Factors Affecting Status and Reproduction of Ospreys in Yellowstone National Park

Abstract

Factors affecting status and reproduction of ospreys (Pandion haliaetus) in Yellowstone National Park were studied during 1972-77. The population declined from an estimated 120 breeding pairs in 1917 to 100 adults (about 45 pairs) in 1974. Nest success at occupied nests averaged 44% and productivity averaged 0.73 young fledged per occupied nest. Reproduction was higher (P < 0.05) along streams with little human disturbance than on Yellowstone Lake which received more human use. Reproduction at active nests located more than 1 km from a backcountry campsite on Yellowstone Lake was comparable to that for stream nests. Reproduction in undisturbed nests was apparently sufficient to maintain an osprey population, whereas reproduction in disturbed nests was not. Residues of environmental pollutants present in osprey eggs did not appear to affect population stability. Proper management of human use of Yellowstone Lake should permit the osprey population to increase. J. WILDL. MANAGE. 43(3):595-601 This study was conducted in Yellowstone National Park from 1972 to 1977 to ascertain status and trends of ospreys and to identify factors affecting reproductive performance. R. L. Eng supervised the study and J. Stradley, D. Nuss, and E. Jones assisted in the field. R. L. Eng, M. Meagher, D. Houston, G. F. Cole, S. Postupalsky, C. J. Henny, and W. Matthews critically read drafts of the manuscript and offered valuable suggestions. Many other National Park Service personnel were helpful, especially N. Ary, R. Fong, and R. Kelly. STUDY AREA AND METHODS The study area encompassed Yellowstone National Park which occupies 8,995 km2 in the northwestern corner of Wyoming and adjacent Montana and Idaho. The geology of the Park has been described by Keefer (1972), the vegetation zones by Despain (1973), and the climate by Lowery (1959). Yellowstone Lake, the largest lake in the Park (355 km2), lies at an elevation of 2,357 m and is oligotrophic (Benson 1961), as are the other large lakes. Within the Park, ospreys nest primarily around Yellowstone Lake, in the Grand Canyon of the Yellowstone River, and along smaller streams. Seventeen aerial surveys were conducted with a 150-hp Piper Super Cub between late April and mid-August during 3 summers (1972-74), when the osprey population was studied intensively. Two flights were conducted in June and August 1977. Data on population size and distribution, nest location, and reproduction were collected during these flights, and Park personnel reported the success of stream nests based on numerous ground observations during 1975-77. Efforts were made to keep disturbance to a minimum, and nests were rarely approached on the ground during incubation. Osprey nests in the Grand Canyon of the Yellowstone were checked periodically with a 15-60x spotting scope. Reproductive terminology follows Postupalsky (1974). Briefly, an occupied nest was one with a pair present, and an active nest was one in which eggs were present. SA joint contribution from Montana State University, Agricultural Experiment Station Project 416, Paper 881, Journal Series; and the National Park Service, RSP Project YELL-N-66. 2 Present address: Montana Department of Fish and Game, Box 36, Rosebud, MT 59347. J. Wildl. Manage. 43(3):1979 595 This content downloaded from 207.46.13.124 on Wed, 22 Jun 2016 05:18:22 UTC All use subject to http://about.jstor.org/terms 596 OSPREYS IN YELLOWSTONE NATIONAL PARKISwenson Productivity was the number of young raised to an advanced prefledging stage of development, and was calculated separately for occupied and active nests. Five osprey eggs that failed to hatch were collected.