The swallow project had a high-profile publication appear in the Proceedings of the National Academy of Sciences. This paper reported the results of our 30-year mark-recapture study of cliff swallows to show that the annual survival advantages of different colony sizes fluctuated among years.
Colony size was under both stabilizing and directional selection in different years and reversals in the sign of directional selection regularly occurred.
Directional selection was predicted in part by drought conditions: birds in larger colonies tended to be favored in cooler and wetter years and birds in smaller colonies in hotter and drier years.
Oscillating selection on colony size likely reflected annual differences in food availability and the consequent importance of information transfer, and/or the level of ectoparasitism, with the net benefit of sociality varying under these different conditions.
Across years, there was no net directional change in selection on colony size.
The wide range in cliff swallow group size is probably maintained by fluctuating survival selection and represents the first known case in which fitness advantages of different group sizes regularly oscillate over time in a natural vertebrate population. The results have broad significance because most animal groups vary extensively in size, but why wide group-size variation persists in most natural populations remains unexplained.
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