Why come back home? Breeding-site fidelity varies with group size and parasite load in a colonial bird

Abstract

Fidelity to a past breeding site is widespread among animals and may confer both costs and benefits. Colonial species occur at specific sites that can accommodate multiple breeders, and the choice of whether to return to last year's site or disperse elsewhere can affect colony site use, the colony size distribution and individual fitness. For the colonial cliff swallow, Petrochelidon pyrrhonota, which occupies colonies of widely different sizes, we used a 30-year field study in western Nebraska to investigate how the extent of infestation by ectoparasites and colony size affected breeders' colony site fidelity between years. We compared philopatry at colonies where parasitic swallow bugs, Oeciacus vicarius, had been removed by fumigation with that at nonfumigated sites exposed to natural levels of ectoparasites. About 25% of birds at nonfumigated colonies returned to their previous year's site, whereas about 69% of birds at fumigated colonies did so. Site fidelity was greatest at nonfumigated sites that changed the least in size between years. Birds were less likely to return to a nonfumigated site as the colony there became increasingly larger. Individuals philopatric to both nonfumigated and fumigated sites resided in colonies more similar in size between years than did dispersing birds. Most cliff swallows settled within 6 km of their previous year's site, indicating that many nonphilopatric birds still may have had some familiarity with the local landscape surrounding the site to which they moved. Removal of ectoparasites at a site allows large colonies to persist there perennially, probably contributing to higher philopatry because such large colonies are rare and would have been difficult to find had the residents dispersed. Cliff swallows are likely to be sensitive to both colony size and general familiarity with a given site or landscape region, and probably integrate these with other cues to select breeding colonies.

Ecological correlates of group integrity among dispersing cliff swallows

“…Parasite‐free (i.e., fumigated) colony sites are well known to be attractive to cliff swallows in our population (Brown et al. ), probably because of enhanced reproductive success there (Brown and Brown , ). If fumigated sites in general attract more settlers from other colonies, higher group integrity could reflect, in part, simply more birds moving to such sites.…”

“…They are also likely attractive because they are perennially used, vary less in size from year to year, and thus provide a temporally stable colony location with which many birds in the population are likely familiar (Brown et al. ). However, they do not function solely as sinks, attracting dispersers and retaining them over time, because these colonies also provide settlement groups that show high levels of group integrity when settling elsewhere.…”

“…Correlates associated with philopatry at the individual level have been explored previously for cliff swallows (Brown et al. ), and in this study, we test whether the same factors (and others) predict group integrity among dispersing birds. Because so many colony sites of different sizes and characteristics (>200) are available each year to cliff swallows in our population (Brown et al.…”

“…These variables were identified a priori as likely to be relevant (e.g., Brown et al. ), and we used them to construct a global model for each analysis. Inference was based on the global model, and we did no model selection.…”

“…Cliff swallows are sensitive to both colony size and extent of parasite load at a colony site in making settlement decisions (Brown and Brown , , Brown et al. , , Roche et al. ), and thus, we emphasize these variables in studying group integrity.…”