Technical Report #82-3-08B DIETS of LONG-EARED OWLS from THREE HABITATS in NORTH-CENTRAL OREGON Oregon Department of Fish and Wi
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FISH &WILDLIFE REFERENCE LIBRARY Technical Report #82-3-08B 1985 DIETS OF LONG-EARED OWLS FROM THREE HABITATS IN NORTH-CENTRAL OREGON ik Aq 16, 0 J' 4", Oregon Department of Fish and Wildlife Nougame Wildlife Program DIETS OF LONG-EARED OWLS FROM THREE HABITATS IN NORTH-CENTRAL OREGON John M. Barss Portland State University Oregon Department of Fish and Wildlife Nongame Wildlife Program Technical Report #82-3-08B 1985 CONTENTS Page FIGURES TABLES ABSTRACT INTRODUCTION STUDY AREA 2 METHODS 5 Work Field 5 Analysis of Pellets 6 .............................................. RESULTS 7 DISCUSSION ............................................................ 22 Diet ............................................................. 22 Seasonal Variability 25 ............................................. Foraging ......................................................... 26 Movements of Long-eared Owls ..................................... 27 Management of Long-eared Owls .................................... 28 ACKNOWLEDGMENTS ....................................................... 28 REFERENCES ............................................................ 29 APPENDIX A ............................................................ 31 FIGURES Number Page 1. Locations of the study areas in Wasco and Wheeler Counties, Oregon 3 ............................................................ 2. Diets of the West Ravine long-eared owl pair of the Lawrence 1978-1982 13 Grassland for the years ................................. 3. Diets of the East Ravine long-eared owl pair of the Lawrence 1981 and 1982 13 Grassland for the years ............................. 4. Diets of the Antelope Valley long-eared owl pair for the years 1975, 1977, 1979, 1981, and 1982 14 .................................. 5. Diets of the Clarno Basin long-eared owl pair for the years 1974-1980 14 ......................................................... 6. Comparison of the predation of northern pocket gophers (T. by of owls 15 talpoides) four pairs long-eared ................... 7. Comparison of the predation of sagebrush voles (L. curtatus) by of owls 15 four pairs long-eared .................................. 8. Comparison of the predation of montane voles (M. montanus) by of owls 16 four pairs long-eared .................................. 9. Comparison of the predation of deer mice (P. maniculatus) by of long-eared owls 16 four pairs .................................. 10. Comparison of the predation of Great Basin pocket mice (P. parvus) by of owls 17 four pairs long-eared ................................... 11. Comparison of the predation of all other species captured by four pairs of owls 17 long-eared .......................................... 12. Seasonal variation in the predation of northern pocket gophers (T. talpoides) by long-eared owls on the Lawrence Grassland from IT M_a_rc_hT_98O 23 March 1983 19 to .................................... 13. Seasonal variation in northern pocket gophers (1:. talpoides) by owls 1982 20 captured long-eared during ........................... 14. Annual variation in the predation of northern pocket gophers (T. talpoides) 1979-1982 21 for the years ............................ 15. Seasonal, variation in the predation of sagebrush vole (L. curtatus) by long-eared owls in the Lawrence Grassland from 16 March 1980 to 23 March 1983 22 ................................ ..................... TABLES Number Page 1. Long-eared owl food data from 1978 to 1983 on the Lawrence Grassland Preserve (West Ravine) 8 .............................. 2. Long-eared owl food data from 1981 to 1983 on the Lawrence Grassland Preserve (East Ravine) 9 .............................. 3. Long-eared owl food data from 1975, 1978, 1979, 1981, and 1982 in the Antelope Valley 10 ................................... 4. Long-eared owl food data from 1974 to 1980 in the Clarno Basin 11 ......................................................... 5. Variation in the biomass of five prey species consumed by long-eared owls from study 18 three sites ........................ 6. Variation in the prey consumed by two pairs of long-eared owls from the Lawrence Grassland 18 ................................... 7. Seasonal variation in the diet of a pair of long-eared owls from the Lawrence Grassland 19 ................................... 8. Shannon-Weiner indices of annual species diversity of prey of long-eared owls (1975-1983) 23 ................................... DIETS OF LONG-EARED OWLS FROM THREE HABITATS IN NORTH-CENTRAL OREGON John M. Barss ABSTRACT Seasonal variability in the diet of long-eared owls (L-io otus) was investigated by analysis of prey remains in pel7-ets collected from nest, roost, and loafing sites of four pairs of owls in three habitats in north-central Oregon. No difference in prey selection was noted between two pairs of owls from the same grassland habitat, but prey selection was significantly different between habitats. The owls consumed a variety of prey species, but obtained two-thirds of their dietary requirements from only two species in each habitat, primarily northern pocket gophers (Thomomys talpoidde:rs), Great Basin pocket mice ) (Perognathus parvus , or mice (Peromyscus manicutatus). Long-eared owls in north-centrar-Oregon exploited those small mammal populations that were most common in the habitat encountered. Prey selection was restricted more by the availability of certain weight classes than by the availability of a particular species. The owls adapted rapidly to seasonal changes in prey availability. Juvenile pocket gophers were taken more often, when they were available in spring and summer, than any other mammalian prey. Microtines and pocket mice were captured more often than deer mice, and were the primary winter prey. Deer mice were captured in high enough numbers in some habitats to be the primary food source, especially in years when microtine numbers were low. INTRODUCTION Previous research on the feeding ecology of long-eared owls (Asio otus) has rarely considered the importance of seasonal fluctuations of prey populations or how those fluctuations might affect an owls' foraging strategies. Studies of many of the species eaten by long-eared owls reveal diverse behavioral, patterns which would affect the availability of prey to the owls (Getz 1961, Goszczynski 1977, Hansen 1960, Hansson 1969, Turner et a!. 1973). There are also seasonal differences in the behavior of a given prey species, including differences in reproductive behavior, foraging behavior, or or time spent above ground. For example, emergence times for estivating or hibernating animals will affect availability of those animals to predators. -1- Changes in shelter for the prey and food resource availability also vary seasonally. This spectrum of variables should be reflected in modified foraging strategies and significant differences in prey selection by avian predators. The only research that has attempted to address seasonal variability of food habits of long-eared owls, as well as interpair habitat differerences, were studies by Marti (1974) and Nilsson (1981). Unfortunately, neither Marti nor Nilsson used allometric equations to determine the approximate body mass of captured prey. Hence, estimates of larger prey biomass were biased. Of the few studies conducted on the feeding ecology of long-eared owls in Washington and Oregon (Maser and Brodie 1966, Maser et a!. 1970, Reynolds 1970, Knight and Erickson 1977) none was conducted for more than three years, nor did any deal with seasonal variability of food habits. Most studies of food habitats of long-eared owls were conducted during the winter when the owls gathered in flocks. Seasonal shifts in prey importance are not revealed in such studies. This study assesses the food preferences of pairs of long-eared owls found in three different floral communities in north-central Oregon and determines the effects of seasonality on prey selection. Study objectives were to: (1) analyze seasonal trends in prey selection by long-eared owls, (2) determine if habitat differences influenced prey slection, and (3) study the variability of prey size selection within a prey species population. STUDY AREA Three areas in north-central Oregon were selected for this study, including the Lawrence Memorial Grassland Preserve, range and agricultural lands within the Antelope Valley, and highly disturbed grazing lands in the Clarno Basin (Figure 1). The total study area covers 311 sq km and includes the communities of Antelope and Clarno. -2- co CO. Shaniko 0 + Antelope Clarno + + Study Sites WHEELER CO. Figure 1. The locations of the study areas in Wasco and Wheeler Counties, Oregon. -3- in the town of Antelope, The weather station nearest the study area is 40 years has averaged 32.2 cm. Average Oregon. Rainfall there over the past period range from 19.5% in July to monthly temperatures for this same time below -18'C and summer temperatures -0.90C in January. Winter temperatures averages recorded in Antelope are above 380C are not uncommon. The rainfall and are the best available for the most applicable to the Antelope Valley There is a trend toward higher Lawrence Grassland and the Clarno Basin. as goes from west to east through the temperatures and decreased rainfall one Camp Hancock Field Station in the study Temperature records taken at area. in 3* 80C in spring and summer than those the Clarno Basin are to higher the in both areas. Antelope Valley. Winter temperatures are similar 10 km in Wasco County, approximately The Lawrence