Animals use multiple signals to attract mates, including elaborate song, brightly coloured ornaments and physical displays. Female birds often prefer both elaborate male song and intense carotenoid-based plumage coloration. This could lead less visually ornamented males to increase song production to maximize their attractiveness to females. We tested this possibility in the highly social and non-territorial house finch (Haemorhous mexicanus), in which females discriminate among males based on both song and on the intensity of red carotenoid-based plumage coloration. We manipulated male plumage coloration through carotenoid supplementation during moult, so that males were either red or yellow. Males were then housed under three social environments: (i) all red birds, (ii) all yellow birds or (iii) a mixture of red/yellow birds. We recorded song after presentation of a female. Red males produced more song than yellow males. But when yellow males were housed with red conspecifics, they produced more song relative to yellow males housed with equally unattractive yellow males. This study provides novel evidence that a male's plumage coloration and the plumage colour of his social competitors influence investment in song.
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