A Comparison of Direct Observations and Collections of Prey Remains in Determining the Diet of Golden Eagles

Abstract

Two techniques were used to determine the diets of 4 pairs of nesting golden eagles (Aquila chrysaetos) in southwestern Idaho during 1978 and 1979; direct observations of prey deliveries to nests were compared with estimates of food habits derived from analyses of systematic collections of pellet and prey remains. There was no difference (P > 0.05) between the 2 methods in the estimated species composition, either by percent frequency or percent biomass. Comparisons of the daily capture rates derived using the 2 techniques demonstrated that collections of pellet and prey remains consistently underestimated observed prey delivery. Estimates of the time period collections reflected prey deliveries ranged from 1.6 to 5.5 days, but were consistent for each nest. Periodic observations of food delivery at nest sites can be used to correct for prey biomass unaccounted for in the collections; this procedure would enable researchers to use collections of pellet and prey remains to estimate prey biomass delivered to nests. J. WILDL. MANAGE. 47(2):360-368 Early work on the diets of birds of prey was based largely on analyses of stomach contents (Fisher 1893, McAtee 1935, Arnold 1954). More recently, investigators have concentrated research efforts on assessing the food habits of free-living raptors. The vast majority of these studies depended on interpretation of pellet and prey remains collected at nests (Craighead and Craighead 1956, Smith and Murphy 1973) and reported species composition qualitatively as percent frequency or biomass. Unfortunately, these studies were unable to estimate the daily and total biomass of prey and delivered to nests. Recently some investigators have used direct observations from blinds to assess the biomass of prey delivered to nests (Snyder and Wiley 1976, Newton 1978). Although direct observation is the most reliable method to assess the frequency and biomass of prey delivered to nests, it is too time-consuming to be a viable technique for most population studies. Most research on the food habits of golden eagles has depended on analyses of pellet and prey remains found on nests (Carnie 1954, Lockie 1964, Arnell 1971, Mollhagen et al. 1972, Lockhart 1976, Olendorff 1976). A few studies have derived quantitative estimates of the food requirements of golden eagles by using captive (Fevold and Craighead 1958) and wild (Brown and Watson 1964, McGahan 1967) birds. However, no study has tested the relationship between species composition estimates (percent frequency and biomass) derived from collections of pellet and prey remains and those based on observed prey deliveries. Furthermore, no previous studies have tested the feasibility of using periodic direct observation in concert with pellet and prey remains collections to generate prey biomass estimates. This paper evaluates these 2 methods and discusses ways in which collections of pellet and prey remains may be used to estimate the biomass of prey brought to the nest. This study is part of the Snake River Birds of Prey Research Project and was funded largely by the Bureau of Land Management. Partial funding also was I Contribution 3655 of the Journal Series, Florida Agricultural Experiment Station, Gainesville, FL 32611. 360 J. Wildl. Manage. 47(2):1983 GOLDEN EAGLE FOOD HABITS * Collopy 361 provided by the National Wildlife Federation, Sigma Xi, and the University of Michigan. Many individuals assisted throughout this study; in particular, I thank D. McCullough, R. Storer, G. Fowler, J. Diana, and R. Drobney. M. Kochert, D. Blasdell, T. Edwards, K. Steenhof, G. Smith, and A. Bammann also assisted throughout the study. I especially thank L. Farley for care in analyzing the collections of pellet and prey remains. S. Beissinger, M. Kochert, R. Labisky, W. Marion, T. O'Meara, and K. Steenhof reviewed the manuscript. STUDY AREA AND METHODS The study area was along the Snake River Canyon and surrounding upland desert plateau south of Boise, Idaho, in Owyhee and Elmore counties. This 195,063-ha area, known as the Snake River Birds of Prey Area (BPA), was administered by the Bureau of Land Management. The BPA lies within the Great Basin semidesert scrub biome described by Whittaker (1975); the major vegetation communities in the area have been previously described (U.S. Dep. Inter. 1979). Four pairs of golden eagles were observed throughout the nesting season at 4 sites in both 1978 and 1979. Photographs showing unique plumage characteristics of the breeding adults in 1978 and in 1979 revealed that the same pairs nested at the same sites in both years. Thus, betweenyear comparisons of results at each study site were possible. Because each breeding pair successfully fledged young in 1978, and subsequently nested in the same nest in 1979, I assumed that investigator interference was minimal. Systematic observations of eagles were made during the brood-rearing period from blinds placed 15-40 m away. Dawnto-dark observations were not possible because the entry to each blind was in full view of the nest; it was believed that activity near the blinds during early morning and late evening would frighten the adults from the nest, unnecessarily exposing the young to chilling temperatures. Consequently, I entered each blind at approximately midday and observed the nest until dark. I then slept in the blind and continued observations the next morning from first light until approximately noon. At the end of an observation period at 1 nest, I immediately moved to the next study site and continued observations using the same procedure. This sampling strategy was used to monitor each of the 4 study nests once every 6th day. All prey items delivered to the nest during each observation period were identified to species, assigned to a size class, and if possible, classified by sex. The proportion of the carcass delivered, time of day, and sex of the eagle delivering the prey were also recorded. Black-tailed jack rabbits (Lepus californicus) were known to be the most common prey item found in nests of this eagle population (U.S. Dep. Inter. 1979). To obtain greater resolution in the estimate of lagomorph biomass delivered to the nest, 6 adult jack rabbits were dismembered and the mean percentage of total body weight contained in 5 portions of the carcass was determined: entire (100%), headless (90.5 ? 1.2%), back 3/4 (83.5 + 1.4%; head and forelegs missing), back ?1/ (53.5 ? 1.1%; head, forelegs, and part of rib cage missing), and hindquarters (38.3 ? 1.0%; pelvis and hindlegs only). The actual biomass of black-tailed jack rabbit and Nuttall's cottontail (Sylvilagus nuttallii) carcasses delivered to nests was estimated using the portion of the body present and its fraction of the mean weight of the entire animal. The actual biomass of other prey delivered was estimated using the proportion of the total J. Wildl. Manage. 47(2):1983 362 GOLDEN EAGLE FOOD HABITS * Collopy carcass present as the proportion of total prey biomass I assumed that at least a portion of all prey by adult eagles during the brood-rearing phase was delivered to the nest. This assumption may have slightly underestimated the total number of prey captured, but did not significantly affect the species composition of the diet at the nest site. Both the biomass of an entire prey item and the actual amount delivered to the nest were estimated. For clarity these will be referred to as the biomass of prey captured and delivered. After each observation period, I entered the nest and verified the identification of all prey items previously All inedible prey remains and reguritated pellets found at the nest were collected (only the head, feet, and tail of fresh prey remains were collected) and later identified. The number of prey items in the pellets was determined by the maximum number of body parts present (e.g., femurs, feet, mandibles). The species and size class of each prey item were identified by comparison with study skins, skeletons, and hair specimens. The number of prey items counted in the pellets was compared with the number counted from fresh remains and those counted in the previous collection. Whenever a prey item in the pellet appeared to duplicate a previously counted item (e.g., in fresh remains or previous collection), it was not considered an additional individual. The total biomass represented by each species identified in the collections of prey remains was calculated using published weight data. Mammal weights were obtained from Hall (1946), Burt and Grossenheider (1964), Schreiber (1973), Armitage et al. (1976), and U.S. Dep. Inter. (1979). Bird weights were taken from Esten (1931), Galbreath and Moreland (1953), Nelson and Martin (1953), Craighead and Craighead (1956), Anderson (1969), Vermeer (1970), Fimreite (1971), Balph (1973), Marti (1973), and U.S. Dep. Inter. (1979). Weights of reptiles were obtained from U.S. Dep. Inter. (1979). All statistical comparisons of percent frequency and biomass of prey delivered to nests were made using 6 major prey categories: Lepus californicus, Sylvilagus nuttallii, Marmota flaviventris, small rodents (Heteromyidae, Cricetidae, and all other Sciuridae), large nonpasserines (Phasianidae and Corvidae), and miscellaneous birds and reptiles. All data were analyzed using an Amdahl 470/V7 computer and the University of Michigan Terminal System (MTS). Statistical analyses were conducted using the Michigan Interactive Data Analysis System (MIDAS). Statistical procedures used to analyze the data included both paired and 2-sample t tests (Remington and Schork 1970). To meet the assumptions of the statistical models, percentage data were arcsine transformed prior to analysis whenever they were outside the interval between 30 and 70%.