Variation in age composition among colony sizes in Cliff Swallows

Area of Science:

• Behavioral Ecology
• Animal Social Behavior
• Population Ecology

Background:

• Group size variation is common in social species.
• Understanding age-based sorting in groups can reveal costs and benefits of social environments.

Purpose of the Study:

• Investigate age composition in Cliff Swallow colonies of varying sizes.
• Determine factors influencing age-based sorting in social groups.

Main Methods:

• Analyzed 18 years of data from over 194,000 Cliff Swallows in Nebraska.
• Used years since banding as a proxy for bird age.
• Compared age composition at fumigated and non-fumigated colony sites.

Main Results:

• Younger swallows (age-class-1) were more prevalent in larger colonies and later-forming colonies.
• Older birds showed higher site fidelity, returning to colonies even when size decreased.
• Age composition variation was linked to colony size and initiation date, but only at sites with ectoparasites.

Conclusions:

• Cliff Swallow colony size variation is partly driven by age-based sorting.
• Ectoparasites (Oeciacus vicarius) appear to influence this age-based group composition.
• Age-specific benefits likely shape individual settlement decisions in social groups.