SG/LTREB Renewal: Spatiotemporal Fitness Variation and Avian Group Size

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Many social animals live in groups of different sizes. Individuals often seem to survive or reproduce better in groups of particular sizes in a given year, and ecologists have thus been puzzled as to why groups with less successful sizes continue to exist. One possibility is that changes in ecological conditions such as food availability or the presence of parasites, and/or climatic conditions such as how wet a year is, lead to certain group sizes being most successful in some years or in some areas, but different ones being most successful in other circumstances. However, whether or not the advantages of different sized groups fluctuate in this way over the long term and across broad geographic areas is unknown. This can only be addressed with long-term research that measures the success of animals in different groups in different years and geographic regions. This research will address this by measuring the reproductive success of individuals of a social bird, the cliff swallow, which breeds in colonies ranging from 1 to 6000 nests. Cliff swallows are found throughout much of western North America and are one of the most highly social land birds in the world. The work is investigating whether the costs and benefits of particular colony sizes change regularly enough among years to explain the long-term persistence of variation in group size. This research involves undergraduate students and offers them intensive research experience and training in field ecology. Public outreach is conducted by giving annual lectures at the Swallows Day festival at Mission San Juan Capistrano in California, appearance on national media associated with that event, and maintenance of a web site about the research and cliff swallows generally. This proposal builds on a long-term study of a social bird, the cliff swallow, that lives in colonies of widely different sizes, and is investigating whether fluctuating selection on reproductive success can explain variation in group size. The data gathered will be applied to study annual reproductive success in colonies of different sizes from year to year. This is done by systematically collecting data on fledging success at up to 40 cliff swallow colonies each year. The work also focuses on the proximate ecological drivers of variation in reproductive success by examining the extent of ectoparasitism, breeding phenology, and other nest- and colony-based variables, allowing identification of climatic and other factors that lead to annual variation in reproductive success in different groups. The information gathered in this project, along with existing data on annual survival of breeding adults and first-year birds, will enable estimation of relative lifetime reproductive success of birds occupying different sized colonies. This study will allow better understanding of how group size affects fitness over the long term, assist in interpreting the extensive variation in group size that is characteristic of most social animals, reveal the potential influence of climatic variability on animal social behavior, and provide a test of the importance of fluctuating selection in generating and maintaining behavioral variation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.