Removal of Yellow Warbler Eggs in Association with Cowbird Parasitism

In this study, I quantified the removal of eggs by the Brown-headed Cowbird (Molothrus ater) from parasitized Yellow Warbler (Dendroica petechia) nests. Laying cowbirds removed one warbler egg from about one in three parasitized nests. Assuming the same individual was involved in both acts, cowbirds removed host eggs on a day before or the same day they laid their own eggs (33.3%, n = 8 nests), on the same day (20.8%, n = 5) but after they laid, or on the same or subsequent days (46%, n = 11). As almost half (41%) of the parasitized nests received cowbird eggs before host eggs appeared, many cowbirds that removed eggs had to return to the nest to do so, although burial precluded that at some nests. Return trips increased the chances that a cowbird might mistakenly remove its own egg, especially if it was the only egg in the nest, and that interactions might occur between host and parasite. Explanations for the removal of eggs at the time of parasitism are summarized and two hypotheses are tested. The likelihood of warblers accepting a cowbird egg experimentally introduced into their nests was not influenced by the removal of a host egg at the time of parasitism, as predicted by the "host deception" hypothesis. Yellow Warblers tolerated five- and four-egg clutches being reduced to three eggs (36/37 nests), but abandoned nearly one-third of clutches reduced to two eggs (13/42) and most clutches reduced to one egg (20/22). Only one control nest was deserted (n = 41 and 59 clutches of five and four eggs, respectively). Only four of 13 (31%) clutches reduced to one egg of each of the warbler and cowbird were abandoned, but 11 of 14 (79%) clutches reduced to only one cowbird egg were deserted. None of the 10 clutches reduced to two cowbird eggs was deserted. These results suggest that cowbirds can remove at least two eggs without risking desertion by the warblers. It is not clear why only one egg is removed, when removal occurs in the first place. Cowbirds that manage to ingest eggs they remove undoubtedly gain nourishment.