromosome[END_REF] showed that interspecific gene flow occurred during the Acrocephalus radiation, and not only between sister species. Here we found evidence of gene flow between A. percernis and A. mendanae, two species that inhabit distinct but close island groups in the Pacific Ocean, with a different phylogenetic origin. One island in particular, Nuku Hiva, seems to act as the main contact zone between the two taxa. The only individual with a conflicting mitochondrial genotype (out of 143 sequences) was caught on this island, and six other individuals showed evidence of admixture from Southern nuclear markers. This island is one of the largest in the archipelago, with 380 km² and a summit at 1,227 m. Its reed warbler population is the largest today in the Marquesas [START_REF] Thibault | Birds of Eastern Polynesia. A biogeographic Atlas[END_REF]. Its large coastline and high mountains might have facilitated the arrival of birds from other islands. Interestingly, the asymmetry of gene flow between the two taxa, more important from South to North than in the opposite direction, is consistent with the current prevailing winds that blow from the southeast below the equator (Galzin et al. 2016).

In terms of plumage, birds from Nuku Hiva cannot be distinguished from the Eiao population (supplemental Figure S5). The non-admixed individuals from Nuku Hiva are identical to the Eiao/Hatutaa birds for nuclear markers (cluster blue in Figure 2); these three islands also share mitochondrial haplotypes (Figure 1). But despite their genetic relatedness, the birds from Hatutaa exhibit distinct plumage, having a light brownish yellow (high values for hue and low values for brightness). On the other hand, the distinctiveness of birds from Ua Huka based on molecular markers (original haplotypes in Figure 1 and the yellow cluster in Figure 2) was not reflected in comparisons of their plumage (see their large overlap in the PCA space in supplemental Figure S5). These examples highlight the overall discrepancy between genetic and plumage characters found in the Marquesan reed warblers. Additionally, island size and distance between islands seemed to have little influence on the observed genetic diversity.

Convergence in Plumage Colouration

Both species have strong yellow colouration, especially in the underparts. A. mendanae does not share this colour with its closest related species as it belongs to a group of brown reed warblers ranging from Line Islands up to Australia and Saipan in the Mariana Islands (the "Micronesian Clade" in [START_REF] Cibois | Charting the course of reed-warblers across the Pacific islands[END_REF] tree; see supplemental Figure S1). On the other hand, A. percernis belongs to a group of mostly grey or brown species from the Cook Islands to Hawaii (the "Polynesian Clade"; supplemental Figure S1) that includes also a few species with yellow colouration, mostly found in the Society islands, the brightest being the Tahiti Reed Warbler. We showed that, in terms of plumage colouration, the two Marquesan reed warblers are more similar to each other than either is to the Tahiti Reed Warbler. Reflectance spectrometry measures demonstrate that they share similar carotenoid-based plumage colouration, but local variation (i.e. among islands) is important. This variation is not correlated to the habitat diversity of the islands. In contrast, size is influenced by resource availability: reed warblers on dry and smaller islands tend to be smaller than birds on large islands, irrespective of their phylogenetic origin [START_REF] Cibois | Uniform phenotype conceals double colonization by reed-warblers of a remote Pacific archipelago[END_REF][START_REF] Leisler | Evolution of island warblers: beyond bills and masses[END_REF]. Having two different phylogenetic origins, the two Marquesan reed warbler lineages acquired their yellow colour independently but show stochastic variation in plumage among islands.

The evolution of convergent plumage colour often implicates selection, as suggested for melanistic insular flycatchers [START_REF] Uy | Island size predicts the frequency of melanic birds in the color-polymorphic flycatcher Monarcha castaneiventris of the Solomon Islands[END_REF][START_REF] Uy | Convergent melanism in populations of a Solomon Island flycatcher is mediated by unique genetic mechanisms[END_REF] or in sexual dichromatism for Petroica robins [START_REF] Kearns | Distinctiveness of Pacific Robin subspecies in Vanuatu revealed from disparate patterns of sexual dichromatism, plumage colouration, morphometrics and ancient DNA[END_REF]. Many correlations between melanistic plumages and traits under selection have been suggested, for instance aggressiveness [START_REF] Ducrest | Pleiotropy in the melanocortin system, coloration and behavioural syndromes[END_REF], immunity [START_REF] Gangoso | MC1R-dependent, melanin-based colour polymorphism is associated with cellmediated response in the Eleonora's falcon[END_REF], or for the melanic form of the Tahiti Reed Warbler, camouflage in dark understories [START_REF] Cibois | The molecular basis of the plumage colour polymorphism in the Tahiti reed-warbler Acrocephalus caffer[END_REF]. Similarly, carotenoid compounds can be under selection during speciation (mostly sexual selection for colouration) and they also have important roles in vision (von Lintig 2012), as antioxidants [START_REF] Fiedor | Potential role of carotenoids as antioxidants in human health and disease[END_REF], or in the immune system [START_REF] Sepp | Effects of carotenoids, immune activation and immune suppression on the intensity of chronic coccidiosis in greenfinches[END_REF].

Several hypotheses could thus be proposed to explain colour variation in the Marquesan reed warblers system, involving the source of carotenoid, selection on the deposition of this pigment on the plumage, or a combination of the two. The diet of the Marquesan reed warblers is known to be rather eclectic, including mainly insects but also small vertebrates (some of them introduced), fruits and seeds [START_REF] Leisler | The Reed Warblers: Diversity in a Uniform Bird Family[END_REF]pers. obs.). The level of endemicity of insects and plants is high in the archipelago (Lorence et al. 2016;Roderick and Gillespie 2016), so one can wonder whether the "Marquesan yellow" pigment could be specific to an endemic prey or plant. An analysis of debris found in feces might improve our knowledge of the diet of these birds (Ramage et al. in prep.).

Native predators (raptors, mammals, snakes) are all absent from the archipelago, so predation is probably not a leading factor in the Marquesan reed warbler system. Interspecific competition can impact trait evolution such as by character displacement in closely related species [START_REF] Pfennig | Evolution's wedge: competition and the origins of diversity[END_REF]). In the Marquesas Islands, competition might have been more important in the past before the extinction of several terrestrial birds [START_REF] Thibault | Birds of Eastern Polynesia. A biogeographic Atlas[END_REF]. However, most of these were parakeets and pigeons and their frugivorous diet would have only marginally overlapped with that of the reed warblers. The monarch flycatchers are the only other native insectivorous passerines in the Marquesas Islands. They are forest-dwelling birds, found in both dry and wet forests, whereas reed warblers prefer today dry scrub habitats and avoid humid forests. However, we note that most of the littoral and low altitude habitats have been highly modified in the archipelago, only relictual native xerophylous forests remaining (Lorence et al. 2016). It is difficult to evaluate how much space was shared between monarchs and reed warblers in the past. Thus, we suggest that sexual selection and pleiotropic effects, rather than predation and competition, have been the drivers of colour divergence of these birds. Alternatively, plumage colour variation can be an adaptive response to variation in light environments across habitats [START_REF] Mcnaught | Interspecific variation in plumage colour among birds: species recognition or light environment?[END_REF] and similar light environments might have influenced the convergent plumage evolution of reed warblers in the Marquesas. Analysis of whole genomes, however, would be necessary to explore the genetic basis and the evolutionary processes that mediate convergent plumage colouration in the Marquesan reed warblers, like recent studies on Holarctic redpoll finches [START_REF] Mason | Differentially expressed genes match bill morphology and plumage despite largely undifferentiated genomes in a Holarctic songbird[END_REF] or Eurasian crows [START_REF] Poelstra | The genomic landscape underlying phenotypic integrity in the face of gene flow in crows[END_REF]. [START_REF] Fiedler | Die Evolution des Gesanges der Acrocephalinae (Hippolais, Acrocephalus und Chloropeta) unter Einbeziehung der Phylogenie und morphologischer[END_REF] showed that song characteristics in the Acrocephalidae are heavily influenced both by phylogeny and natural history. For example, males of migratory species have larger repertoires than tropical, sedentary species, and often include more mimicry. While a single species from the Pacific Ocean was included in Fiedler's study (the Australian Reed Warbler Acrocephalus australis), Leisler and Schulze-Hagen (2011) used her work, along with isolated observations, to infer that insular species have in general simpler and shorter songs than continental species. Although examples of simpler, almost call-like songs have been reported in two Polynesian species [Henderson Reed Warbler A. taiti;[START_REF] Graves | The endemic land birds of Henderson Island, southeastern Polynesia: notes on natural history and conservation[END_REF]; and Rimatara Reed Warbler A. rimitarae; [START_REF] Thibault | The natural history and conservation of Acrocephalus rimitarae, the endemic reed-warbler of Rimatara island, Oceania[END_REF]], the global pattern is probably more complex and idiosyncratic. For instance, [START_REF] Bell | From wetlands to islands: morphological variation, plumage and song in Pacific island Acrocephalus warblers[END_REF] analysis of numerous Pacific Acrocephalus showed that the mean frequency of song syllables was inversely correlated with mean body size. He also suggested that the environment though which the sound is transmitted also influences the song, lower frequencies being prevalent in forest and scrub habitats.

Vocalizations

Although a comparison of the complexity of Marquesan reed warblers' songs with continental species was beyond the objectives of this study, we noticed that the Marquesan reed warblers provide long and complex vocalizations that include repetitions of similar motifs. The multifactorial analysis showed that vocalizations of the Marquesan reed warblers differ slightly between species and that significant differences between islands led to different dialects. However, these dialects retained some common elements within each taxon, suggesting a phylogenetic component in the vocalization of Marquesan reed warblers. Differences in habitat richness, as broadly defined by the number of native plant species, were not significantly associated to their vocalization diversity.

Marquesan reed warblers live in open habitats, which should have similar abiotic acoustic properties. This might have led to similar types of songs arising across the archipelago. In contrast, "continental" species of reed warblers vocalize from inside dense vegetation such as reed beds. The variation in their songs has been linked to differences in mating system and habitat quality [START_REF] Leisler | The Reed Warblers: Diversity in a Uniform Bird Family[END_REF]. In oscine passerines, males often learn singing behaviour from parental or neighbouring tutors, and elements inherited in this fashion should favour the evolution of local dialect [START_REF] Wada | The Development of Birdsong[END_REF]. In relatively small and isolated populations, the consequences of founder effects and of vocal drift have also been shown to favour the creation of local dialects, sometimes associated with a reduction of vocalization diversity compared to a larger source population [START_REF] Valderrama | Effects of Population Size on Singing Behavior of a Rare Duetting Songbird[END_REF], but not always [START_REF] Baker | Differing effects of isolation on evolution of bird songs: examples from an island-mainland comparison of three species[END_REF].

Conclusion

In summary, the Marquesan reed warblers, two taxa that originated from independent lineages in the Pacific Ocean, exhibit convergent plumage colouration while retaining differences in songs. Vocal characteristics may act as the main pre-zygotic barriers when birds from different lineages come into contact. In a majority of passerines with a monogamous breeding strategy, dispersal is femalemediated [START_REF] Greenwood | Mating systems, philopatry and dispersal in birds and mammals[END_REF], as shown in migratory species [START_REF] Guerrini | Large-Scale Patterns of Genetic Variation in a Female-Biased Dispersing Passerine: The Importance of Sex-Based Analyses[END_REF] and in at least one insular situation [START_REF] Paris | Female-biased dispersal in a spatially restricted endemic island bird[END_REF]. Although our data set was not designed to test sex-biased dispersal within the Marquesas archipelago, the occurrence of mitochondrial haplotype introgression is consistent with dispersal of at least one female between the two groups. Further work, in Nuku Hiva in particular, might be necessary to unravel the mechanisms in action in the contact zone between the two taxa. Until recently, mitochondrial DNA sequences and microsatellite loci (SSR, simple sequence repeats) were the gold standards in population genetics. Now, the use of genomic methods can provide hundreds or thousands of nuclear single-nucleotide polymorphisms (SNPs) generated by restriction-site associated DNA sequencing (RAD-seq), leading to a broader evaluation of the genome. While these new data set are not devoid of drawbacks, in particular related to evolutionary noise [START_REF] Mesak | Hundreds of SNPs vs. dozens of SSRs: which dataset better characterizes natural clonal lineages in a self-fertilizing fish[END_REF], the study of the Marquesan reed-warblers would benefit from a genomic approach in the future.

Ornithological

  Society of Polynesia (covenant n° 6-0056/mdd), the Collection Study Grant from the Frank M. Chapman Memorial Fund (AMNH), the European Commission's Research Infrastructure Action via the SYNTHESYS Project (applications GB-TAF-1846/2002), the Systematic Research Fund of the Systematics Association and Linnean Society, the Swiss Academy of Sciences (Commission of travel grant), the G. and A. Claraz Foundation, the Basel Foundation for Biological Research, are gratefully acknowledged.

Acknowledgements

We are very grateful to Joel Cracraft and Paul Sweet [American Museum of Natural History (AMNH),

New York] for access to the collections in their charge and for the sampling of specimens. We thank Jean-Yves Meyer (Research Delegation, Tahiti), Claude Serra (DIREN Tahiti), Philippe Raust (Manu, Ornithological Society of Polynesia), Josie Lambourdière (MHNH), the Institut pour la Recherche et le Développement (Papeete), the Service du Développement Rural (Iles Marquises), and the Service de Systématique Moléculaire [UMS2700-CNRS, Muséum National d'Histoire Naturelle (MNHN), Paris]. Financial support by the Research Delegation (covenant n°5992/MEE/REC), the DIREN Tahiti and the