Ecological correlates of group integrity among dispersing cliff swallows

Abstract

Breeding colonies of birds represent groups of individuals that associate during one breeding season, at least partially dissociate for the non‐breeding season, and may re‐associate the next year through collective settlement at another breeding site. Little is known about the extent to which colonial birds maintain group integrity when occupying different sites in different years or the benefits of long‐term associations among colonial individuals. For cliff swallows (Petrochelidon pyrrhonota) in western Nebraska, USA, we examined ecological correlates and potential benefits associated with group integrity. Using a dataset of over 25,000 individuals, we found that associations between dispersing cliff swallows were greater than would be predicted by purely random settlement among colony sites. The extent of group integrity varied widely among sites, with birds seeming to settle together more often at sites with fewer ectoparasites and at colonies similar in size and closer in physical proximity to the one they had previously occupied. Some associations of birds lasted three years in which they used a different site each year. Successful colonies had higher levels of group integrity among their settlers than did colonies that failed completely. Cliff swallows that were known to have settled with at least one conspecific from the previous year's site had a higher survival the next year than those not known to have settled with past residents. The results are consistent with cliff swallows choosing colonies based in part on parasite load and with sorting among colonies based on the birds’ preferences for colonies of certain sizes.

The cost of ectoparasitism in Cliff Swallows declines over 35 years

“…We used mark‐recapture data collected over a 30‐yr period, 1984–2013, in which we banded ~229,000 Cliff Swallows with United States Geological Survey bands and had ~405,000 total bird captures in mist nets during that time at up to 40 different colony sites each year (Brown et al 2016, Hannebaum et al 2019). As swallows exited their nests, they were captured by putting nets across the entrance of highway culverts or along the sides of bridges that contained swallow colonies.…”

Birds of a Feather Flock Together: Extent of Long-Term Consistency of Colony-Size Choice in Cliff Swallows

“…Over the summer, we typically captured 10-60% of the residents at a colony, as inferred from a colony's capture total and the colony size. Most colonies netted were in a region of about 15 km × 15 km in Keith County (Hannebaum et al, 2019a), although the total study area over which netting occurred was about 20 km × 125 km, from near Paxton, Nebraska, to near Broadwater, Nebraska.…”

“…Independent predictors included the first-year cohort a bird belonged to (Year), its proportion of sites used in 2 or more years (Samesite), its proportion of years occupying a fumigated site (Fumsite), its banding age (Age), the number of years for which its colony size was known (N), the time period over which its colony sizes were measured (Interval, see above), and its mean colony size (Meansize). In order to account for birds that might have preferred to settle at either the same site each year or to not move far from the first-year site (Brown et al, 2017;Hannebaum et al, 2019a), we used the first-year's colony site (Yr1site) as a random effect to account for any sort of spatial dependence.…”

“…The birds had multiple colonies to choose from each year: over 220 colony sites were available within a 50 km × 150 km region (Brown et al, 2013). In parts of the study area, up 20 sites can be within 10 km of each other (Hannebaum et al, 2019a), which is within the distance that radio-tagged cliff swallows often moved early in the year while assessing sites (Brown and Brown, 1996). We use the term "colony" to refer to the birds occupying a structure in a given year, whereas "colony site" refers to the physical substrate.…”