The Clutch Size and Numbers of Eggs of Brown-Headed Cowbirds: Effects of Latitude and Breeding Season

Abstract

Geographic variation in reproduction in certain bird species has been explained in several ways. The larger clutch size at higher latitudes in several species of passerines in the northern regions of the Old World and New World (Lack 1947-48, 1968, Klomp 1970, Hussell 1972) has sometimes been accounted for by the extra hours of daylight for feeding activity by the parents at the higher latitudes (Lack 1947-48) and by the extra hazards faced by adult birds in more northern populations or by higher mortality of eggs and nestlings in the tropics (Skutch 1967). The short breeding season available to birds of the higher latitudes also may help explain the large clutch size of birds in those areas. With time for rearing no more than one brood, one adult may be likely to leave more offspring than another if it lays a large clutch. In more tropical areas the long nesting season, with repeated opportunities for rearing young e.g., Willis 1974) may provide an ecological setting where the long-term breeding success of a pair of birds is higher if little parental investment goes into a single nesting effort (Williams 1966). The numbers of eggs laid in a season and the number in each set or clutch within a season may vary in parasitic birds as well as in birds that rear their own young. Such variation may help provide a general explanation for the trends in the nesting passerines. Parasitic birds such as the cowbirds provide a natural experiment to test the generality of some of the explanations of variation in clutch size, particularly those explanations that are based on the concept that family size in birds has a selective history shaped mainly by the amount of food the parents can provide to their young. The brood parasites lay eggs in the nests of other species that rear their young. As they do not feed their own young, they may be expected to lay more eggs than their nesting relatives, and so to show a different pattern of geographic variation in breeding effort. To test the effectiveness of these general explanations in forecasting the reproductive variation of a parasitic bird, I sampled populations of Brown-headed Cowbirds (Molothrus ater) in Oklahoma, California, and Michigan. These areas differ in (a) latitude, (b) length of the breeding season, and (c) number of years the population has existed. The predictions were as follows: (1) if clutch is imite in birds by the amount of food the arents can find for their brood, then the n rth rn (Michigan) cowbirds should not hav lar er clutches than cowbirds in Oklahoma or California. (2) If clutch size is smaller than the number of young that parents usually can feed because long-term seasonal reproductive success is adversely affected by a big early reproductive effort (Williams 1966), then the northern (Michiga ) cowbirds should have larger clutches because they have a shorter breeding season, and hence should be less restrained in their