Resumen de Bird population dynamics in the wintering season: the case of the mediterranean Gull Larus melanodephalus = Dinàmiques poblacionals en ocells durant la hivernada: el cas de la Gavina Capnegra Larus melanocephalus
The winter period is typically associated with harsh weather conditions and reduced food availability, which may lead to increased mortality. In birds, a common strategy to avoid such risks is to migrate to more favourable grounds, but its demographic costs are largely unknown. The wintering season remains a black box in our knowledge of the life cycle of birds. From the perspective of a wintering area, I study the population ecology of a migratory species, based on the hypothesis that winter aggregations of birds constitute true populations, instead of random aggregations of individuals. I look at the population composition, structure and dynamics across several spatial scales, and the link between the wintering and breeding areas through connecting migratory routes. My study model is a population of wintering Mediterranean Gulls Larus melanocephalus made of birds from multiple origins, a significant proportion of which have been individually marked with individually-coded colour-rings in their natal or breeding colonies. The study system constitutes the species’ main winter quarters globally and is situated on the Mediterranean coast of E-NE Spain. Capture-recapture analyses reveal high estimates of apparent local interannual survival after the first year (0.81 on average), indicating a high individual winter site-fidelity that does not vary with age. This attachment to the wintering area models the distribution of individuals, shaping them into a spatially structured population with metapopulation dynamics, as each subpopulation has independent demography, determined by area-specific survival and low levels of dispersal. Using a probability model, I infer the migratory route followed by first-winter birds; the most likely path is a combination of fluvial and coastal routes, seeking minimal distance along favourable terrain. In the focus species, wintering populations are made of individuals from different sex and age classes, in one of the few documented cases of nondifferential migration. Exceptionally, the species also exhibits synheimy (the population in each distinct wintering area is made of birds from all the breeding populations), possibly as a result of its recent history. Individual choice of wintering area is made in the first migration according to a probability rule determined by distance, and is maintained through life. On a local scale, I find a very high turnover of individuals and a frequent movement, in no particular direction, among sites. The low monthly probability of local residency (0.56) contrasts with the species’ strong site-fidelity to the general wintering area. Individual turnover shows that a single site may be visited by a large proportion of the regional (43%) and global (18%) populations every season. Thus, the population home range extends over an area much larger than a single locality or a protected area, showing evidence of a mismatch between the distribution of protected areas and the spatial needs of the population. Compared with data from 60 years ago, Mediterranean Gulls have undergone a huge expansion of their breeding distribution range (750% area increase) while the wintering distribution has increased only moderately (25%). The global population size has remained stable, indicating that its growth may be limited by events taking place in the wintering season. As the breeding distribution continues to expand, where breeding and wintering populations come in contact, they interact little and follow independent dynamics. This work contributes to a better understanding of bird population dynamics in the wintering season and shows how the detailed study of a species’ ecology during one part of the annual cycle can help comprehend what happens during the rest of the year. The ‘view from the winter quarters’ can bring important and complementary information to the study of birds during the breeding period.
