EXPERIMENTS ON INDETERMINATE LAYING IN HOUSE WRENS AND EUROPEAN STARLINGS

Although long studied, determinate and indeterminate laying patterns in birds have not been related to avian ecology. House Wrens (Troglodytes aedon) often experience egg removal by potential nest usurpers, while European Starlings (Sturnus vulgaris) commonly experience both egg removal and egg addition by conspecific brood parasites. We predicted that House Wrens should respond to egg removal by laying extra eggs but should not respond to egg addition, while European Starlings should both increase clutch size in response to removal and decrease clutch size in response to addition of eggs. We manipulated clutch size at 70 House Wren and 49 European Starling nests by adding or removing two eggs during the laying period. In early wren clutches, birds altered clutch size as predicted: wrens with nests from which eggs were removed had larger clutches than did control birds, while birds with nests to which eggs were added had clutches of the same size as control birds, However, in late clutches wrens did not respond to either egg removal or addition. Starlings showed no significant response to either egg removal or addition in spite of high rates of conspecific egg removal and brood parasitism within the study population.