Date of Award

5-1-1994

Document Type

Independent Study

Degree Name

Doctor of Arts (DA)

Department

Biology

Abstract

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.

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