Interannual variation and long‐term trends in proportions of resident individuals in partially migratory birds

Abstract

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.

Are winter and breeding bird communities able to track rapid climate change? Lessons from the high North

“…Winter feeding, in combination with climate change, was found to allow previously migrating warm‐dwelling species to become resident at the northern edges of their distribution (Carey, ). However, recent work shows that in partially migratory land birds the wintering and breeding population sizes have changed at the same rate; therefore, the proportion of wintering birds has remained the same (Meller et al ., ). Also, the result that CTI w has changed similarly in the whole country despite the stronger temperature increase in the north goes against our initial expectations.…”

Fitness consequences of different migratory strategies in partially migratory populations: A multi‐taxa meta‐analysis

“…It follows, therefore, that in partially migratory populations residency may offer complementary fitness benefits to those offered by migration (Lundberg, ; Zúñiga et al, ). In the case of conditional strategies, these may refer to individual states such as sex or body condition (Hegemann, Marra, & Tieleman, ; Warkentin, James, & Oliphant, ), or external conditions, such as population density (Grayson & Wilbur, ) or environmental conditions (Chapman et al, ; Lack, ; Lundberg, ; Meller et al, ). Additionally, the prevalence of each strategy within a population may itself influence the relative fitness benefits conferred by either (Kokko, ; Lundberg, ).…”

Large‐scale climatic drivers of regional winter bird population trends

“…We hypothesize that winter populations of partial migrants increased more than residents. Partial migrants can benefit from favourable circumstances during the breeding and non‐breeding season, and/or from a shift of wintering ranges towards the north because of more favourable winter conditions (Brommer, ; Meller et al ., ; Välimäki et al ., ).…”