A Comparison of Great Horned Owls and Red-tailed Hawks

Matt Kaplan
AVS 15L Tuesday
Alida Morzenti
10/24/95

Many journal entries concerning the great horned owl (Bubo virginianus) and it's relationship to red-tailed hawk state, "The red-tailed hawk (Buteo jamaicensis) and the great horned owl are the diurnal/nocturnal equivalents of one another" (McInvaille and Keith, 1978), (Mikkola, 1983), (Johnsguard, 1988). But there has yet to be an author to explain why this is. Instead three studies are repeatedly cited to defend the great horned owl/red- tailed hawk equivalency statement. Yet these three studies, discussing nest sites, dietary similarities, and perch hunting techniques have never been examined as a whole. What is even more amazing is the fact that these are the only three studies in the past forty years that test the above statement. The only other documents are merely random sightings of interesting activity between the two species or analysis of one of these three studies. While three investigations may individually support the concept of great horned owls and red-tailed hawks being reflections of one another, there is no documentation that shows that they hold together as a unified whole. Are the red- tailed hawk's and great horned owl's nest cites, diets, and hunting techniques alike enough to call them diurnal/nocturnal equivalents of each other? Through in depth exploration of the three studies and the subjects they raise, this paper explores that question.

Great horned owls are found almost everywhere in North America, and in about 85% of that area red-tailed hawks are found as well (Shupe 1982). With the two species sharing so much of the same territory, it is not surprising that they would end up nesting close to one another. In a study carried out in Saskatchewan, Canada by C.S. Houston (1975), however the nesting of the two species was found to be much closer than what was previously thought possible between two raptors. This study examined the close proximity of red-tailed hawk and great horned owl nests and found that the two species preferred the same 40 acre clump of aspen, nested almost exclusively in aspens, and often reused each other's nests. Six nest sites, in particular, were carefully watched. In each site, the distance between the red-tailed hawk and great horned owl nest's was only 35 to 72 yards. In at least four, and perhaps all six sites, the red- tailed hawk built its new nest or moved into an old one after the great horned owl had already begun incubating. During the five years that these six sites were observed, only once were the remains of a red-tailed hawk found in a great horned owl nest. There appears to be only one reason for the two birds to nest so closely together if the great horned owls are not predatorialy towards the red-tailed hawks, and that is that they both are instinctively searching out the same type of nest sites. This in turn defends the equivalence concept discussed earlier.

The diets of the great horned owl and red-tailed hawk would be expected to overlap, considering the massive amount of territory they share. But in a three year analysis of the diets of the great horned owl and red-tailed hawk done in Green County, Wisconsin by Orians and Kuhlman (1956) the diets were found to be nearly identical. The diet of the red-tailed hawks consisted of 46 adult ring-necked pheasants, 26 cottontails, 20 chickens, 9 American crows, 8 mice, 6 ground squirrels, 4 redwings 2 domestic pigs, 2 domestic turkeys, 1 cowbird, and a partridge (but it wasn't in a pear tree). The diet of the great horned owl consisted of 43 cottontails, 30 ring-necked pheasants, 5 American crows, 3 rock doves, 1 chicken, and a starling. The fact that the two raptors have the same two species as the main constituent of their diets is certainly a correlation that cannot be overlooked. There was one problem with the study, Orians and Kuhlman examined great horned owl castings to figure out what the owls had been eating and searched through red-tailed hawk nests for food remains to discover what the hawks had been eating. Neither of these processes would yield everything that the birds had eaten. What might have been egested by a great horned owl might be dissolved in the low Ph of a red-tailed hawk's stomach. Nonetheless, the thoroughness of these nest and casting examinations as they occurred month after month for three years, certainly presents a pretty accurate picture of what the birds had been eating.

In a study reported by S. Shupe (1982) in Nebraska, the nesting and hunting techniques of the red-tailed hawk and great horned owl were analyzed from Winter, 1980 through Fall, 1981. During this period, three birds of each species were monitored and records were kept of their activities. During the almost year long period, the red-tailed hawks spent an average 68% of their time on vantage points (telephone poles, barren trees, power line stations) hunting, while the great horned owls spent 75% of their time on vantage points hunting. Both were almost always perch hunting. Less than 7% of the time of the birds studied was spent on the wing. The fact that both raptors perch hunt and spent like amounts of time performing their various activities definitely backs the concept of the two species being reflections of one another. The only weakness in the research done was the fact that no time of day or night was given for when observations were made. Either Shupe and his colleagues followed the birds for 24 hours a day for a full year, or they watched several times a week at completely random times, or they had a schedule and watched the birds during their most active periods each day.

While the three studies individually support the concept of the two species being diurnal/nocturnal facsimiles of one another, there is an incongruity between Houston's nesting observations and Orians and Kuhlman's dietary observations. Orians and Kuhlman's dietary analysis showed that the great horned owl and red-tailed hawk primarily ate the same animals. It seems very odd that the two would nest so close to one another when they are competitors for the same food, especially when there were so many acres of the same terrain for them to nest in. Another incongruity is highlighted by a study performed by Klem et al. (1985) intended to help explain why raptors kill other raptors. This study resulted in statistics that are in direct contrast to Houston, Orians, and Kuhlman's findings. Klem and his colleagues found that 44% of young red-tailed hawks, 30% of young spotted owls (Strix occidentalis), and 65% of young great gray owls (S. nebulosa) are slain each year as the result of great horned owl predation. They cite an author who wrote in the Wisconsin Technical Bulletin No. 111. However, the combined efforts of Katharine (librarian extrordinare), Melvyl, and me could not find this document on any U.C. campus (the computer is still doing an inter-library, search but I have been told that I shouldn't get my hopes up). Nonetheless, the great horned owl's activity of hunting down young raptors is a behavior that is never mentioned in the principal three studies that have been cited by so many authors. While Shupe said that both species have a tendency to go for easy, unsuspecting prey, Klem et al. shows that the great horned owl, unlike the red tailed hawk consumes other raptor's chicks almost exclusively.

In light of all of the conflicting data, it appears that the phrase, "the red-tailed hawk and the great horned owl are the diurnal/nocturnal equivalents of one another" is one that can be loosely used. It is clear that the two raptors have many similarities, but it is definitely apparent that characteristics vary with their habitat. Behavioral traits in animals are heavily affected by the environment they live in. After all, one of the five definitions of life is, "that an animal must be capable of reacting to it's environment." I searched for and am still searching for, studies that look into the varying behavioral traits of great horned owls and red-tailed hawks in different parts of the continental United States, but there seems to be little or no information about. I am confident, though, that in the future these horizons will be explored and a greater knowledge of why these birds act the way they do will be present in the scientific community.

Houston, C.S. 1975. Close proximity of red-tailed hawk and great horned owl nests Auk 92:612-614
Johnsguard, P.A. 1988. North American Owls: Biology and Natural History. Smithsonian Institute Press. Washington D.C. 128- 135
Klem, D., B.S. Hillegass and D.A. Peters. 1985. Raptors killing raptors. Wilson Bull. 97:230-231
McInvaille, W.B. and L.B. Keith 1978. Predator-prey reltions and breeding biology of the Great Horned Owl and Red-tailed Hawk in central Alberta. Can. Field-Nat. 88: 1-20
Mikkola, H. 1983. Owls of Europe. Buteo Books, Vermillion, SD. 74-92
Orians. G., and F. Kuhlman. 1956. Red-tailed Hawk and Horned Owl populations in Wisconsin. Condor 58:371-385
Shupe, S. 1982. Hunting behavior in the Red-tailed Hawk and Great Horned Owl. Nebr. Bird Rev. 54:84-86

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