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

Charles R. Brown,Mary Bomberger Brown

doi:10.1002/ecs2.2428

Abstract

AbstractWhat 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.

Highlights

Animals breeding in groups of different sizes can experience very different ecological and social environments (Alexander 1974, Wittenberger and Hunt 1985, Brown and Brown 2001)
In colonial cliff swallows (Petrochelidon pyrrhonota), annual survival selection on colony size was found to vary in direction and form in different years, with larger colonies advantageous for survival in some years, intermediate-sized colonies best in other years, and smaller colonies favored in still others (Brown et al 2016)
2003), the results reported here are among a very few to show a strong advantage for individuals in the smallest and largest group sizes

Summary

Introduction

Animals breeding in groups of different sizes can experience very different ecological and social environments (Alexander 1974, Wittenberger and Hunt 1985, Brown and Brown 2001). In colonial cliff swallows (Petrochelidon pyrrhonota), annual survival selection on colony size was found to vary in direction and form in different years, with larger colonies advantageous for survival in some years, intermediate-sized colonies best in other years, and smaller colonies favored in still others (Brown et al 2016). These fluctuations correlated with broad-scale climatic patterns in the birds’ breeding and wintering areas, such as extent of seasonal drought

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