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We investigated the function of Red-tailed Tropicbird (Phaethon rubricauda) tail streamers by quantifying patterns of variation in streamers and other measurable traits on the basis of measurements of 659 individuals, including 422 of known age and 459 of known sex, measured in the field from 2000 to 2002. Our data were consistent with the idea that Red-tailed Tropicbirds' tail streamer ornaments function for mate attraction, but it suggested that variability in streamer expression is arbitrary and unlikely to provide a meaningful signal of individual quality during mate choice. The two elongated red tail streamers are composed of bilaterally symmetrical filamentous central rectrices with a black rachis and narrow red vane averaging 399 ± 1.8 mm (SE) in length when fully grown. Prior to breeding, individuals of opposite sex conspicuously display elongated red tail streamers during complex aerial courtship. Streamers exhibited measurement asymmetries (the difference between new and old fully grown tail streamers) of up to 144 mm because of wear at the tip of the older streamer, which accounts for most of the difference in the lengths of two fully grown streamers in individual birds. As in other putative sexually selected traits, tail streamers were more variable than nonornamental traits across individuals. Males were slightly larger than females in length of fully grown streamers (ratio 1.04), culmen length (ratio 1.02), and tarsal length (ratio 1.01). There was no correlation between fully grown streamer length and body size (score on first principal component) (r = 0.004), wing length (males, r = 0.05; females, r = 0.10), or mass (males, r = 0.10; females, r = 0.04), which is consistent with the hypothesis that streamers have an ornamental rather than aerodynamic function. There was a significant correlation between the ratio of streamer lengths (growing vs. fully grown, r = 0.50) of male and female pair members, indicating that at least the timing of ornament development was coordinated within pairs. However, tail streamers did not appear to be good indicators of an individual's “quality;” on the basis of 372 adults of known age (3–22 years), streamer length did not increase with age (r = 0.30) and was not correlated with an index of body condition (r = 0.03). The streamer length of pair members was not correlated with chick asymptotic body mass or chick fate, nor was female streamer length with egg size.

Function of Tail Streamers of Red-Tailed Tropicbirds (Phaethon Rubricauda) As Inferred From Patterns of Variation