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 parasitism, such as interference by predators and inclement weather.
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Vol. 112 • No. 2