Movement Patterns in a Partial Migrant: A Multi-Event Capture-Recapture Approach

Partial migration is a pervasive albeit poorly studied phenomenon by which some individuals of a population migrate while others are residents. It has tremendous consequences on seasonal variations of population size/structure and therefore management. Using a multi-event capture-mark-recapture/recovery (CMR) approach, we assessed seasonal site occupancy, survival and site fidelity of a partially migratory diving duck, the Common pochard (Aythya ferina), in an area potentially including both local breeders and winter visitors. The modelling exercise indeed discriminated two different categories of individuals. First, locally breeding females which had a probability of being present in our study area during winter of 0.41. Females of this category were found to be more faithful to their breeding site than males (breeding site fidelity probabilities of 1 and 0.11, respectively). The second category of birds were winter visitors, which included adults of both sexes, whose probability of being present in the study area during the breeding season was nil, and young of both sexes with a 0.11 probability of being present in the area during the breeding season. All wintering individuals, among which there was virtually no locally breeding male, displayed a high fidelity to our study area from one winter to the next (0.41-0.43). Estimated annual survival rates differed according to age (adults 0.69, young 0.56). For both age classes mortality was higher during late winter/early spring than during summer/early winter. Our study is among the first to show how and under which conditions the multi-event approach can be employed for investigating complex movement patterns encountered in partial migrants, providing a convenient tool for overcoming state uncertainty. It also shows why studying patterns of probability of individual presence/movements in partial migrants is a key towards understanding seasonal variations in numbers.