Egg Size, Hatching Asynchrony, Sex, and Brood Reduction in the Common Grackle

Abstract

In clutches of five, the Common Grackle (Quiscalus quiscula) provisions eggs unequally and starts incubation before the clutch is complete, thereby insuring initial advantages to some young which are not accrued to others. Field observations and experiments show: (1) egg wt increases with laying sequence within the clutch, (2) intraclutch variation in egg size results in competitive advantages in growth for young from large eggs, (3) asynchronous hatching favors young which hatch early at the expense of those which hatch late, (4) starvation mediates brood reduction of late—hatching young in many nests, and (5) normal brood reduction produces a sex ratio skewed in favor of ♀ ♀. Egg size variation within the clutch and asynchronous hatching are interpreted as antagonistic forms of parental investment. Presence of both adaptations suggests that the parent attempts to ensure the survival of chicks which hatch late as long as possible within an overall pattern of initial investment which ensures starvation of those individual young if food is insufficient to raise the entire brood. This retention is interpreted as a means by which the parent maximizes reproduction in the event of unexpectedly favorable food conditions. Elimination of late—hatching young produces a sex ratio favoring ♀ ♀ because ultimately investment in that sex is dependable if exceptionally healthy, and hence potentially polygynous, ♂ ♂ cannot be produced. Facultative adjustment of sex ratio within the brood may produce a high variance around a population sex ratio of 1:1 or some kurtoisis necessitated by unequal costs of raising individuals of each sex. The overall pattern of brood reduction in this species is interpreted as an evolved system by which a ♀ parent reduces risks so as to maximize inclusive fitness.