Intial Investment, Clutch Size, and Brood Reduction in the Common Grackle (Quiscalus Quiscula L.)

Female Common Grackles ( Quiscalus quiscula L.) laying clutches of 2, 3, and 4 eggs show different patterns of initial investment in offspring than those laying clutches of 5 and 6. Females laying small clutches do not increase egg size with laying sequence and hatch young synchronously by starting incubation after the last egg is laid. Such parents usually raise the entire brood. Females laying clutches of 5 and 6 increase provisioning of yolk and albumen with laying sequence, start incubation before the clutch is complete, and have variable success in space and time. Few raise the entire brood. Late—hatching young are favored by large egg size, but disadvantaged by hatching order because older siblings are fed first and grow faster. Youngest members of a brood are maintained as long as possible, but differentially starve if food is scarce. A review of the literature suggests that similar patterns may be widespread in passerine birds. Antagonistic forms of parental investment and brood reduction are interpreted as “bet—hedging” tactic for minimizing losses of parents attempting to raise large broods during periods of uncertain food availability. The most common clutch of 5 in Common Grackles is, on the average, the most productive of fledglings. But complete success of small broods and variability in the success of adults laying "the most productive clutch" suggest selection for individual optima related to demographic or ecological circumstances. Population averages emphasized in the literature appear as effects of such selection.