Date of Award
Doctor of Arts (DA)
The study was conducted from early May to late July, 1992 at Oakville Prairie, Grand Forks Co., North Dakota. The clay-colored sparrow (Spizella pallida) was the most common breeding bird at the site. Also abundant at Oakville was the brown-headed cowbird (Molothrus ater) , an obligcite brood parasite. The main objectives of the study were to assess the impact of cowbird parasitism on the reproductive success of this clay-colored sparrow population, and to examine some factors that may influence the probability that a given nest will be parasitized.
Forty-two clay-colored sparrow nests were found, and of these, 16 (38%) were parasitized by cowbirds. The occurrence of cowbird eggs in sparrow nests conformed to a Poisson distribution. Sparrow clutch sizes in non- parasitized nests were significantly greater than in parasitized nests. Apparent fledging rates from non- parasitized and parasitized nests were also different (47% versus 18%); the fledging rate for cowbird chicks was 32%. Mayfield nest success rates for clay-colored sparrows were 31.7% from non-parasitized nests and 17.4% from parasitized nests. The height of a nest above ground apparently did not influence its chance of being parasitized; however, its proximity to potential cowbird perch sites was a key factor in determining whether or not it was parasitized. Mean distance to nearest perch site was significantly greater for non-parasitized nests. Of the 16 parasitized nests, only 3 were abandoned by female clay-colored sparrows, and there were no observed incidents of cowbird egg ejection. Based on these data, the clay-colored sparrows at Oakville Prairie were considered to be cowbird egg acceptors and tolerant hosts.
Romig, Gregory P., "The Impact of Cowbird Parasitism on a North Dakota Population of Clay-Colored Sparrows" (1994). Theses and Dissertations. 3830.