Experimental Modification of Reproductive Performance by Density in Captive Starlings

Abstract

Regulation of animal populations by densitydependent factors has received much attention from ecologists. Lack (1966) has examined this problem extensively in natural populations of birds, and contends that reproductive rate, as measured by the number of eggs per clutch, has evolved through natural selection to correspond, on the average, with the number giving rise to the greatest number of offspring. Since clutch size varies little with density, population regulation must, therefore, be a function of density-dependent variations in mortality which in some wild birds correlates well with food shortage. Lowered fecundity with high population density is rarely reported for avian populations (Kluyver 1951, 1963; Lack 1966; Perrins 1965). It is known, however, that external stimuli associated with breeding behavior in birds can result in changes in the birds' physiological state which might affect fecundity. Birds with a history of domestication are generally used in behavioral studies because of their adaptibility to confinement. Experimental tests of hypotheses concerning population regulation in wild birds have not been conducted perhaps due to difficulties in getting large numbers to breed readily in captivity. Thus mechanisms operating at the physiological and behavioral levels in wild birds which might result in reproductive changes and lead to population regulation remain unclear.