RAPID LAYING BY BROWN-HEADED COWBIRDS MOLOTHRUS ATER AND OTHER PARASITIC BIRDS

We compared the length of time parasitic and nonparasitic female birds spent on nests while laying eggs (laying bouts) to evaluate the hypot hesis that rapid laying by parasitic Brown-headed Cowbirds Molothrus a ter and other parasitic birds is a specialization for brood parasitism . Brown-headed Cowbirds typically spent less than 1 min on host nests while laying (41.0 +/- 4.58 [mean + s.e.] s, n = 21). In contrast, mea n laying bouts of six nonparasitic icterine species ranged from 21.5 m in to 53.4 min, and laying bouts of 13 other passerine species ranged from 20.7 min to 103.7 min. By spending only a few seconds on the nest while laying, brood parasites probably increase their chances of para sitizing nests unnoticed by hosts or, if noticed, are harassed by host s for less time. Rapid laying may be adaptive if aggression by hosts c an thwart attempted parasitism by chasing away the parasite, preventin g the parasite from entering the nest or injuring the parasite. Rapid laying may increase the likelihood that the parasitic egg will be acce pted. We tested some of these hypotheses by recording the responses of three frequently parasitized species to a stuffed female cowbird plac ed on their nests for 1 min. All species attacked the model vigorously ; however, the mean time for discovery of the model ranged from 3 min to 17 min, ample time for female cowbirds to parasitize the nests. We concluded that rapid laying by parasitic birds is an adaptation for pa rasitism and, in Brown-headed Cowbirds, reduces the chances that the p arasite will be attacked by hosts.