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In response to brood parasitism by the Brown-headed Cowbird (Molothrus ater), some female Yellow Warblers (Dendroicapetechia) bury cowbird eggs and sometimes their own eggs, whereas other females desert parasitized nests and renest at new sites. We identified circumstances that elicit burial or desertion by analyzing the histories of 132 naturally parasitized nests inspected over 13 breeding seasons in Manitoba. Damaged nests and clutches reduced to zero, one, or two host eggs were deserted, whereas the clutch was buried when zero, one, or two host eggs were present the morning cowbirds laid and the probability of hatching was high. Response times for burial (2.3 ± 0.1 [SE] days) and desertion (2.5 ± 0.3 days) were similar, but the variance differed significantly (1.29 days2 for burial versus 2.58 days2 for desertion). Burial is the Yellow Warbler's more frequent method of rejection, though desertion is used about one-third of the time, and it may be elicited by factors unrelated to brood parasi...

Their mates continued to feed the chicks: one male raised two to fledging while the other raised only one. This difference in success reflected differences in performance between the males as judged by other criteria. One chick survived a period of food-shortage by stealing fish from neighboring broods. These observations show the importance of food availability and parental competence in breeding success, the role of asynchronous hatching in limiting destructive competition between siblings, and the importance of food-stealing in competition between families. We thank G. Faddoul of the Suburban Experiment Station, University of Massachusetts, for conducting the autopsies, and the Patuxent Wildlife Research Center, U.S. Fish & Wildlife Service, for pesticide analyses. This work was supported by a grant from the Frederick W. Beinecke Fund. This note is Contribution no. 147 from the Scientific Staff, Massachusetts Audubon Society.