Date of Award:

1975

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Advisor/Chair:

Michael L. Wolfe

Abstract

Forty-three and 54 Ferruginous hawk (Buteo regalis) pairs were found occupying territories in northern Utah and southeastern Idaho during 1972 and 1973, respectively. Of these 38 and 27 nesting pairs laid eggs. Nesting success was 77.1 percent in 1972 and 74.6 percent in 1973. for successful nests, an average of 2.9 and 2.6 young hatched and 2.7 and 2.3 young fledged during the respective years. This population is reproductively comparable to others in Utah and Colorado. Analysis of prey items collected from the nests indicated that black-tailed jack-rabbits (Lepus californicus) constitute 86 percent of the biomass (by weight) of three major prey species consumed by ferruginous hawks in this area. Jackrabbit density may be a major determinant of the number of young produced in a given year. Weight gained by the nestlings showed a marked sexual dimorphism. Female fledgelings weighed up to 1.43 times as much as males. Criteria were developed for sexing ferruginous hawks by measuring the diameter of the hallux. Mortality of 17 birds from the study area was recorded, of which 47 percent were immature birds. A total of 108 fledglings were banded and marked with color-coded patagial wing markers. Band reports of five (10 percent) of these birds were received. Utah juniper (Juniperus osteosperma) provided nest sites for 96.0 percent of the nests while three percent were built on the ground. Plant community types were determined at 63 nesting sites from aerial photographs. Dominant vegetation around nest sites were desert shrub types and crested wheatgrass (Agropyron cristatum) seedings. The possible impact of land management practices on ferruginous hawks is discussed.

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