Intraspecific Brood Parasitism in the European Starling

We studied intraspecific brood parasitism in the European Starling (Sturnus vulgaris) in New Jersey. We used three criteria to detect brood parasitism. These were: (1) the appearance of more than one egg/day while the host was laying, (2) the appearance of extra eggs after the host completed its clutch, and (3) the mismatching of protein phenotypes between parent and putative offspring using polyacrylamide gel electrophoresis of blood and pectoral muscle tissue. Based on these methods of detection, one in every four early nests (i.e., clutches initiated in April) contained at least one parasitic egg. Later nests experienced little parasitism. Hosts from parasitized nests of the electrophoresis sample fledged fewer young than nonparasitized nests. Hosts exhibited behaviors that minimized the potential cost of brood parasitism. These behaviors included throwing out the first parasitic egg, initiating incubation midway through laying, laying synchronously, and laying one less egg than the apparent optimal clutch size. Parasites searched for unattended nests in which to lay their eggs. Those parasites laying eggs synchronously with the population fledged more young than those that did not. However, parasites probably laid only one egg to the hosts' four or five. Parasites were probably birds without nests and were therefore adopting a reproductive strategy that was better than total reproductive failure. Intraspecific brood parasitism may be an evolutionary force shaping the current reproductive strategies of our population of European Starlings.