SUNRISE NEST ATTENTIVENESS IN COWBIRD HOSTS

We recorded vigilance around sunrise at 140 nests of 10 potential host -species (six ''accepters'' and four ''rejecters'' of cowbird eggs) of the brood parasitic Brown-headed Cowbird (Molothrus ater) at Delta Ma rsh, Manitoba, to determine if hosts are present on or near their nest s when cowbirds come to lay. We predicted that accepter species parasi tized infrequently would be found to be more vigilant at their nests d uring this time than accepter species more heavily parasitized. During our watches we observed seven acts of parasitism, all between 03:44 C ST (sunrise - 44 min) and 4:00 CST (sunrise - 25 min). We found no cor relation between frequency of parasitism of accepter hosts and nest at tentiveness. There was no significant difference between accepters and rejecters in early-morning vigilance. However, females of all species that roosted on their nests were more likely to be present during the critical period for parasitism than females that did not roost. Indiv iduals that did not roost first arrived at their nests, on average, af ter cowbirds presumably would have arrived to lay and this difference was significant in six species. We suggest roosting on the nest may pl ace the host in the best position to guard against parasitism, althoug h this behavior does not always prevent parasitism.