THE ROLE OF EXTRINSIC AND INTRINSIC FACTORS ON THE DEPARTURE DECISIONS OF A LONG-DISTANCE MIGRATORY PASSERINE

Factors determining departure decisions of migrants from a stopover site can be extrinsic and/or intrinsic but the relative role of each of these factors on departure decisions is still poorly known. Date and wind should be the main factors determining departure decisions in a long-distance migrant, which is expected to minimise duration of migration. Date was considered as an intrinsic factor and wind as an extrinsic one. We analysed the capture-recapture data of a long-distance migrant European songbird, the sedge warbler Acrocephalus schoenobaenus, from a stopover site in northern Iberia during the autumn migration period to quantify the relative importance of several factors on emigration likelihood. Cormack-Jolly-Seber (CJS) models were used to estimate the emigration likelihood. From 107 models tested, only the model with an additive effect of date, tailwind assistance and population size substantially supported the data. As expected, sedge warblers were more likely to depart with high tailwind values and late in the season and, contrary to expectations, with decreasing sedge warbler abundance.