Extreme group sizes in a colonial bird favored during a rare climatic event

What maintains stasis in animal group-size distributions is an unresolved problem in behavioral ecology. One potential driver could be rare climatic events that favor certain group sizes in ways that do not occur in normal conditions. We investigated mortality among colonially nesting cliff swallows (Petrochelidon pyrrhonota) during a rare climatic event in western Nebraska in 1996 that led to the starvation of thousands of adult birds. Colonies at the extreme end of the size distribution exhibited less size reduction (higher adult survival) than those of intermediate size. That this event resulted in disruptive selection on colony size was suggested by an underrepresentation of locally produced yearling birds that recruited into colonies of intermediate size the following year. There was no evidence that the colony-size-related mortality could be explained by differential sorting of birds among colonies based on body size or differing patterns of selection on morphology. The selection on colony size was likely driven by lower competition for food in the smallest colonies and better quality foraging habitat associated with the largest colonies, with these advantages enhanced in severe weather. Selection on colony size during rare climatic events can reinforce or oppose selection occurring during other times of the annual cycle. Whether such selection results in long-term change in the colony-size distribution may depend on the frequency and severity of these climatic events