SEXUAL SELECTION FOR DELAYED SIZE MATURATION IN A BIRD EXHIBITING DELAYED PLUMAGE MATURATION

In Purple Martins Progne subis, adult males were found to have signifi cantly longer wings and tails than subadult males, whereas females of both age classes were equal in flight feather length. This male age cl ass difference was not caused by higher mortality of long-winged subad ult males because the pattern also existed for the same individuals me asured at age one and two years, indicating that males postponed full feather growth for their first breeding year while females did not. An other male age class difference was that adult males were paired to th eir mates assortatively by wing-length while subadult males were not. This difference suggests either that females paired to subadult males were constrained in their choice of males or they did not prefer to pa ir with longer-winged males. A possible lack of female preference for longer-winged subadult males is consistent with two apparent costs tha t females suffered by pairing with subadult males that had longer wing s than themselves: (1) The greater the difference in wing-length betwe en subadult males and their mates, the less the male provisioned the b rood relative to the female, and (2) in a parallel study, females pair ed with longer-winged subadult males obtained many fewer extra-pair fe rtilizations from adult males. These findings suggest that, contrary t o previous hypotheses, ''delayed size maturation'' may be primarily se xually selected in some species, rather than naturally selected.