LOW-FREQUENCY OF OBSERVED COWBIRD PARASITISM ON EASTERN KINGBIRDS - HOST REJECTION, EFFECTIVE NEST DEFENSE, OR PARASITE AVOIDANCE

Eastern kingbird (Tyrannus tyrannus) nests rarely are parasitized by brown-headed cowbirds (Molothrus ater). Kingbirds are one of a dozen or so species known to eject cowbird eggs from their nests. We hypothesized that either kingbirds eject cowbird eggs so quickly that researchers normally do not detect the eggs during daily nest inspections, or that cowbirds avoid parasitizing kingbirds. We tested these alternative hypotheses by experimentally introducing real cowbird eggs into eastern kingbird nests during the pre-egg, early laying, late laying, and incubation stages. We recorded the interval between ''parasitism'' and ejection of the cowbird eggs. Although kingbirds ejected 87 of 88 cowbird eggs placed in their nests, about 40% of the eggs remained in nests for more than 24 h. Thus, during daily nest inspections we should have observed cowbird eggs if nests were parasitized at all. In fact, we detected only one parasitized nest among the 402 inspected daily. The time for ejection was longest at nests parasitized early in laying, and shorter at nests parasitized before and after. This variation in ejection times may reflect the time kingbirds require to learn to recognize their own eggs. Although kingbirds defend their nests aggressively, they do not respond to female cowbirds as unique threats and do not guard their nests before sunrise when cowbirds lay. We conclude that cowbirds avoid parasitizing eastern kingbirds because their eggs most likely will be wasted. The rejection behavior persists possibly because it is almost cost-free (a maximum of 0.07 kingbird egg lost or damaged per cowbird egg ejected), or it evolved in response to conspecific rather than cowbird parasitism. Foreign kingbird eggs introduced into nests at different nest stages were ejected only during the pre-egg stage. This result supports the hypothesis that rejection behavior in eastern kingbirds evolved in response to cowbird parasitism.