The first winter influences lifetime wintering decisions in a partially migrant bird

Abstract

In facultative partial migration, flexibility of wintering behaviour throughout the bird's lifetime (i.e. switching between migrant and resident tactics and vice versa) may be considered an advantage because it allows individuals to cope with environmental conditions. Quantifying the extent of flexibility of wintering behaviour and identifying the related factors (environmental and individual variables) are crucial issues. The present study addressed these issues in a European subpopulation of a shorebird species, the pied avocet, Recurvirostra avosetta, that displays three distinct wintering tactics: strictly resident, locally resident and migrant. We explored tactic fidelity over consecutive years from the bird's first winter by testing the influence of age and estimating more specifically the long-term consistency of the first wintering tactic. We also explored tactic-related survival. Based on a 10-year ringing study carried out on five French colonies and on capture–recapture modelling, we showed that birds were highly faithful to their first wintering tactic over consecutive years and when a change occurred, it was mainly to wintering in the French Atlantic area. We also found a moderate decrease in fidelity to the migrant tactic over consecutive years with age. Complementarily, the fidelity of locally resident and migrant individuals to their first wintering site was remarkably high. Finally, survival over winters was particularly high and did not depend on tactic. At the individual scale, flexibility of wintering tactic was therefore limited over the study period. Despite a slight age effect, other biological information failed to support the ʽarrival time and dominance hypothesesʼ, commonly suggested to explain age-differential migration. Additional factors, including the absence of harsh winters, habitat quality and advantages of site familiarity, can be put forward to interpret the main results.