Behaviour 2019
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Ornamentation conveys reproductive benefits but is socially costly for male Red-backed Fairywrens
Joseph F Welklin1,2, Sarah Khalil3, Samantha M. Lantz3, Jordan Boersma4, Hubert Schwabl4, Jordan Karubian3, Michael S Webster1,2. 1Cornell University, Ithaca, NY, United States; 2Cornell Lab of Ornithology, Ithaca, NY, United States; 3Tulane University, New Orleans, LA, United States; 4Washington State University, Pullman, WA, United States

The presence of unornamented male phenotypes in nature is puzzling because unornamented males typically obtain lower reproductive success than ornamented males. Social costs have been proposed as a potential cause of unornamented male phenotypes, especially in birds, but few studies have tested this hypothesis using free-living animals in a natural context. The Red-backed Fairywren (Malurus melanocephalus) exhibits unornamented and ornamented male phenotypes, and ornamented plumage is associated with increased reproductive success. We tested whether ornamented plumage is socially costly in the Red-backed Fairywren by experimentally implanting one-year-old males with testosterone during the non-breeding season to induce premature molt into ornamented plumage and then tracking post-treatment social interactions. We found that testosterone-implanted (ornamented) males were more likely to receive aggression from other males than were control-implant (unornamented) males. This indicates that young males do experience social costs when molting into ornamented plumage, and suggests that social costs likely help maintain the honesty of ornamented plumage in Red-backed Fairywrens.