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Seasonal variation in the diet of great horned owls (Bubo virginianus) on shortgrass prairie.

INTRODUCTION

Diet studies are important to conservation and management programs for raptors because prey abundance may limit populations where nest sites and other requisites are available (Village, 1983; Newton, 1991). Furthermore, knowledge gained from studies of raptor diets may be used to detect shifts in prey populations (e.g., Korschgen and Stuart, 1972), and this information may be useful for studies of human impacts on wildlife populations (Steenhof, 1983). Raptors such as owls have large home ranges that encompass many vegetation types, and this heterogeneity may complicate investigations of prey selection (Reynolds et al., 1992). Food habits also vary over time as a result of seasonal changes in the owls' energy demands and with prey abundance and availability.

Despite these complications, it is possible to determine owl diets because the birds regurgitate pellets containing the less digestible parts of their prey. Identification of prey remains from bones, hair, scales and chitin therefore provide qualitative and quantitative information on food habits. Although these estimates may be biased by the size and body composition of prey and the level of taxonomic identification employed, pellet analysis is generally considered to be the most reliable technique for determining owl diets (Marti, 1987).

The great horned owl (Bubo virginianus) is a common year-round resident and the dominant strigiform on shortgrass prairie (Marti, 1969; Olendorff, 1972). Leslie (1992) reported higher numbers of great horned owls in northern Colorado than were found in the early 1970s, and suggested that the increase in owl numbers may affect other breeding raptors. Great horned owls respond numerically to increasing prey abundance (Rusch et al., 1972; McInvaille and Keith, 1974), but there is no recent information on the diet of owls inhabiting shortgrass prairie or the distribution and abundance of prey. We investigated seasonal trends in owl's diet by examining pellets from roosts on shortgrass prairie in N-central Colorado. We compared this information to the relative abundance of small mammals on the study site to assess whether diet composition could be used to determine where owls forage.

STUDY AREA

Our study was conducted on the Central Plains Experimental Range, located approximately 60 km NE of Fort Collins, Colorado. The site is administered by the United States Department of Agriculture Agricultural Research Service and is the location of the Shortgrass Steppe Long-Term Ecological Research project. The site is grazed by cattle and divided into fenced 130-ha pastures, which are bordered by approximately 20 km of unpaved roads. The topography consists of rolling hills separated by broad swales. The vegetation on upland prairie is low in stature and dominated by perennial warm-season shortgrasses such as blue grama (Bouteloua gracilis) and buffalograss (Buchloe dactyloides). The broad basin of Owl and Cow creeks comprises the eastern portion of the study area and contains abundant cover of four-wing saltbush (Atriplex canescens) and western wheatgrass (Pascopyron smithii). Saltbush cover is particularly dense on the loamy soils of the floodplain adjacent to Owl Creek. In the shrub-grassland area where soils are coarsely textured, the saltbush is more widely spaced and there are numerous small shrubs (Artemisia frigida, Eriogonum effusum, Gutierrezia sarothrae, Chrysothamnous nauseous, Ceratoides lanata) and perennial bunch-grasses (Stipa comata, Oryzopsis hymenoides, Aristida longiseta, Sitanion hystrix). Taller grasses and herbaceous weedy annuals grow along roadsides and around the few buildings and stock corrals at the site headquarters. There is a small stand of cottonwoods (Populus sargentii) along Owl Creek at the northern boundary of the site, and a few isolated trees occur along the creek bed and near buildings.

A variety of small mammals occurs on our study site. Black-tailed jackrabbits (Lepus californicus) are considerably more abundant than white-tailed jackrabbits (L. townsendii) and are most numerous in saltbush areas, whereas desert cottontails (Sylvilagus audubonii) occur throughout the study area and are common in overgrown areas near buildings and corrals (Donoho, 1971; Flinders and Hansen, 1973). Thirteen-lined ground squirrels (Spermophilus tridecemlineatus), northern grasshopper mice (Onychomys leucogaster), deer mice (Peromyscus maniculatus) and Ord's kangaroo rats (Dipodomys ordii) are the most abundant rodents (Flake, 1971), although only grasshopper mice, ground squirrels, and northern pocket gophers (Thomomys talpoides) are common on upland shortgrass areas. Western harvest mice (Reithrodontomys megalotis), prairie voles (Microtus ochrogaster), house mice (Mus musculus), plains harvest mice (R. montanus), and three species of pocket mouse (hispid pocket mouse Chaetodipus hispidus, silky pocket mouse Perognathus flavus, and plains pocket mouse P. fasciatus) have also been captured on the site.

METHODS

On the Central Plains Experimental Range, great horned owls roost in trees along riparian areas as well as in isolated trees and buildings (Marti, 1969; Olendorff, 1972; Leslie, 1992). We collected intact pellets and pellet fragments from roosts that were known to be used by great horned owls during 1992, 1993 and 1994. Other owl species were occasionally sighted, but were rare. We collected pellets periodically, and assigned the collection periods to four seasons: spring (March-May), summer (June-August), autumn (September-November) and winter (December-February). All pellets gathered from beneath a roost during a collection period were kept in a single plastic bag until analysis. We calculated the number of pellets from a given collection period as the number of intact pellets collected plus a rough estimate of the number of fragmented pellets (obtained by comparing the amount of material remaining in a collection bag with the volume of a typical intact pellet). Because we were primarily interested in seasonal changes in diet, we pooled samples of each season from all years and thus assumed no variation among years.

We carefully removed bone fragments from pellets and identified them by comparisons with museum collections at Colorado State University and the Denver Museum of Natural History. Great horned owls generally consume smaller prey items whole, but may make several meals from larger prey (Seidensticker, 1968; Marti, 1987; Weir and Hanson, 1989). To account for this potential bias, remains of small rodents and pocket gophers were enumerated for each pellet collected. Bones of lagomorphs were pooled together from a given collection period (i.e., all pellets beneath a roost) and enumerated. We counted the number of right and left mandibles present for each species, and then assigned identifiable post-cranial bones (tibia, fibula, femur, humerus and pelvic girdles) to the appropriate species using a key to postcranial bones (Olsen, 1973). The number of individuals in a given pellet was usually estimated as the number of bones from a side. Postcranial bones were used to enumerate individuals only when identifiable bones outnumbered the mandibles. One of us (GZ) performed all pellet analyses to avoid potential observer bias.

Prey remains were divided into three categories (small rodents, lagomorphs and pocket gophers). The small rodent category was subdivided into its component species to compare owl diets with the distribution and abundance of prey on the site. Results were expressed as both the percentage of prey biomass in the diet and as the percentage of prey individuals consumed. We estimated prey masses for biomass calculations using either average values or range midpoints from Armstrong (1972) and Jones et al. (1983). We also calculated the modified Hill's ratio (E5; Ludwig and Reynolds, 1988) to describe seasonal differences in the evenness of the distribution of small-rodent taxa in owl diets. E5 is the ratio of the number of very abundant to abundant taxa in the diet and approaches one for even diets and zero as the diet becomes dominated by a single species.

We live-trapped rodents in saltbush-floodplain and shrub-grassland vegetation to estimate prey abundance. One trapping grid, encompassing 2.3-3.2 ha and consisting of 100-144 Sherman traps spaced at 15-m intervals, was established in each vegetation type. We trapped grids for 5 consecutive nights, once each during August 1992 and 1993 (summer), December 1992 and 1993 (winter) and May 1993 (spring). We counted the number of individuals of each species captured during trapping sessions as an index of abundance. These numbers were divided by the total number of rodents captured during a given season to calculate the proportional representation of each species on a site. We did not trap rodents in autumn and did not attempt to census lagomorphs or pocket gophers.

RESULTS

We collected 24 pellets in five collections during spring, 14 in three summer collections, 90 in eight autumn collections, and 35 in four winter collections, for a total of 163 pellets. Our analyses indicated that great horned owls took a wide array of prey types. All pellets contained mammalian remains, 33% contained insects, 9% crayfish, and 3% remains of birds. Seven genera of nocturnal rodents and both genera of lagomorphs were taken. We also found the remains of one thirteen-lined ground squirrel, a diurnal species, in one pellet.

Lagomorphs comprised the highest proportion ([greater than] 70%) of the diet biomass in all seasons [ILLUSTRATION FOR FIGURE 1A OMITTED]. Because our quantification technique was conservative for large prey such as rabbits, these results probably underestimate the actual number of lagomorphs consumed and our estimates of biomass. The greatest biomass of pocket gophers was consumed in winter and spring, and the greatest biomass of small rodents, in autumn and summer [ILLUSTRATION FOR FIGURE 1A OMITTED].

In terms of numbers of individual prey consumed, small rodents were the dominant taxa in all seasons [ILLUSTRATION FOR FIGURE 1B OMITTED]. The numbers of individuals taken among the three taxa differed among seasons ([[Chi].sup.2] = 10.83, df = 6, P = 0.09). To explore the source of these seasonal differences further, we compared the proportion of individual small rodents and pocket gophers in the diet by season, ignoring lagomorphs. The proportions of small rodents and gophers differed among seasons ([[Chi].sup.2] = 9.57, df = 3, P = 0.02); the consumption of gophers declined from spring to winter, whereas relatively few small rodents were taken in spring.

Deer mice were the most common small rodent taken by owls, and comprised from 2544% of individuals in the diet, depending on season (Table 1). Kangaroo rats were also frequent prey and were consumed most often in spring and winter. Small rodents in winter diets were mostly deer mice, kangaroo rats, harvest mice and house mice, and winter pellets contained the most even distribution of rodent taxa consumed of all seasons (E5 = 0.87). Deer mice and kangaroo rats dominated in summer and autumn diets (E5 = 0.77 for both seasons), whereas spring diets were intermediate in evenness (E5 = 0.81). Seasonal differences in the distribution of the six small-rodent taxa identified in the diet, however, were not statistically significant ([[Chi].sup.2] = 16.13, df = 15, P = 0.37).

Deer mice and harvest mice comprised more than 90% of the captures on the saltbush-floodplain grid and there was little seasonal variation in species composition and population size (Table 1). Rodents were relatively abundant in all seasons on this site, but were less numerous on shrub grassland, especially in winter (Table 1). Grasshopper mice, kangaroo rats and deer mice were the most commonly captured rodents on the shrub-grassland grid and showed little seasonal variation in proportional abundance on this site (Table 1). Hispid pocket mice were present on shrub grassland during spring and summer, but were not active aboveground during winter.

DISCUSSION

Great horned owls are typically considered generalist predators whose food habits reflect the relative abundance of prey (Baumgartner and Baumgartner, 1944; Rusch et al., 1972; [TABULAR DATA FOR TABLE 1 OMITTED] Schuster, 1974; Wink et al., 1987; Llinas-Gutierrez et al., 1991). The preponderance of rabbits in owl diets from our work is similar to results reported in other diet studies of great horned owls in North America (e.g., Errington et al.,1940; Seidensticker, 1968; Wink et al., 1987; Weir and Hanson, 1989), but we have no information on the seasonal abundance of lagomorphs for the years of our study. Roadside surveys conducted on the Central Plains Experimental Range in 1994 and 1995 suggested that lagomorph densities were lowest in winter and highest in summer and autumn (Lindquist et al., 1995). If this pattern is consistent among years, then owls may respond to the reduction in rabbit abundance by consuming more of the larger rodents, such as pocket gophers [ILLUSTRATION FOR FIGURE 1B OMITTED] and kangaroo rats (Table 1).

Deer mice were the primary small rodents consumed during most seasons and usually were the most common rodents on the areas we trapped (Table 1). Kangaroo rats were also frequent prey, but on our study site tend to occur only in shrub-grassland areas where soils are sandier (Lindquist et al., 1995). Neither species is abundant on upland prairie, the predominant vegetation type of shortgrass prairie. Conversely, grasshopper mice inhabit both open prairie and shrub-dominated areas, but are uncommon in dense vegetation, such as that found on our floodplain trapping site. Predation on grasshopper mice was infrequent relative to their abundance on the study area as a whole, which was somewhat surprising given the relatively large size of this species (40 g; Jones et al., 1983) compared to that of some of the other species taken, such as deer mice (22 g) and harvest mice (12 g).

The importance of deer mice and kangaroo rats in the diet of great horned owls has been noted in other studies on grasslands in Colorado (Marti, 1974) and Oklahoma (Schemnitz and Ables, 1962). Marti (1974) also reported that harvest mice and voles (Microtus sp.) were common prey. Voles were uncommon on our study area, but were taken regularly by owls (Table 1). Furthermore, although harvest mice were abundant on the floodplain trapping site, they tend to be restricted to areas with dense vegetation (Webster and Jones, 1982) and are considered an unusual species for shortgrass prairie (Abramsky, 1978). Compared to Marti's (1974) work conducted nearby, in our study owls consumed fewer rabbits and birds and more gophers, deer mice and harvest mice. Marti (1974) noted that deer mice and rabbits were abundant during his study, but no comparable estimates of prey numbers are available to evaluate whether changes in prey abundance contributed to the increase in owl numbers reported by Leslie (1992) on our study site.

The frequency of harvest mice, deer mice and voles in owl diets and the relative scarcity of grasshopper mice indicate that owls forage intensively in or near areas with dense vegetation. The high proportion of kangaroo rats in the diet implies that owls also hunt along roadside ditches, as kangaroo rats are common at night in the vegetation bordering gravel roads (Flake, 1971; Abramsky, 1978). Such areas may also support high numbers of rabbits (Flinders and Hansen, 1973). Fence posts and utility poles associated with roads probably serve as valuable hunting perches for sit-and-wait predators such as great horned owls (Marti, 1974). Additionally, the presence of house mice in owl diets (Table 1) indicates that owls also forage near buildings and stock corrals. House mice were rare in natural vegetation (only one individual was captured from the floodplain site), but likely are abundant in disturbed areas associated with human activities. The trees and vertical structures found in these areas provide numerous perching, roosting and nesting opportunities for owls.

Pellets contained remains of several small-rodent species that were uncommon during our trapping studies and relatively few remains of the most widespread species (grasshopper mice) on our study area. These observations suggest that the owls foraged in relatively restricted areas, including roadsides, buildings and corrals, and along the narrow floodplain that comprises a small proportion ([less than]10%) of the total vegetation of the site. These areas have denser vegetation than the surrounding grassland and likely support higher rodent densities (Abramsky, 1978) and perhaps, more lagomorphs. Vertical structures such as fence posts, utility poles and trees are often associated with these sites, and we speculate that the location and abundance of perches may determine owl foraging habitat in open vegetation such as that on shortgrass prairie and other grasslands. As was the case in our study, prey abundance for species such as owls is frequently determined by sampling mammal populations on sites that represent a broad area or vegetation type (e.g., Rusch et al., 1972; Marti, 1974). These areas may be important to foraging owls (especially vegetation types where prey numbers are consistently high), but researchers seeking to quantify overall prey abundance should not overlook the potential value of less common cover types that may be associated with other important habitat features.

Acknowledgments. - Special thanks to Jan Saysette for assisting with identification of postcranial bones and to Sheri Jones (Denver Museum of Natural History) and Bruce Wunder (Department of Biology, Colorado State University) for allowing us to use their specimen collections. We thank Pat Ward and Bob Schooley for technical and statistical assistance. The U.S. Dep. Agric. Agricultural Research Service permitted us to use the Central Plains Experimental Range site. Our work was supported in part by the NSF Shortgrass Steppe Long-Term Ecological Research project (BSR-9011659, Ingrid Burke and William Lauenroth, Principal Investigators). Constructive comments from John Wiens, Michael Morrison, Stanley Anderson, Jeff Kelly, Nancy McIntyre and Laura Stapp greatly improved the manuscript.

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Author:Zimmerman, Guthrie; Stapp, Paul; Horne, Beatrice van
Publication:The American Midland Naturalist
Date:Jul 1, 1996
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