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Egg size and asymmetric sibling rivalry in red-winged blackbirds

Oecologia volume 163, pages 361–372 (2010)Cite this article

Abstract

How big to make an egg is a life history decision that in birds is made coincident with a series of other similar decisions (how many eggs to have, whether to fortify them with maternally derived hormones or immune system boosters, whether to hatch the eggs synchronously or asynchronously). Though within-population variation in egg size in birds has been well studied, its adaptive significance, if any, is unclear. Here we examine within-population variation in egg size in relation to asymmetric sibling rivalry in a 17-year study of red-winged blackbirds (Agelaius phoeniceus), an altricial songbird. Egg mass showed a twofold range of variation, with roughly 80% of the variation occurring across clutches. By commencing incubation before the clutch is complete, mothers create advantaged core and disadvantaged marginal elements within their brood. Previous work on this system has shown that sibling competition is asymmetric, and that core offspring enjoy priority access to food, and as a consequence show higher growth and lower mortality than marginal offspring. Here we examine the effect of initial egg size on nestling growth and survival in relation to these competitive asymmetries. Egg mass was strongly linked to hatchling mass, and remained significantly related to the mass of both core and marginal nestlings; the effect of egg size was stronger for core offspring early in the nestling period, but the disparity between core and marginal nestlings narrowed as they approached fledging age, and slower growing marginals fell victim to brood reduction. The effect of egg mass on survival differed dramatically between core and marginal nestlings. Egg mass was significantly related to the survival of marginal but not core nestlings: below average egg mass was associated primarily with very early mortality. Asymmetric sibling competition is clearly a strong determinant of the consequences of egg size variation.

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Acknowledgments

This work complies with the current laws of Canada. This work would not have been possible without the assistance of a very large number of enthusiastic field assistants. Space does not allow us to acknowledge them all but we would like to thank in particular the following members of the Swamp Crew: Kristin Tuchscherer, Kristjana and Jessie Lee Cameron, Dobryan Tracz, Nagu Atmuri, Dean Swedlo, Saumya Jayakumar, Aaron Trachtenberg, Leanne Grieves, James Rogers, Jodi Griffith, Suzanne Thornton, Richard Grosshans and Barb Glassey. Two anonymous reviewers provided helpful comments on the manuscript. This work was funded by the Natural Sciences and Engineering Research Council of Canada.

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

    Scott Forbes & Mark Wiebe

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  1. Scott Forbes
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  2. Mark Wiebe
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Correspondence to Scott Forbes.

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Communicated by Chris Whelan.

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Forbes, S., Wiebe, M. Egg size and asymmetric sibling rivalry in red-winged blackbirds. Oecologia 163, 361–372 (2010). https://doi.org/10.1007/s00442-010-1629-x

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  • DOI: https://doi.org/10.1007/s00442-010-1629-x

Keywords

  • Egg size
  • Family structure
  • Asymmetric sibling competition
  • Core offspring
  • Marginal offspring