The impact of increased food availability on survival of a long-distance migratory bird.

Temperature-, rainfall- and habitat-driven change in food availability is one likely mechanism by which anthropogenic factors may affect animal population dynamics and species distributions. Long-distance migratory birds must synchronize their migrations with food availability at locations hundreds or thousands of kilometers apart, so changes in the overall abundance of food or the phenology of peaks in food availability may be critical factors influencing annual survival. In this study we used experimental food supplementation at a breeding site to examine and compare the impact of changing food supply on annual survival of adult migratory birds and their offspring. We provided supplemental food to Northern Wheatears (Oenanthe oenanthe) breeding on Fair Isle, UK, to infer the sensitivity of annual survival to increases in natural food availability. Food-supplemented wheatears exhibited higher rates of annual survival than control wheatears, and the strength of this effect varied with age. Food supplementation led to 1.56 times higher annual survival of juveniles and 1.22 times higher survival of adults. Survival of juveniles was related to their own food availability as fledglings, but not to whether their parents were food-supplemented or unfed control adults. This increased survival, combined with increased breeding productivity associated with food supplementation, implies that an increase in natural food availability, of the magnitude simulated in our experiment, would increase the population growth rate of wheatears on Fair Isle from approximately lambda = 0.93 (a contracting population) to lambda = 1.14 (an expanding population).