Abstract
In facultative partial migrants some individuals in a population are migratory and others are resident and individuals decide each year anew which strategy to choose. While the proportion of birds migrating is in part determined by environmental conditions and competitive abilities, the timing of individual departure and behaviours on route are little understood. Individuals encounter different environmental conditions when migrating earlier or later. Based on cost/ benefit considerations we tested whether behaviours on route were affected by time constraints, personality and/or age in a partially migrating population of Blue tits (Cyanistes caeruleus). We captured female Blue tits on migration at the Southern tip of Sweden during early, peak and late migration and measured latency to feed in an unfamiliar environment, exploration of a novel object and hesitation to feed beside a novel object (neophobia). Lean birds and birds with long wings started feeding earlier when released into the cage indicating that foraging decisions were mainly determined by energetic needs (lean and large birds). However, juveniles commenced feeding later with progression of the migratory season in concordance with predictions about personality effects. Furthermore, lean birds started to explore earlier than birds with larger fat reserves again indicating an effect of maintaining threshold energy reserves. Moreover, late migrating juveniles, started to explore earlier than early migrating juveniles possibly due to time constraints to find high-quality foraging patches or a suitable winter home. Finally, neophobia did not change over the migratory season indicating that this behaviour is not compromised by time constraints. The results overall indicate that decisions on route are mainly governed by energetic requirements and current needs to learn about the environment and only to a small extent by differences in personality.
Highlights
Partial migration describes the phenomenon that some individuals in a population are migratory during the non-breeding season, whereas other individuals of the same population remain on their breeding ground [1]
The restricted ANOVA model for latency to feed in the cage on the day of capture in female blue tits in Falsterbo autumn 2007. doi:10.1371/journal.pone.0163213.t002
The restricted model with fat score, migratory season and the interaction between age and migratory season showed a significant influence on the latency to touch the novel object (Tables 3 and 4)
Summary
Partial migration describes the phenomenon that some individuals in a population are migratory during the non-breeding season, whereas other individuals of the same population remain on their breeding ground [1]. Which individuals migrate in a population can be a) genetically determined, i.e. be fixed across lifetime, b) condition (age, sex, competition) or environmentally (resources, temperature, predation) dependent and change within an individual’s life or c) be a mixture of both, genetic and condition/environment [7]. The latter has received increasing behavioural and genetic support [8,9]. While median migration dates can vary with the magnitude of migration in a given year [15], the variation around this median in each year is little understood
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