ADAPTIVE TOLERANCE OF COWBIRD PARASITISM BY PROTHONOTARY WARBLERS - A CONSEQUENCE OF NEST-SITE LIMITATION

In a study of prothonotary warbler, Protonotaria citrea, breeding ecology in which nest-site densities were manipulated, 21% of 172 warbler nests were parasitized by cowbirds, Molothrus ater; a relatively high frequency of parasitism compared to the majority of hole-nesting species. Cowbirds reduced warbler nest success by approximately 25%, primarily through egg removal by female cowbirds and lowered hatching success of warbler eggs. However, the impact of cowbird parasitism on warbler nesting success was relatively low compared to that reported for the majority of host species. Although female warblers accepted parasitism in 81% of cases, parasitized nests were abandoned significantly more frequently than were unparasitized nests, with the probability of nest desertion being contingent upon the numbers of nest-sites available on the territory. Similarly, the overall tendency for females to renest following either a successful or, especially, a failed nest was related to nest-site availablity. Timing of parasitism in relation to stage of the nest or time of the season did not significantly influence nest desertion by warblers. Thus, prothonotary warbler females may usually be forced to accept cowbird parasitism as a consequence of nest-site limitation. Relatively low costs associated with accepting cowbird eggs may be chosen by warblers only as an alternative to not breeding. These reuults support the hypothesis that acceptance of parasitism may be adaptive when the costs of rejection outweigh the costs of acceptance of parasite eggs. © 1991.