Resource Partitioning in Breeding Populations of Marsh Hawks and Short-Eared Owls

During the 1979 breeding season four pairs of northern harriers, or marsh hawks (Circus cyaneus) and four pairs of short-eared owls (Asio flammeus) were studied in Cache Valley, Utah. The study was concerned solely with diurnal resource utilization, and did not examine the owls\u27 nocturnal activities. The home range of each harrier pair overlapped substantially with that of an owl pair. Percent habitat overlap for hawk-owl pairs varied from 39 percent to 72 percent. Observations were made to determine if differences existed in their utilization of habitat and food resources, or in their daily and seasonal activity patterns. Both species utilized mainly wet old field and pasture habitat types for their hunting efforts. In general wet old fields were utilized more than expected based on their availability, while pasture, bare ground, and harvested field habitats were used less than expected. Pairs of hawks and owls sharing common habitats generally showed differences in preferred hunting habitats. An analysis of variance showed that hawks and owls were making strikes in different habitat types and to some extent in different parts of the habitat. Harriers and owls nested in different habitat types. Breeding seasons of the two species overlapped almost totally, but interspecific differences were detected in time-activity budgets. Overall, the owls were more sedentary than the hawks. Both species spent approximately 10 percent of the day in hunting-related activites, but timing of hunting varied from pair to pair. Overlapping pairs generally differed in their daily distribution of hunting time. The analysis of variance showed that there was a significant difference in the timing of strikes made by harriers and owls. Both species were feeding primarily on small mammals in the study area, and food resources were probably not a limiting factor for either population. Though northern harriers and short-eared owls appear to have a high degree of niche overlap, this study showed that where eight individual pairs of the two species came into contact they differed in time-activity budgets and habitat utilization. Coexistence between these two species may be enhanced by the fact that they both feed on an abundant prey resource. By subtle habitat and time budget preferences, reinforced through interspecific aggression, they can avoid competition