REACTIONS OF 4 PASSERINE SPECIES TO THREATS OF PREDATION AND COWBIRD PARASITISM - ENEMY RECOGNITION OR GENERALIZED RESPONSES

Four host species of the parasitic brown-headed cowbird (Molothrus ate r) were exposed to taxidermic mounts of a female cowbird, fox sparrow (Passerella iliaca), and common grackle Quiscalus quiscula) at their n ests during their egg-laying or nestling stage. Red-winged blackbirds (Agelaius phoeniceus), a species that accepts cowbird eggs laid in the ir nests, responded more aggressively to cowbird models early in their nesting cycle, indicating that they recognized the unique threat the cowbird posed. Gray catbirds (Dumetella carolinensis), northern oriole s (Icterus galbula), and cedar waxwings (Bombycilla cedrorum) can remo ve cowbird eggs from their nests and for the most part they responded similarly to cowbird models and the ''nonthreatening control,'' i.e. a fox sparrow. Cedar waxwings were nonaggressive to all the models and may rely on concealment to protect their nests from enemies. Removal o f cowbird eggs by puncture ejection is more risky than grasp ejection. Despite this, orioles and waxwings (puncture ejectors) were not signi ficantly more aggressive to cowbird models at egg laying than catbirds (grasp ejectors). Responses of the three rejector species toward the cowbird model did not change over the nesting cycle, indicating furthe r that they do not recognize cowbirds as a unique threat. Rejector spe cies may not recognize, cowbirds because they have little experience w ith them. With the exception of waxwings, all of the hosts recognized the grackle as an enemy and increased their levels of defence from the laving to nestling stages. Three of the host-species did not simply r espond in a generalized manner to any intruder at their nests but inde ed recognized specific enemies. Considerable interspecific variability exists amongst the four species in defensive behaviours, which may re flect their different nesting habitats.