| Summary: | Climate-driven changes in the overall abundance or phenology of food resources may be critical factors affecting migratory populations, which depend on suitable conditions at widely-spaced locations across the planet. Numerous studies have described associations between climatic change and breeding success, survival and the timing of migration among migrant birds, but we used an experimental approach to investigate the causal mechanisms underlying these widely-reported correlations. Specifically, we used food-supplementations to simulate experimentally the impact of changes in food availability across the annual cycle of a model species of long-distance migrant bird, the northern wheatear (Oenanthe oenanthe). Food supplementation modified migratory schedules of wheatears by affecting migratory fuelling, but the strength of this impact varied between stages of the annual cycle and between two subspecies with contrasting migration strategies. Food supplementation advanced hatching date of wheatears in the UK, and led to some supplemented individuals increasing their reproductive output through multiple breeding attempts. Food-supplemented wheatears exhibited higher rates of annual survival than control wheatears, and the strength of this effect was most pronounced on survival of young immediately following fledging. These experiments highlight aspects of wheatear performance that are currently limited by food availability and are thus most sensitive to climate-driven changes in food supply (primarily migratory fuelling, number of breeding attempts and survival). We used experimental manipulations of temperature and soil moisture of upland grassland turfs to simulate the impact of climate change on the abundance and emergence phenology of the wheatear’s arthropod prey. Warming by 2 ºC and low soil moisture levels led to a reduction in arthropod abundance later in the season, indicating how such climate-driven changes would affect food availability to wheatears across the breeding season. These studies together provide experimental evidence for the mechanisms by which climate change is expected to influence population changes in migratory taxa.
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