Philippe Delaporte

Origins and Age Structure of Black-Tailed Godwits Limosa limosa on the Central Atlantic Coast of France

Summary. Coastal shorebird species overlap at many wintering sites or migratory stopovers. However, each species is morphologically and physiologically specialised for reducing competition for space, time or habitat. A species can encompass several populations, sometimes designated as subspecies. Differences between populations, ages and sexes within the same species can also lead to segregation between individuals in a foraging area. In this study, we explored the non-breeding spatial and temporal overlap between two subspecies of the black-tailed godwit Limosa limosa on the central Atlantic coast of France (the Pertuis Charentais). The proportions of individuals of each sex and each age within the populations were assessed to explore differences in the wintering distribution. The subspecies L.l. islandica winters in the area from July to March whereas the subspecies L.l. limosa may stopover in the area during south- and northward migration. The two subspecies occasionally overlap at some sites just after the breeding period, and frequently in February and March. We also highlight two previously unknown features of the L.l. islandica population in this part of its distribution area. Firstly, we found a high proportion of juveniles among the individuals caught by mist-net throughout the winter period. An unexpectedly high proportion of the 10,000 individuals wintering in the Pertuis Charentais were juveniles. Secondly, there was a significant predominance of males in both age classes, but particularly among juveniles throughout the wintering period. That 59% of the godwits caught during the winter were male and that 56% of these were juveniles, suggest (1) strong age- and sex-related population structuring, and (2) a possible redistribution of these juveniles from the first winter to the next, or unequal survival of juveniles across the wintering grounds.

Tracing changes in the diet and habitat use of black-tailed godwits in Western France, using a stable isotope approach

ABSTRACT Western France is at the crossroads of the migratory routes of two subspecies of black-tailed godwit, Limosa limosa. After leaving Iceland, the godwit L.l. islandica Icelandic black-tailed godwit (IBTG) winters on the coast of western Europe, while the continental black-tailed godwit (CBTG) L.l. limosa can stop in France when migrating between Iberia or Africa and their main breeding grounds in the Netherlands. In this study, we analysed δ15N and δ13C from flight feathers and whole blood throughout the non-breeding period to trace variations in habitat use for both subspecies on the western French coast. Adults and juveniles of IBTG adopt the same feeding habitats as soon as they arrive in the study area, progressively losing the Icelandic freshwater habitat signal, and becoming clearly restricted to marine habitats in winter. Some individuals begin to move locally to freshwater habitats, joining CBTG in a stopover at the end of the wintering period in preparation for northward migration.