Dawn song of the eastern kingbird: an honest signal of male quality?

To be used as honest signals of quality, phenotypic traits must vary markedly within populations and show repeatable differences between individuals. Eastern kingbirds, Tyrannus tyrannus, sing a distinctive dawn song in the predawn darkness, and one hypothesis for its function is that it serves as an honest signal of male quality that females use during choice of within-pair or extrapair mates. Over a 2-year period, we quantified dawn songs of males to measure between- and within-season repeatability of the timing (start and end time) and length of song bouts, and song rate. We also measured morphological characters to measure repeatability and describe individual differences in body size and plumage quality. All song traits varied considerably among males, start time and song rates were repeatable between years, and all song rates were repeatable within both years. All morphological characters were significantly repeatable between years. Moreover, early singing males were larger and had relatively long flight feathers, and males that sang at high rates had relatively long flight feathers. Early singing males were also paired to the earliest breeding females, and in one year, male song rate was positively correlated with their social mate's clutch size. Relatively long flight feathers in other species are typical of older males and individuals of higher body condition, suggesting that, despite being suboscine passerines with relatively simple and innate songs, eastern kingbirds use song performance as an honest signal of male quality that females may use during mate choice.