Author's personal copy Intersexual and intrasexual consequences of epaulet colour in male red-winged blackbirds: an experimental approach Ken Yasukawa a, * , Luke K. Butler b, 1 , David A. Enstrom c, 2 a Department of Biology, Beloit College b Department of Biology, Tufts University c Illinois Natural History Survey, University of Illinois article info Article history: Received 15 May 2008 Initial acceptance 3 October 2008 Final acceptance 4 November 2008 Published online 21 December 2008 MS. number: A08-00320 Keywords: Agelaius phoeniceus badge epaulet colour female choice intersexual selection intrasexual selection male–male competition red-winged blackbird We conducted experiments on the intersexual and intrasexual consequences of epaulet colour in male red-winged blackbirds, Agelaius phoeniceus. In the female choice experiment, we gave captive females a choice between males with normal or dulled epaulets, and between males with normal or reddened epaulets. Females tended to associate more with normal males than with dulled males, but were equally likely to perform precopulatory displays to the two types of males. Females associated significantly more with redder-than-normal males and were more likely to perform precopulatory displays to reddened males. In the male–male competition experiment, we presented free-living territorial males with a male model to which we could attach wings with dulled, normal or reddened epaulets. Presence of the model with reddened epaulets resulted in significant increases in display rates by territory owners, but the increase in display rates was probably in response to males other than the territory owner flying through and trespassing rather than to the reddened epaulet model per se. Normal epaulets produced a signifi- cant decrease in song rates and approach distances of owners. Responses of territorial and other males did not differ significantly during normal and dulled epaulet presentations. These results support the hypothesis that epaulets of male red-winged blackbirds are salient inter- and intrasexual signals: unusually red epaulets were attractive to females and attracted more attention and aggression from male conspecifics than normal epaulets. Epaulet colour may thus be an example of intrasexual selection opposing intersexual selection. Ó 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Studies of coloration in avian signalling have received much attention (e.g. Hill & McGraw 2006a, b), especially colour signals that may have evolved by sexual selection (e.g. Andersson 1994; Searcy & Nowicki 2005; Hill 2006; Senar 2006). Colour displays provide many well-known examples of intrasexual signals of aggression and dominance (see Senar 2006) as well as intersexual signals used in mate choice (see Hill 2006). For example, Senar (2006) listed 26 avian species in which plumage traits have been associated with success in agonistic encounters, and 18 avian species for which a relationship between plumage traits and status signalling has been tested experimentally. Interest in the impor- tance of colour in mate choice in birds is even more widespread. Hill (2006) listed more than 50 avian species that have been studied (some multiple times) for the relationship between colour and mate choice. The red-winged blackbird, Agelaius phoeniceus, is one species that has received considerable attention with respect to both inter- and intrasexual selection (see Searcy & Yasukawa 1995). The common name ‘red-winged blackbird,’ refers to an exten- sively studied feature of the male’s plumage: the red-and-yellow epaulet at the wrist of the wing. Observational evidence is consistent with the hypothesis that the epaulet functions intrasexually in territory defence. Territory owners display their epaulets in response to male intruders, but trespassing and newly establishing males keep their epaulets covered (Nero 1956b; Orians & Christman 1968; Hansen & Rohwer 1986). Some experimental evidence also supports the territory defence hypothesis. Males whose epaulets were experimentally blackened (‘black-winged blackbirds’) experienced elevated rates of trespassing, had difficulty evicting trespassers and were more likely to lose their territories than sham-manipulated controls (Peek 1972; Smith 1972; Morris 1975). On the other hand, males whose epaulets had been reduced in size were able to defend their territories (Westneat 2006). Thus, despite a number of pub- lished studies, the territory defence function of male red-winged blackbird epaulets remains unclear. There is also some evidence that the epaulet functions inter- sexually in attracting females. Male red-winged blackbirds display their epaulets conspicuously during courtship and precopulatory * Correspondence: K. Yasukawa, Beloit College, Department of Biology, Beloit, WI 53511, U.S.A. E-mail address: yasukawa@beloit.edu (K. Yasukawa). 1 L. K. Butler, Department of Biology, Tufts University, Medford, MA 02155, U.S.A. 2 D. A. Enstrom, Illinois Natural History Survey, University of Illinois, Champaign, IL 61820, U.S.A. Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/yanbe 0003-3472/$38.00 Ó 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2008.11.013 Animal Behaviour 77 (2009) 531–540