Multiple Paternity and Multiple Male Feeders in Bicknell's Thrush (Catharus Bicknelli)

Abstract

Although multiple male feeders and multiple paternity for a single brood are taxonomically widespread among nonmigratory birds, they appear to be exceedingly rare among migrants. During a demographic study of Bicknell's Thrush (Catharus bicknelli) in Vermont, we observed groups of 2–4 males and one female feed each of 14 broods (78% of 18 broods observed). One male and one female fed each of the remaining four broods. Three males fed broods of two females concurrently or consecutively. Genetic analysis using microsatellite markers revealed multiple paternity in 9 of 13 broods (69%). Four males had offspring in more than one brood in the same year, but no females did. Ten of 17 males (59%) sired offspring in the broods they fed, and 10 of 13 sires (77%) fed the broods in which they sired young. Females fed nestlings more than twice as often as males. Both males and females tended to increase feeding rates as brood size increased. Possible factors influencing the reproductive behavior of Bicknell's Thrush include harsh montane weather, food limitation, a male-biased sex ratio, and reproductive synchrony. Additional research is needed to examine the interaction of ecological and demographic variables that shape the complex breeding ecology of Bicknell's Thrush.