Experiments on defenses Cedar Waxwings use against Cowbird parasitism

TitleExperiments on defenses Cedar Waxwings use against Cowbird parasitism
Publication TypeJournal Article
Year of Publication1976
AuthorsRothstein SI
JournalThe Auk

Single real or artificial Brown-headed Cowbird eggs were experimentally added to 58 Cedar Waxwing nests. Rejection occurred at 69.0% of the nests. Nests experimentally parasitized during the laying period or shortly thereafter had a significantly higher rejection rate than nests parasitized later in the cycle (87.5% versus 40.0%). Waxwings rejected by nest desertion (60.0%), ejection (25.0%), or by damaging cowbird eggs (15.0%). Control nests showed that most nest desertions were in response to the cowbird egg. Twelve naturally parasitized nests also demonstrate that rejection is the usual response. Some but not all waxwing eggs frequently disappeared or were found broken in experimentally or naturally parasitized nests. Egg breakage and disappearance were significantly rarer at control nests. Among seven species known to reject experimental cowbird parasitism only the Cedar Waxwing shows the following: (1) frequent nest desertions, (2) any rejection by damaging cowbird eggs, (3) a higher rejection rate early in the nesting cycle than later in the cycle, (4) frequently delayed rejection throughout the nesting cycle, and (5) frequent disappearance and breakage of host eggs. The likely reason for the waxwing's nest desertion and egg damage and for the other peculiarities of its responses is that it cannot easily eject cowbird eggs. Not only is the Cedar Waxwing the smallest rejecter species but it also has a disproportionately small beak. Waxwings trying to eject cowbird eggs may have accidentally broken and then removed some of their own eggs. Thus, waxwings trying to remove cowbird eggs apparently endanger their own eggs. If parasitized early in the cycle, when the cowbird egg is likely to hatch, selection probably favors rejection as the reproductive loss from waxwing eggs damaged during ejection attempts is less than that from the nestling cowbird. But selection may favor acceptance of cowbird eggs laid later in the cycle as such eggs will not hatch and attempting to eject them results in the loss of some of the waxwing's own eggs.