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Robin’s egg blue: does egg color influence male parental care?

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A recent hypothesis suggests that birds’ blue-green egg colors may be a sexually selected signal of female (and potentially nestling) quality that males use to make parental investment decisions. While there is some empirical support for this idea, both theory and observations question its validity. To test this hypothesis experimentally, we examined the influence of egg color on male American robin Turdus migratorius behavior by replacing natural clutches with four artificial eggs that were all either pale or vividly colored, close to the extremes in natural egg coloration. At the end of the incubation period, three unrelated nestlings were fostered into each experimental nest, and parental provisioning behavior was monitored when nestlings were 3, 6, and 9 days old. Male provisioning rate for 3-day-old nestlings was significantly higher in the vivid egg treatment compared to both the pale egg treatment and untreated controls, but there was no effect of egg color on paternal behavior at the older nestling stages. Male feeding rate at unmanipulated nests was only weakly positively related to natural egg color (chroma) when nestlings were 3 days old. These results suggest that blue-green egg color may act as a post-mating signal of female quality or investment in this species, but our findings do not exclude the possibility that egg color pigmentation also serves other adaptive functions.

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We thank Lori Parker, Fraser Cameron, Sara Calhim, Susan Crowe, and Mark Conboy for their time and support throughout the field season; Fraser Cameron for watching the many hours of recorded feeding behavior; and dozens of private landowners who gave permission to work on their properties, including the Queen’s University Biological Station. We are also grateful for P. R. Martin and V. L. Friesen’s comments and suggestions throughout this project and for the suggestions made by M. Hauber and anonymous reviewers. Funding was provided by the Natural Sciences and Engineering Research Council of Canada in the form of Graduate Scholarships, Discovery Grants and equipment (RTI) grants.

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Correspondence to Robert Montgomerie.

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Communicated by M. Hauber

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The online supplement to this paper presents some additional details of our “Methods” and “Results”, especially to provide full details of the statistical methods used and the models evaluated using the IT approach. (DOC 133 kb)

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English, P.A., Montgomerie, R. Robin’s egg blue: does egg color influence male parental care?. Behav Ecol Sociobiol 65, 1029–1036 (2011).

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