Data from: Inter-annual variation and long-term trends in proportions of resident individuals in partially migratory birds

Partial migration – a part of a population migrates and another part stays
resident year-round on the breeding site – is probably the most common
type of migration in the animal kingdom, yet it has only lately garnered
more attention. Theoretical studies indicate that in partially migratory
populations, the proportion of resident individuals (PoR) should increase
in high latitudes in response to the warming climate, but empirical
evidence exists for few species. We provide the first comprehensive
overview of the environmental factors affecting PoR and the long-term
trends in PoR by studying 27 common partially migratory bird species in
Finland. The annual PoR values were calculated by dividing the winter bird
abundance by the preceding breeding abundance. First, we analysed whether
early-winter temperature, winter temperature year before or the abundance
of tree seeds just before overwintering explains the interannual variation
in PoR. Secondly, we analysed the trends in PoR between 1987 and 2011.
Early-winter temperature explained the interannual variation in PoR in the
waterbirds (waterfowl and gulls), most likely because the temperature
affects the ice conditions and thereby the feeding opportunities for the
waterbirds. In terrestrial species, the abundance of seeds was the best
explanatory variable. Previous winter's temperature did not explain
PoR in any species, and thus, we conclude that the variation in food
availability caused the interannual variation in PoR. During the study
period, PoR increased in waterbirds, but did not change in terrestrial
birds. Partially migratory species living in physically contrasting
habitats can differ in their annual and long-term population-level
behavioural responses to warming climate, possibly because warm winter
temperatures reduce ice cover and improve the feeding possibilities of
waterbirds but do not directly regulate the food availability for
terrestrial birds.