Ecology of parasitism of Brown-headed Cowbirds by haematozoa

Abstract

We quantified haematozoa infections in 964 Brown-headed Cowbirds (Molothrus ater) sampled over three summers in eastern Ontario. Our objective was to use the cowbird–haematozoa system to assess practical and theoretical aspects of testing Hamilton and Zuk's parasite hypothesis of sexual selection. We found that most individuals resampled within and between years were consistently scored as parasitized or unparasitized, although some individuals changed status, including going from parasitized to unparasitized. Many hatching-year birds were parasitized, which suggests substantial early exposure to parasites. Significant variation within and between years in general parasite prevalence, specific parasite prevalence, and intensity of infections indicated that population estimates of parasitism would be highly dependent on when birds were sampled and on the age and sex composition of the sample. Analysis of body condition and within-season recapture patterns indicated that parasitism did not have a negative effect on the health of cowbirds. Parasitized birds were also recaptured at the same rate as unparasitized birds between years, indicating that there were no viability effects due to parasitism. Our results suggest a number of pitfalls, some of which are potentially critical, to using haematozoa infections in passerines to test the Hamilton and Zuk hypothesis.