Factors Affecting the Productivity of Ospreys Nesting in West-Central Idaho

Abstract

-An Osprey (Pandion haliaetus) population nesting in the vicinity of Cascade Reservoir in west-central Idaho was studied for three years. The area supported about 50 nesting pairs, which laid an average of 2.58 eggs and fledged an average of 1.37 young per active nest throughout the study. These productivity estimates suggest a healthy, increasing population. Most nests were atop snags (66%) and on private land (70%). Ospreys nesting on artificial sites and those nesting more than 1,500 m from human disturbances produced more offspring. Fish in the 11-30 cm range constituted the bulk of the diet (89%) with brown bullheads being the most important prey species (38%). Osprey captures reflected prey availability. Establishment of Cascade Reservoir increased the availability of fish, which, in turn, allowed the Osprey population to increase. Productivity of these hawks appears to be chiefly related to reservoir level and prey availability. A large concentration of Ospreys (Pandion haliaetus) nest in Long Valley, Valley County, Idaho. Early census data indicate that this area contained fewer than a dozen Osprey nests in the late 1940's (Larrison et al. 1967). Although the nests were not studied intensively at that time, comparisons with recent data suggest that the population has increased since Cascade Reservoir was formed in 1948. In the past decade, the human population of Long Valley has increased 56% (State of Idaho 1980) and recreational use of Cascade Reservoir has intensified. In light of expanding human pressure on this unique Osprey habitat, the U.S. Bureau of Reclamation sponsored a three-year (1978-1980) investigation to collect base-line information on this population. The objectives were to: 1) census the population and determine its productivity, 2) identify factors influencing nest site selection and nesting success, and 3) ascertain feeding habits.