The size of the home range seems to depend largely on the type of habitat and the available food supply. In the West and Midwest, where horned owl prey is more abundant, ranges of horned owls are often quite small, a square mile or less being sufficient in many cases. But in the East, where food is not so plentiful, the owls must hunt over a larger area to satisfy their needs. Undoubtedly the home ranges of some eastern great horned owls are considerably larger than is generally supposed, probably as much as two or three square miles.
Pairs of great horned owls maintaining large home ranges usually confine most of their activities to a fairly limited portion of that range for one or two seasons at a time. Then they move on to other parts for comparable periods until, finally, they return to the initial tract after a lapse of several years. Accordingly, they may appear to be impermanent residents in an area. From their absences, a casual observer might well conclude that the original pair of owls had deserted the area altogether, to be later replaced by a second pair. Actually, the same birds may have been present all the time in some distant part of their large range.
The size of the habitat ranged over by a pair of great horned owls is governed also by topography, distribution and types of plant life, and other environmental conditions. Normally the home range of a pair of owls will include a rather large woodland area together with adjacent fields and smaller wood lots. There the birds will carry on their life functions until death severs the pair-bond which unites them. In this event the survivor usually occupies the same territory until a new mate happens along. Eventually both original birds may be replaced in this way, but generally there is at least one bird associated with the home range, holding it until a new mate is secured and then continuing to occupy it.
Defense of the home range is subject to considerable variation: it is affected not only by the same factors which govern the size and extent of the range itself but also by the seasonal changes in the temperaments of the birds themselves. As a general rule, a pair of horned owls will keep their range clear of individuals of their kind; however, they frequently permit other birds of prey to establish ranges within their territory's invisible boundaries. Such tolerance is often greatly heightened in areas where prey is abundant and where most hawk and owl ranges are correspondingly small. A great abundance of prey-mice or rats or rabbits, for example-usually results in the overlapping of ranges and territories of many owls and hawks, even of the same species. Under such conditions, each pair may actively defend only a very small portion of the over-all range.
Within the home range, great horned owls have favorite nesting and roosting areas which they use with great regularity from year to year. Some birds adhere so rigorously to a seasonal roosting routine that it is sometimes possible to predict even the exact trees in which the they may be found at certain times of the year. This is particularly true when dense cover is scarce, or during the fall and winter, when deciduous trees are bare. These factors reduce the number of places in which a bird as large as a great horned owl can find suitable cover. At such coniferous needle-bearing trees are favored over all other locations. In the absence of pines, hemlocks, spruces, or other conifers, the owls must find seclusion elsewhere. This may be in certain deciduous trees, such as oaks and beeches, some of which tend to hold clusters of dried leaves throughout the winter; in vine tangles or under broken limbs hanging against the side of a tree; or they may simply perch on a small horizontal branch on the trunk of a sizable tree. Great horned owls may seek shelter in Just about any place that offers them concealment or at least tends to make them as inconspicuous as possible in their surroundings.
In northeastern Massachusetts, pines, spruces, and hemlocks are fairly common in most woodlands, even stands of predominantly deciduous trees are usually interspersed with some conifers. With such a number of preferred roosting sites available, it is interesting to note some of the peculiar hiding or roosting places which seem to strike the fancy of many eastern great horned owls. Several specific examples from our own experience illustrate this: a small group of white pines in a red maple swamp, set apart from an extensive hillside stand of pines; the largest white pine in a white-pine grove; the smallest white pine in a similar grove; a lone hemlock tree in a white pine stand; an isolated white pine in a red-pine plantation; a single white pine surrounded by pitch pines; and a solitary pitch pine in a grove of white pines and type hemlocks.
In each of the foregoing examples, the roost tree itself had certain broad characteristics that tended to distinguish it from the surrounding growth. Not all great horned owls are quite so fastidious in their selection, but there is a strong tendency for them to be attracted to roost trees which are more or less segregated, either by size, type, or location. It almost seems as though the birds do so in an effort to make it easier for them to identify their individual roosting trees.
The great horned owl and the red-tailed hawk are usually considered complementary species. Each hunts similar prey over the type of habitat, one by night, the other by day. Each is essentially a woodland as well as an open-country bird. Physically each has long broad wings and a short tall. Great horned owls and red-tailed hawks of the eastern United States usually prefer substantial tracts of heavy timber for nesting and seclusion, but in the Far West more open country is commonly used by both species. Both are generally birds of dry upland country, whether it is the hilly and mountainous regions in the East, the flat well-drained farmlands of the Midwest, or the deserts of the Southwest.
A similar complementary relationship seems to exist between the red-shouldered hawk and the barred owl. Both are primarily woodland species, with shorter wings and longer tails in relation to their body length than open-country hawks and owls; they often live quite harmoniously together in the same woods and normally occupy swampy woodlands, river bottoms, and wet, wooded lowlands throughout the country.
In contrast is northeastern Massachusetts, where the topography of the land is generally low and swampy, but the nesting population of large predatory birds consists predominantly of the red-shouldered hawk and the great horned owl. In this region, however, the association of these two species is far from a close one. The owls tend to occupy tracts of mature white pine trees, and they range only slightly into the surrounding deciduous woods. The hawks, on the other hand, usually occupy extensive deciduous swamps as far removed from any great horned owl range as possible. As a result, the paths of the two birds seldom cross; but should a red-shoulder return in the spring to find its nest of the previous season taken over by horned owls, it often refuses to nest under such disturbing circumstances, being unable to adjust to the owl's sudden invasion of its former domain.
