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Department of Biology, Cameron University, Lawton, Oklahoma
Prey species were identified from 169 pellets cast by a pair of great horned owls (Bubo virginianus) and their young in Jackson County, southwestern Oklahoma. Pellets were collected monthly between February and August, 1977. In decreasing order of importance, prey species were: cottontails (Sylvilagus spp.), cotton rats (Sigmodon hispidus), and mice (Perognathus hispidus, Peromyscus spp., and Reithrodontomys spp.).
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Even though great horned owl (Bubo virginianus) food preferences are generally known throughout much of North America (1, 2, 3, 4), no definitive work has been published concerning the food habits of this species in Oklahoma. There have been several reports of predation by this owl on free-tailed bats, Tadarida brasiliensis (5, 6, 7) at locations within the state. The objectives of our study were to assess the types and quantities of prey eaten and the seasonal changes in preference or availability of food consumed by a pair of great horned owls and their young. A total of 169 regurgitated owl pellets, numerous pellet fragments, and some larger prey remains were analyzed and prey items were tabulated. We collected pellets during the first few days of each month between February and August 1977 from the loft of an abandoned barn 3.2 km north and 1 km east of Duke, Jackson County, Oklahoma, where, in the spring of 1977, a pair of owls roosted and reared three young birds atop a bale of hay.
The loft was cleared of all pellets on 5 February 1977 so that pellets deposited thereafter would be of known age. Pellets were placed in a plastic bag with a collection data tag. In the laboratory, pellets were teased apart and their component skulls, bones, hair, and debris were sorted. With the aid of a mammal skull key (8) and a reference skull collection in the Cameron University Museum of Zoology, skulls were identified.
For purposes of this study, winter foods were from pellets deposited before or during the period 5 February through March, spring foods from April through May, and summer foods from June through July. Average weights for prey species were calculated from the numbers of study specimens in the Cameron Museum as parenthetically indicated in Tables 1 to 4. These values were then multiplied by the number of individuals of each species found in the pellets to obtain an estimated biomass of food consumed.
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The cotton rat (Sigmodon hispidus) was decidedly the most important winter food (Table 1). It comprised over 66% of the total biomass, occurred in 56% of the 115 pellets, and of 153 individual prey animals identified, 42% (65) were of this species. It probably was the most abundant and easily obtainable prey in winter. Peromyscus was second in percent occurrence but only fourth in percent of biomass. Both Peromyscus maniculatus and P. leucopus were undoubtedly taken by the owls, but their skulls were indistinguishable. Craighead and Craighead (3) found that these were the major winter food of Michigan horned owls by percent occurrence in 1942 (49.4%) and 1948 (58.2%). Rattus ranked third in percent of biomass, but sixth in percent occurrence. By this latter measure, it ranked third in Michigan in 1942 and fifth in 1948 (3). Both Bent (1) and Fisher (2) mentioned that Bubo is an excellent ratter. In 1942 and 1948, voles (Microtis spp.) ranked second by percent occurrence in winter in Michigan (3). Perognathus hispidus and Reithrodontomys spp. (predominantly R. montanus and R. fulvescens) were taken often, but the former constituted over twice the percentage biomass of the latter, owing to its larger size. Numerically, the least shrew (Cryptotis parva) was an
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important midwinter food item, occurring in 15% of the pellets, but contributed less than 1% of total biomass. Fisher (2) listed Blarina brevicauda and Sorex spp. as shrews known to be eaten by Bubo. Although only five Rattus and two Sylvilagus were found during the period, these larger mammals constituted far more bulk than did the smaller prey species. Both Sylvilagus auduboni and S. floridanus occur in this part of Oklahoma. Errington (4) found that 68.5% of 4,838 winter pellets and stomachs contained lagomorph remains in the northcentral states and stated that rabbits constituted the staple winter fare of the great horned owl over most of its range. But Craighead and Craighead (3) found rabbit remains in only 20% of 539 winter pellets from Michigan, and stated that during the winters of 1942 and 1947-49, rabbits were not a staple there. This was probably attributable to the fact that rabbit populations were low then.
Sylvilagus was the most important food item in both percent occurrence and percent of biomass during spring, and Rattus was second (Table 2). Sigmodon ranked third in percent of biomass.
Shrew remains appeared in 9% of the spring pellets. Only two prey species occurred in May, Sylvilagus and Cryptotis. These findings agree with those of Bent (1) and Fisher (2), who indicated that rabbits were the primary prey of great horned owls throughout the year over much of the U.S.
Because of its large size, Sylvilagus ranked highest in percent of biomass in summer but only third in percent occurrence (Table 3). Conversely, the much smaller Perognathus, although first in percent occurrence, was third in percent biomass. The only Neotoma found in this study appeared in June.
By comparison, during the spring and summer of 1942 in Michigan, rabbits constituted but 8.1% of 99 food items and only 3.1% of 161 items in 1948 (3). Small to medium-sized birds made up 29.3% of foods in 1942 and 11.8% in 1948. Interestingly, the percent occurrence of pheasants in 1942 was 20.2% and 13% in 1948. But voles, contributing 21.2% in the former and 49.1% in the latter year, seem to have been a year-round staple. In their study, Peromyscus made up only 6.1% and 0.6% of spring and summer foods in 1942 and 1948, respectively. Comparable figures for rats during these two years were 5% and 6.2%.
Table 4 summarizes our findings in all 169 pellets. From these data, it is apparent that Sigmodon hispidus ranked highest in over-all percent occurrence and a close second to Sylvilagus in total percent of biomass, this despite the fact that Sylvilagus is approximately seven times larger than Sigmodon. According to numbers of individuals recovered by season, Sigmodon was extremely important to the owls in winter, whereas preference (or availability) shifted to Sylvilagus in spring when the young owls were growing rapidly. Mice constituted a stable supply of food at all seasons. Other species were probably taken opportunistically.
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1. A. C. BENT, Life Histories of North American Birds of Prey,Part 2, Bull. U.S. Natl. Mus. 170, Washington, D.C., 1938, pp. 306-319.
2. A. K. FISHER, The Hawks and Owls of the United States in Their Relation to Agriculture, U.S. Govt. Print. Off., Washington, D.C., 1893, pp. 174-182.
3. J. J. CRAIGHEAD and F. C. CRAIGHEAD, Hawks, Owls and Wildlife, Stackpole Co., Harrisburg, Pennsylvania, and The Wildlife Management Institute, Washington, D. C., 1956.
4. P. L. ERRINGTON, F. HAMERSTROM, and F. N. HAMERSTROM, JR., Iowa Agric. Exp. Sta. Res. Bull. 277: 757-850 (1940).
5. A. E. PERRY and G. ROGERS, Southwest. Nat. 9: 205 (1964).
6. R. J. TAYLOR, J. Mammal. 45: 300-301 (1964).
7. R. K. CHESSER and M. L. KENNEDY, Bull. Oklahoma Ornith. Soc. 9: 1-3 (1976).
8. B. P. GLASS, A Key to the Skulls of North American Mammals, Stillwater, Oklahoma, 1960.
