Diet of the Great Horned Owl in Central Saskatchewan

DIET OF THE GREAT HORNED OWL IN CENTRAL SASKATCHEWAN DYLAN D. VON KUSTER, 303 Avenue F North, Saskatoon, Saskatchewan. S7L 1V9 and DAVID SCHNEBERGER, 111 7th Street East, Saskatoon, Saskatchewan. S7H 0W7 Introduction In our Grade 12 year at Aden Bowman Collegiate, we needed a project for our science fair. Dr. Stuart Houston suggested analysing Great Horned Owl pellets that he and his climbers had collected over four years from various areas in central Saskatchewan, from Prince Albert south almost to Regina. This paper reports the results of that study. (2) variations in prey captured for the years 1986, 1987 and 1989; (3) dif¬ ferences in diet between vegetation zones; and (4) the average size of an owl pellet. Because there are such great differences in the mass (size) of prey such as Deer Mice and Snowshoe Hares, we then recalcu¬ lated our initial data to determine the relative importance of each prey animal in the diet. We had four objectives: To dis¬ cover (1) the overall diet of this bird; The pellets had been bagged and labelled as to place and date found. Great Horned Owl pellets. 50(3). September 1992 C. Stuart Houston 195 Great Horned Owl 196 Gary Seib Blue Jay Table 1: DIET OF THE GREAT HORNED OWL IN SASKATCHEWAN BY YEAR AND NUMBER OF PREY ITEMS 1986 1989 1987 Total # % # % # % # % 126 Deer Mouse Northern Pocket Gopher 50 48 Meadow Vole 14 Snowshoe Hare Red-backed Vole 3 Richardson’s Ground Squirrel 0 0 Least Weasel 0 Muskrat 241 Total Mammals 44.5 17.7 17.0 5.0 1.1 0.0 0.0 0.0 85.3 344 93 54 39 4 3 0 1 538 60.0 16.2 9.4 6.8 0.7 0.5 0.0 0.2 93.8 315 40 64 20 6 2 3 1 451 66.5 8.4 13.5 4.2 1.3 0.4 0.6 0.2 95.1 785 183 166 73 13 5 3 2 1,230 59.0 13.8 12.5 5.5 1.0 0.4 0.2 0.2 92.6 3.2 2.8 2.5 2.1 0.9 1.2 0.7 0.5 0.7 1.1 0.4 0.5 0.0 0.0 0.0 0.2 6.2 8 6 1 2 3 1 1 0 0 0 1 0 23 1.7 1.3 0.2 0.4 0.6 0.2 0.2 0.0 0.0 0.0 0.2 0.0 4.8 22 21 12 11 11 8 5 4 3 1 1 1 100 1.7 1.6 0.9 0.8 0.8 0.6 0.4 0.3 0.2 0.1 0.1 0.1 7.6 100.0 474 99.9 1,330 100.2 MAMMALS BIRDS House Sparrow Blue-winged Teal Mallard Ruffed Grouse Red-necked Grebe Gadwall Black-billed Magpie American Robin American Crow American Coot Gray Partridge Western Meadowlark Total Birds Grand Total 9 8 7 6 4 1 2 1 3 1 0 0 42 1.4 0.4 0.7 0.4 1.0 0.4 0.0 0.0 14.9 5 7 4 3 4 6 2 3 0 0 0 1 35 283 100 573 We borrowed skeletons of the prey species from W. Bruce McGillivray of the Provincial Museum of Alberta, Edmonton, and later, with the assis¬ tance of Dr. Nigel Mathews, photographed them because they had to be returned prior to comple¬ tion of our study. Then we measured and dissected the pellets, bagging the bones and some of the fur and feathers for later identification from skeletons and photos. 50(3). September 1992 Results Overall, Deer Mice made up 59% of the 1,330 prey items (Table 1). Northern Pocket Gophers comprised 14%, Meadow Voles 12% and Snowshoe Hares nearly 6%, these four mammal species comprising 90% of the prey items. Birds, ranging in size from House Sparrow to Mallard, formed only 8% of the prey items. On an annual basis, the major 197 Table 2: DIET OF THE GREAT HORNED OWL IN SASKATCHEWAN BY DIFFERENT VEGETATION AREAS # in O/ /o # in o/ /o # in Dry Wet Hill Lands Lands Lands O/ /o # in % Plains MAMMALS Deer Mouse 43 39.8 1 25.0 131 52.2 610 63.1 Northern Pocket Gopher 19 17.6 1 25.0 49 19.5 114 11.8 Meadow Vole 17 15.7 1 25.0 26 10.4 122 12.6 Snowshoe Hare 14 13.0 1 25.0 23 9.2 36 3.7 2 1.9 0 0.0 2 0.8 9 0.9 Richardson’s Ground Squirrel 0 0.0 0 0.0 3 1.2 2 0.2 Least Weasel 0 0.0 0 0.0 1 0.4 2 0.2 Muskrat 1 0.9 0 0.0 0 0.0 1 0.1 96 88.9 3 75.0 235 93.7 896 92.6 House Sparrow 1 0.9 0 0.0 3 1.2 18 1.9 Blue-winged Teal 1 0.9 0 0.0 5 2.0 15 1.6 Mallard 3 2.8 1 25.0 0 0.0 8 0.8 Ruffed Grouse 1 0.9 0 0.0 1 0.4 9 0.9 Red-necked Grebe 2 1.9 0 0.0 3 1.2 6 0.6 Gadwall 1 0.9 0 0.0 0 0.0 7 0.7 Black-billed Magpie 0 0.0 0 0.0 1 0.4 4 0.4 American Robin 1 0.9 0 0.0 1 0.4 2 0.2 American Crow 0 0.0 0 0.0 1 0.4 2 0.2 American Coot 1 0.9 0 0.0 0 0.0 0 0.0 Gray Partridge 0 0.0 0 0.0 1 0.4 0 0.0 Western Meadowlark 1 0.9 1 25.0 0 0.0 0 0.0 Total Birds 12 11.0 1 25.0 16 6.4 71 7.3 Grand Total 108 99.9 4 100.0 251 100.1 967 99.9 Red-backed Vole Total Mammals BIRDS mammals held the same positions except for pocket gopher and vole in 1989 (Table 1). Birds varied from 5% in 1987 to 15% in 1986. However, one would expect these results to vary at different points during the ten-year population cycle of the Snowshoe Hare, which was scheduled to (but failed to) peak in 1990. The top three mammals (Meadow 198 Vole, Northern Pocket Gopher, and Snowshoe Hare), continued to be the top three prey items in all vegetation areas (Table 2). However, Snowshoe Hares were absent in the southern drylands. Birds made up almost twice as many prey items in the drylands (11%) as in the hills or ran¬ gelands (6%) and agricultural plains (7%). The foregoing calculations were Blue Jay Table 3: DIET OF THE GREAT HORNED OWL IN SASKATCHEWAN BY MASS OF PREY Number Average Weight (grams) Total Weight Species % Diet by Weight MAMMALS Deer Mouse 785 25.0 19,625 8.9 Northern Pocket Gopher 183 100.0 18,300 8.3 Meadow Vole 166 78.5 13,031 5.9 Snowshoe Hare 73 1,500.0 10,9500 49.5 Red-backed Vole 13 30.0 390 0.2 Richardson’s Ground Squirrel 5 450.0 2,250 1.0 Least Weasel 3 70.0 210 0.1 Muskrat 2 1,200.0 2400 1.1 1,230 3,453.5 165,706 75.0 House Sparrow 22 26.7 587 0.3 Blue-winged Teal 21 375.6 7,888 3.6 Mallard 12 13,32.4 15,989 7.2 Ruffed Grouse 11 626.6 6,783 3.1 Red-necked Grebe 11 1,304.1 14,345 6.5 Gadwall 8 793.8 6,350 2.9 Black-billed Magpie 5 170.1 851 0.4 American Robin 4 85.0 340 0.2 American Crow 3 503.2 1,510 0.7 American Coot 1 538.6 539 0.2 Gray Partridge 1 382.7 283 0.1 Western Meadowlark 1 104.6 105 0.1 100 62,33.4 55,570 25.3 1,330 9,686.9 221,276 100.3 Total Mammals BIRDS Total Birds Grand Total based on numbers of prey items identified as eaten by the owls. How¬ ever, because of differences in size, they do not reflect each species’ con¬ tribution to the Great Horned Owl’s diet. Applying the average weight of live animals to the numbers caught, it turned out that Snowshoe Hares make up nearly 50% of the owl’s diet in terms of biomass (Table 3). Deer Mice and Pocket Gophers drop to 9% and 8%, respectively, and the Meadow Vole to 6%. Birds went from 50(3). September 1992 8% of the prey items to 25% of the biomass — and excepting hares, a larger proportion than all other mam¬ mals combined. Whole pellets varied considerably in length — from 2.0 to 12.5 cm (Table 4). The average length as well as the most frequent length was 6.0 cm. A pellet which contained only Deer Mice averaged between 2.6 and 3.0 mice per pellet. 199 Table 4: AVERAGE SIZE OF PELLETS OF THE GREAT HORNED OWL IN SASKATCHEWAN Size Number Size Number (cm) (cm) Size Number (cm) 2.0 2 5.5 31 7.7 1 2.2 1 5.6 2 7.8 2 2.5 1 5.7 6 7.9 1 2.6 1 5.8 6 8.0 18 3.0 12 5.9 4 8.2 3 3.5 7 6.0 62 8.5 6 3.6 2 6.1 4 8.6 2 3.9 1 6.2 6 8.9 2 4.0 27 6.3 2 9.0 8 4.2 3 6.4 7 9.2 2 4.3 1 6.5 42 9.4 1 4.4 1 6.6 4 9.5 2 4.5 27 6.7 3 9.6 1 4.6 2 6.8 2 9.7 1 4.7 3 6.9 3 9.9 1 4.8 1 7.0 36 10.0 2 4.9 3 7.1 5 10.5 1 5.0 43 7.2 6 10.8 1 5.2 6 7.3 2 11.0 1 5.3 1 7.5 18 12.0 1 5.4 3 7.6 4 12.5 1 2. AUSTING, G.R., and J.B. HOLT, JR. 1966. The world of the Great Horned Owl. J.B. Lippincott, New York. Our display at the Saskatoon Regional Science Fair, and later at the Canadian National Science Fair in Windsor, Ontario, included a map of the landscapes of central Sas¬ katchewan, photographs of owl pel¬ lets, and skeletons of prey animals.1 The project won first in the Senior Life Science category and Best of Show at Aden Bowman Collegiate, first in Senior Life Science and Students’ Choice at the Regional Science Fair, and a Bronze Medal at the National Science Fair. 4. GODFREY, W.E. 1986. The birds of Canada, revised edition. National Museum of Canada, Ottawa. 1. ACTON, D.F. 1977. Landscapes of Southern Saskatchewan. (map) Department of Mines, Energy and Resources, Ottawa. 7. PERRINS, C M. and MACDONALD, D. 1985. All the world’s animals: rodents. Torstar Books, Toronto. 200 3. BANFIELD, A.W.F. 1974. The mammals of Canada. University of Toronto Press, Toronto. 5. KEITH, L.B. 1977. Great Horned Owl. Hinterland Who’s Who. Canadian Wildlife Service, Ottawa, 4 pp. 6. MIDDLETON. 1985. All the world's animals: birds, owls, parrots and waders. Tors tar Books, Toronto. Blue Jay