Abstract
Many populations of migratory songbirds are declining or shifting in distribution. This is likely due to environmental changes that alter factors such as food availability that may have an impact on survival and/or breeding success. We tested the impact of experimentally supplemented food on the breeding success over three years of northern wheatears (Oenanthe oenanthe), a species in decline over much of Europe. The number of offspring fledged over the season was higher for food-supplemented birds than for control birds. The mechanisms for this effect were that food supplementation advanced breeding date, which, together with increased resources, allowed further breeding attempts. While food supplementation did not increase the clutch size, hatching success or number of chicks fledged per breeding attempt, it did increase chick size in one year of the study. The increased breeding success was greater for males than females; males could attempt to rear simultaneous broods with multiple females as well as attempting second broods, whereas females could only increase their breeding effort via second broods. Multiple brooding is rare in the study population, but this study demonstrates the potential for changes in food availability to affect wheatear breeding productivity, primarily via phenotypic flexibility in the number of breeding attempts. Our results have implications for our understanding of how wheatears may respond to natural changes in food availability due to climate changes or changes in habitat management.
Highlights
Many animal populations are limited by food availability [1], which can itself be influenced by variation in climate, habitat quality and competition
This study tested the impact of experimentally manipulated food availability on the reproductive performance of the northern wheatear, a long-distance migratory insectivorous bird, with implications for the potential effects of climate- and land-usedriven changes in food in the breeding areas
The experimental increase in food availability increased the annual reproductive output of the wheatears breeding on Fair Isle by increasing the frequency of pairs raising second broods and males raising broods with more than one female
Summary
Many animal populations are limited by food availability [1], which can itself be influenced by variation in climate, habitat quality and competition. Temperature and rainfall have strong effects on migrant bird populations by changing the abundance and phenology of their invertebrate food supply [2,3,4,5]. The availability of invertebrates to birds may be affected by temperature and rainfall via changes in activity of invertebrates and birds or in foraging efficiency of birds [6] This may affect the ability of insectivorous birds to obtain sufficient energy reserves for reproduction or to provide adequate food for their young [2]. Very few studies have addressed experimentally the impacts of changes in food availability on breeding productivity of long-distance migrant songbirds [19,20] and these have focused on Nearctic-Neotropical migrants
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