Clutch Size and Numbers of Eggs Laid by Brown-Headed Cowbirds

The evolution of reproductive rates in birds has often been explained on the basis of either one of two somewhat contrasting theories.Lack (1954)) on the one hand, has maintained that the numbers of eggs laid by each species of birds has been established by natural selection to correspond with the largest number of young for which the parents can, on the average, find and provide sufficient food.Darwin (1859), on the other hand, pointed out that animal species with a high mortality rate have a high birth rate, and some Darwinian evolutionists have held recently that high mortality rates through selective action cause high birth rates.The concept that parental provision of food sets the upper limit for reproductive activity has been adopted by many ecologists who work with birds.Observational evidence in support of it is rather substantial (Lack, 1954; Lack, Gibb, and Gwen, 1957; Kluyver, 1963; Perrins, 1963Perrins, , 1964)).The numbers of eggs laid by brood parasites provide a source of information useful in evaluating theories of the evolution of reproductive rates.One might expect brood parasites to lay more eggs than birds which have to feed their own young, if food with which the parents nourish the young is the main determinant of the numbers of eggs laid by birds.Larger clutches, more clutches, or the abandonment of laying eggs in distinct clutches are possible adaptations in the brood parasites.The present study is a report on the clutch sizes and the numbers of eggs laid by Brown-headed Cowbirds (Molothrus ater) during two breeding seasons in Michigan.