Date of Award
Master of Science
Dr. Liana Zanette
Brood parasitism of song sparrow (Melospiza melodia) nests by brown-headed cowbirds (Molothrus ater) leads to a 50% reduction in the number of female offspring that survive to day 6 post-hatch and to fledging age. It has been proposed that this effect can be explained by size-based competition. The cowbird is larger than male sparrow nestlings, which are in turn larger than females. It is thought that the females cannot compete successfully against these two larger classes of nestling competitors, and suffer higher rates of mortality as a result. I tested this hypothesis by adding a large conspecific nestling to just-hatched sparrow nests to determine whether large competitor size alone is sufficient to cause female-biased mortality. I found no evidence that a large added nestling caused increased female mortality, but I observed decreased growth of host nestlings in general, and an increase in the mortality rate.
DeCaire, Robert Peter, "Do Brown-Headed Cowbirds Skew Host Offspring Sex Ratios Because They Are Large?" (2010). Digitized Theses. 3736.