%0 Journal Article %J Ornithological Science %D 2016 %T Relatedness constrains virulence in an obligate avian brood parasite %A Rivers, J.W. %A Peer, B.D. %K begging %K brown-headed cowbird %K Kin competition hypothesis %K Molothrus ater %K Virulence %X

Virulence, the amount of harm a parasite inflicts on its host, is integral to elucidating the evolution of obligate avian brood parasitism. However, we lack information regarding how relatedness is linked to changes in behavior and the degree of harm that brood parasites cause to their hosts (i.e., virulence). The kin competition hypothesis combines theory from offspring signaling and parasite virulence models and states that the begging intensity of co-infecting parasites is driven by their relatedness, with concomitant changes in the degree of virulence expressed by parasitic young. We tested this hypothesis using the Brown-headed Cowbird Molothrus ater, an obligate brood parasitic bird whose virulence at the nestling stage is mediated by vigorous begging displays that are used to outcompete host young during feeding bouts. We found support for both predictions of the kin competition hypothesis: first, the begging intensity of cowbirds was greater in a population where cowbirds typically competed against unrelated host nestmates, relative to a population where they often competed against kin. Second, the greater intensity of begging in cowbirds was positively associated with decreased growth in host offspring during the developmental period. Given the dearth of studies on virulence in avian brood parasites, our results notably extend our understanding of how relatedness is linked to parasite behavior and virulence, and they highlight how spatially-isolated host populations can harbor different levels of virulence that are driven by competitive interactions between co-infecting parasites.

%B Ornithological Science %V 15 %P 191 - 201 %G eng %U https://doi.org/10.2326/osj.15.191 %N 2 %M KNZ001754 %R 10.2326/osj.15.191 %0 Journal Article %J Animal Behaviour %D 2013 %T The exaggerated begging behaviour of an obligate avian brood parasite is shared with a nonparasitic close relative %A Rivers, J.W. %A M.A. Blundell %A Loughin, T.M. %A Peer, B.D. %A Rothstein, S.I. %K Agelaius phoeniceus %K begging %K brood parasitism %K brown-headed cowbird %K Molothrus ater %K offspring signalling %K red-winged blackbird %X Offspring signalling models predict that the begging displays of obligate brood parasites are more intense than nonparasitic species because parasitic young are never reared by their genetic parents and often compete against unrelated host young during development. The brown-headed cowbird, Molothrus ater, has been described as having exaggerated begging relative to nonparasitic species, but an effective test of this idea is lacking because previous studies have not controlled for evolutionary history while simultaneously standardizing rearing conditions. We quantified the begging intensity of cowbirds and the closely related, nonparasitic red-winged blackbird, Agelaius phoeniceus, when both species experienced identical rearing conditions in two distinct nest environments: reared alone by a small cowbird host, or reared with two host young by a moderate-sized cowbird host. Against theoretical predictions, we found that in both nest environments four components of the cowbird begging display were similar to (or less intense than) blackbird begging displays (i.e. latency to beg, begging score, call rate and call amplitude) when nestlings were tested across a gradient of short-term need. Our results provide the first experimental evidence that a closely related, yet nonparasitic, species shares an exaggerated begging display with a brood-parasitic species when reared under conditions typically experienced by parasitic offspring. We discuss three nonexclusive explanations for our findings: (1) relatedness among cowbird nestmates reduced cowbird begging intensity (kin selection hypothesis), (2) reduced body condition of blackbirds elevated their begging intensity (body condition hypothesis) and (3) intense competition in blackbird nest environments led to increased blackbird begging intensity (competitive environment hypothesis). %B Animal Behaviour %V 86 %P 529 -536 %G eng %U http://dx.doi.org/10.1016/j.anbehav.2013.06.004 %M KNZ001591 %R 10.1016/j.anbehav.2013.06.004