Ectoparasitism shortens the breeding season in a colonial bird.

Charles R Brown,Mary Bomberger Brown

doi:10.1098/rsos.140508

Charles R Brown, Mary Bomberger Brown

Open Access

https://doi.org/10.1098/rsos.140508

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Abstract

When blood-feeding parasites increase seasonally, their deleterious effects may prevent some host species, especially those living in large groups where parasites are numerous, from reproducing later in the summer. Yet the role of parasites in regulating the length of a host's breeding season—and thus the host's opportunity for multiple brooding—has not been systematically investigated. The highly colonial cliff swallow (Petrochelidon pyrrhonota), a temperate-latitude migratory songbird in the western Great Plains, USA, typically has a relatively short (eight to nine week) breeding season, with birds rarely nesting late in the summer. Colonies at which ectoparasitic swallow bugs (Oeciacus vicarius) were experimentally removed by fumigation were over 45 times more likely to have birds undertake a second round of nesting than were colonies exposed to parasites. Late nesting approximately doubled the length of the breeding season, with some birds raising two broods. Over a 27 year period the percentage of birds engaging in late nesting each year increased at a colony site where parasites were removed annually. This trend could not be explained by changes in group size, climate or nesting phenology during the study. The results suggest that ectoparasitism shortens the cliff swallow's breeding season and probably prevents many individuals from multiple brooding. When this constraint is removed, selection may rapidly favour late nesting.

Highlights

Blood-feeding ectoparasites often exert strong selective pressure on their hosts and may affect the evolution of life-history traits such as clutch size, immune investment and offspring quality/quantity trade-offs [1,2,3,4,5]
We investigated how the frequency of late nesting changed over a period of more than 20 years at a parasite-free site and use the results to gain insight into how ectoparasites potentially constrain the length of the cliff swallow’s breeding season and the number of breeding attempts it makes
Year was a significant predictor of the percentage of nests re-occupied for late nesting (F1,15 = 31.6, p < 0.001, β = 0.49), with Whitetail colony initiation date (F1,15 = 0.08, p = 0.79), annual first juvenile capture date in the study area (F1,15 = 0.0, p = 0.99), Whitetail colony size (F1,15 = 0.46, p = 0.51) and annual Palmer drought severity index (PDSI) (F1,15 = 0.94, p = 0.35) not explaining any significant variation

Summary

Introduction

Blood-feeding ectoparasites often exert strong selective pressure on their hosts and may affect the evolution of life-history traits such as clutch size, immune investment and offspring quality/quantity trade-offs [1,2,3,4,5]. A number of studies have demonstrated negative effects of ectoparasites on nestling growth and survival, especially in colonial hosts where grouping enhances parasite transmission [6,7,8,9,10,11]. Ectoparasitism may affect the number of nesting attempts hosts are able to undertake, if infestations increase seasonally to the extent that. Breeding later—when more parasites are present—is often unsuccessful [12,13]. Ectoparasitism 2 could be one of the ecological drivers for the commonly observed seasonal decline in reproductive success for many birds of temperate latitudes [14,15,16,17,18] and might serve to truncate the typical length of the breeding season

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