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 Marsh, Manitoba, to determine if hosts are present on or near their nests when cowbirds come to lay. We predicted that accepter species parasitized infrequently would be found to be more vigilant at their nests during this time than accepter species more heavily parasitized. During our watches we observed seven acts of parasitism, all between 03:44 CST (sunrise - 44 min) and 4:00 CST (sunrise - 25 min). We found no correlation between frequency of parasitism of accepter hosts and nest attentiveness. 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. Individuals that did not roost first arrived at their nests, on average, after cowbirds presumably would have arrived to lay and this difference was significant in six species. We suggest roosting on the nest may place the host in the best position to guard against parasitism, although this behavior does not always prevent parasitism.