Growth and Sexual Dimorphism of the Boat-Tailed Grackle

Abstract

-At hatching, male and female Boat-tailed Grackles (Quiscalus major) are similar in size. By day 12, just before fledging, they differ in manus and tarsal lengths, tarsal thickness, and weight, but not in culmen, primary remex 7, and tail lengths. Male and female nestlings attain asymptotic weights of 102.4 g and 65.6 g, respectively, at a rate faster than predicted from the general passerine pattern. Furthermore, these asymptotes are only 54.8% and 65.3%, respectively, of adult weight. The combination of a faster growth rate and an asymptote below adult size means that grackles fledge sooner than would be expected for the general passerine pattern. Clutches of three eggs hatch asynchronously; the hatching of the third chick does not decrease growth rates of the early-hatching chicks. Thirdhatched young fledge at weights significantly below those of early-hatching young. Compared to males hatching from the first or second egg, males from the third egg require several more days to attain sizes larger than females. The pattern of growth represents an important component of an animal’s life history. Bird species differ widely in the rate at which young attain independence and adult size. Much of this variation is correlated with body size (Ricklefs 1968) and, presumably, fitness. For species in which the sexes differ in size, the pattern of growth often differs between sexes. Typically, at hatching, males and females of many size-dimorphic species do not differ significantly in size, but by fledging, one sex is significantly larger (Ricklefs 1968, Newton 1978, Moss 1979, Howe 1979, Fiala 1981). Frequently the smaller sex develops its plumage and motor skills sooner than the larger sex (Holcomb and Twiest 1970, Newton 1978, Moss 1979, Richter 1983). The larger sex may be more variable in growth and take longer to reach adult size (Howe 1979, Richter 1983). Asynchronous hatching leads to a size hierarchy among the nestlings (Howe 1976, Bryant 1978) which influences how the parents feed the nestlings and the nestlings’ subsequent growth. I have found little information about the effects of asynchronous hatching on the development of sexual dimorphism. Boat-tailed Grackles (Quiscalus major) are sexually dimorphic as adults; this dimorphism develops during the nestling period (McIlhenny 1937). I could find no detailed study of their growth. In central Florida, Boat-tailed Grackles typically lay clutches of two or three eggs (Bancroft 1983). Clutches of two, and the first two eggs in clutches of three, usually hatch on the same day, but several hours apart. In clutches of three, the third egg hatches one day later. Relative to adult size, grackles have a short nestling period (13-l 5 days). This suggests that they grow rapidly and/or fledge before they reach adult size. I investigated the development of sexual dimorphism in Boattailed Grackles and examined the effect of brood size and asynchronous hatching on their growth.