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Asymmetric sibling rivalry extends to hosts and brood parasites

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Abstract

Obligate brood parasites rely upon unrelated hosts to provide parental care for their offspring. Specialist brood parasites use visual and vocal mimicry of host offspring to induce host parents to care for the parasite offspring. Generalist brood parasites, however, use different tactics to obtain host parental care. Here, we examine how brown-headed cowbirds (Molothrus ater) restructure host red-winged blackbird (Agelaius phoeniceus) broods and the consequent effects on the expected survival of the brood parasite nestling. Competition within the host brood is governed by an asymmetric sibling rivalry, with the size of the core brood driving the survival of both core and marginal nestlings. Competitively inferior marginal nestlings conversely have little effect on core nestlings. Cowbirds primarily removed core host nestlings when altering the size and structure of host broods. Host brood size at hatching was reduced in conjunction with parasitism by cowbirds. The resulting brood structures differed markedly from those in unparasitized broods with most notably fewer core nestlings, the most potent competitors for the cowbird nestling. Marginal brood size, however, differed only slightly. This reflects the competitive dynamics: core but not marginal brood size affected cowbird survival. The same asymmetric nestling rivalry that governs competition among host nestlings applies to these brood parasitic cowbirds as well; by altering host family structure, they enhanced the effective environment for their nestlings.

Significance statement

Obligate brood parasites must purloin parental care from unrelated host species to rear offspring. Generalist brood parasites, such as cowbirds, face an especially steep challenge, as the offspring are not visual or vocal mimics of host offspring: they are inserted into the host brood and must compete against host offspring. Here, we show that changes in host brood size and structure associated with cowbird parasitism enhance the effective environment for the brood parasite nestling. Through the removal/damage of host eggs or interference with host egg hatching, host brood size is reduced, and most importantly, the number of first-hatched “core” offspring that are the most potent competitors for the brood parasite offspring. The same asymmetric sibling rivalry that exists within the host brood extends to competition between host nestlings and brood parasites.

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Data availability

The data for the paper are available at: https://dataverse.scholarsportal.info/privateurl.xhtml?token=90060b78-200c-4d3a-a85d-d781d0325115 (brood structure); https://dataverse.scholarsportal.info/privateurl.xhtml?token=daac8b1a-29b6-48b5-a5a1-09854a030fc8 (cowbird nestling survival); https://dataverse.scholarsportal.info/privateurl.xhtml?token=4b027565-f750-4e26-9523-eaa02300288b (egg removal analysis); https://dataverse.scholarsportal.info/privateurl.xhtml?token=f3a49627-c4a7-465c-9a81-c84c6cbc3b06 (demographic data).

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Acknowledgements

The authors thank a very large number of field assistants—too numerous to name here—who made this study possible. We thank Douglas Mock and two anonymous reviewers for providing constructive comments that significantly improved the manuscript.

Funding

Funding for this work was provided by the Natural Sciences and Engineering Research Council of Canada and the University of Winnipeg.

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Authors and Affiliations

  1. Department of Biology, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB, R3B 2E9, Canada

    Scott Forbes & Mark Wiebe

  2. Department of Biology, Cape Breton University, 1250 Grand Lake Rd, P.O. Box 5300, Sydney, NS, B1P 6L2, Australia

    Barb Glassey

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  1. Scott Forbes
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  2. Barb Glassey
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  3. Mark Wiebe
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Correspondence to Scott Forbes.

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The work reported here complied with the laws of Canada, and ethical approval was obtained under Animal Care Protocols issued by the University of Winnipeg. All applicable international, national, and/or institutional guidelines for the use of animals were followed.

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The authors declare no competing interests.

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Communicated by M. Soler.

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Forbes, S., Glassey, B. & Wiebe, M. Asymmetric sibling rivalry extends to hosts and brood parasites. Behav Ecol Sociobiol 76, 43 (2022). https://doi.org/10.1007/s00265-022-03137-1

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