Brian D. Peer

Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Eric K. Bollinger


We attempted to determine the factors associated with the lack of parasitism of the Common Grackle (Quiscalus quiscula) by the Brown-headed Cowbird (Molothrus ater). We investigated the breeding phenology of the two species, the responses of colonial- and noncolonial-nesting grackles to female cowbird models, the frequency of artificial egg rejection by grackles, incubation success of cowbird eggs transferred into grackle nests, and the survival rates of cowbirds cross-fostered into grackle nests.

By the time cowbirds began egg-laying at our study sites, 88.5 % of all grackle nests were beyond the point of successful parasitism. Grackles responded much more aggressively toward female cowbird models than to Fox Sparrow (Passerella iliaca) models. Grackles rejected artificial cowbirds eggs more frequently during the prelay stage of the nesting cycle (13 rejections at 32 nests) compared to later stages. However, the rejection frequency during the later stages of nesting (lay and incubation) was virtually the same as in Rothstein's original study (1975) (12.4 % vs. 11.3 %).

A total of 14 cowbird eggs and nestlings were cross-fostered into grackle nests. Data were collected on six cowbird nestlings, none of which survived to fledging. Five of the nestlings died after two days, and the sixth nestling survived five days despite having two grackle nestmates that were each a day older. Grackle nestlings weighed more than twice as much as cowbird nestlings at hatching (5.4 ± 0.91 g vs. 2.5 ± 0.72 g), and had significantly greater gape widths and culmen lengths for the first two days after hatching. The lack of survival of cowbird nestlings in grackle nests may be partially due to this size asymmetry. However, one cowbird nestling died after two days despite having no grackle nestmates to compete with, thus suggesting the possibility of some behavioral incompatibility. This was unexpected as it is generally believed that nestling passerines have similar dietary requirements, with the exception of those species that feed their young primarily seeds or fruit.

Of the eight eggs that did not hatch, four appeared to be the result of ineffective incubation. These clutches contained between four and six eggs total, whereas the clutches in which cowbird eggs hatched contained a total of three eggs or fewer. These data support the host incubation hypothesis for egg removal by female cowbirds. If Brown-headed Cowbirds preferred larger hosts in the past (as indicated by the fact that all but one of the species that regularly eject cowbird eggs are larger than the cowbird), then it may have been advantageous for a female cowbird to remove at least one host egg to ensure more effective incubation of her own smaller egg.

Mourning Doves (Zenaida macroura) like Common Grackles exhibit a high rate of parasitic egg rejection behavior (31.2 %) for an accepter species. Despite Rothstein’s (1975a) conclusion that no geographic variation in egg rejection behavior exists, we found Mourning Doves in central Illinois rejected artific ial cowbird eggs at nearly twice the rate (58.6 %, x2 = 3.7, df = 1, p < 0.06) of those in Rothstein’s trials. The reason for the lack of geographic variation in Rothstein's trials may be the result of small sample sizes.