Coastal specialised species have naturally restricted distribution areas and may be drastically affected by fragmentation or loss of their habitats due to ongoing changes, such as artificialisation, shoreline erosion, rising water levels or colonisation by invasive species. In this context we characterise the land occupation strategies of a Western Atlantic French coastal endemic bird: the Bluethroat Cyanecula svecica namnetum. Our study focuses on a key period of the life cycle of this species: the post-breeding moult. Capture and recapture sessions in intertidal habitats have allowed us to retrieve 26% of local breeders during their moulting period. The modeling of moult kinetics revealed that moult of flight feathers takes 37-50 days. A radio-tracking survey of moulting birds revealed exploitation by individuals of both the lowest and tallest vegetal formations of intertidal sites and exploitation of small home ranges (0.42-1.34 ha), typical of locations where trophic resources tend to be abundant and predictable. Analyses of droppings highlighted that Coleoptera, Aranea and marine crustaceans (Amphipoda) contributed most of the prey biomass consumed, amphipods being particularly selected by birds in active moult. Our results underline the importance of intertidal wetlands in terms of trophic opportunities to compensate for the energy costs of moult for the Bluethroat. Given the global changes already dramatically affecting coastal habitats, we emphasise that special attention should be given to the conservation of intertidal wetlands for marshland passerines of conservation concern such as the Bluethroat, and that restoration of adjacent coastal terrain is a promising development.

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Our study deals with density estimation in intertidal reed beds of the Gironde estuary (France) of the French endemic subspecies of Bluethroat breeding along the Atlantic coast (Cyanecula svecica namnetum). We analysed count point data collected during the beginning of the breeding period, with information gathered from 94 stations distributed over 100 hectares. Analysis of data integrating bird detection probability as a function of the detection distance (distance sampling) allowed us to estimate an average density of 20.5 ± 3.7 males for 10 hectares (14.4 – 29.5 within the 95 % CI). Up to now, no equivalent densities have been documented for this endemic taxa which is of conservation concern given its relatively high degree of specialization in terms of ecological resources exploited and its limited range and numbers. Despite their singularity given the limited size of the study area, our results confirm the particular importance of coastal marshes for C. s. namnetum. In view of strong alterations of coastal habitats due to the rise in sea level already observed in the breeding grounds of C. s. namnetum, our results highlight the interest in developing conservation strategies for habitats exploited by the taxa, integrating the potential key roles of back littoral areas in order to compensate losses of habitats due to the ongoing and forthcoming effects of global changes.