BURIAL OF COWBIRD EGGS BY PARASITIZED YELLOW WARBLERS - AN EMPIRICAL AND EXPERIMENTAL-STUDY

The responses of yellow warblers, Dendroica petechia, to naturally laid and experimentally introduced eggs of the brown-headed cowbird, Molothrus ater, were recorded. Some female warblers accepted naturally laid cowbird eggs. Rejected eggs were usually buried, sometimes along with warbler egg(s). Yellow warblers more frequently buried cowbird eggs that were laid in their nests before their own clutches were initiated through the day on which they laid their second eggs. Warblers accepted most cowbird eggs laid in their nests later in the laying cycle and during incubation. Warblers were more likely to reject parasitic eggs when they occurred during the first half of the clutch-initiation period. Cowbirds parasitized yearling and older female yellow warblers with equal frequency and rejection was not related to the age of the female warbler. Yellow warblers responded differently at nests parasitized experimentally with real cowbird eggs. At about 25% of the nests, warblers buried cowbird eggs added before clutch initiation. However, nearly all cowbird eggs added during the warbler's laying cycle were accepted, regardless of whether (1) a warbler egg was removed at the time of parasitism, (2) clutches were reduced, or (3) hosts were exposed to a stuffed female cowbird before being parasitized. The dichotomy of responses yellow warblers exhibit to cowbird parasitism may be because of the high cost of rejection. Selection for rejection/acceptance may oscillate over time as costs and benefits of each strategy vary with different environmental conditions.