THE DETECTION OF AND RESPONSES TO EXPERIMENTAL INTRASPECIFIC BROOD PARASITISM IN EASTERN KINGBIRDS

Eastern kingbird, Tyrannus tyrannus, nests were experimentally parasitized to (1) devise a method for determining the frequency of intraspecific brood parasitism and (2) determine the frequency with which foreign, conspecific eggs are accepted. The data indicate that eggs of different females vary in appearance and that experimental intraspecific brood parasitism significantly increased intra-clutch variation in egg length, width, shape and spot density in the upper third of the egg. Based on a discriminant function analysis of egg traits in control and experimental nests (N = 10 for both), eight of 10 experimentally parasitized nests and seven of 10 parasitic eggs were correctly identified. Examination of physiognomic variation among eggs can thus provide an approximate (minimum) estimate of the frequency of intraspecific brood parasitism in kingbirds. Kingbird responses to experimental intraspecific brood parasitism varied with the stage of the laying cycle. Egg ejection predominated in nests parasitized prior to clutch initiation whether parasitized by control (red-winged blackbird, Agelaius phoeniceus) or foreign kingbird eggs. Once egg laying began kingbirds virtually never rejected a kingbird egg, but did eject most blackbird eggs. Hence, despite significant differences in egg appearance among females, and an ability to eject eggs, females will accept virtually all parasitically laid kingbird eggs once they have begun laying. © 1993 The Association for the Study of Animal Behaviour. All rights reserved.