Moulton, Colleen E. – Ecology of Burrowing Owls in southwestern Idaho: Association with agriculture, food habits, and territorial behavior. 2003.
Western burrowing owls (Athene cunicularia hypugaea) are the only species of raptor that show an affinity for agricultural areas in southwestern Idaho. The underlying mechanisms leading to this association have not been previously examined. To determine potential factors underlying the association of burrowing owls with agriculture, I examined the availability of suitable nest burrows (burrow availability hypothesis), abundance of potential prey (prey availability hypothesis), and predation of nest burrows (predation hypothesis) during the 2001 and 2002 breeding seasons. Nest burrow availability did not differ between agricultural and non-agricultural habitats, and occupancy rates of owls in artificial burrows were greater near agriculture. More rodent prey species were live-trapped in agricultural habitat compared to non-agricultural habitat, but there was no difference in relative abundance of prey between habitat types. Pellet remains indicated a greater abundance and biomass of prey being consumed in agricultural habitat compared to non-agricultural habitat. Finally, predation rates of artificial nests did not differ between habitats. These findings allow me to reject the burrow availability and predation hypotheses for the association between owls and irrigated agriculture in southwestern Idaho, while the prey availability hypothesis remains tenable. Thus, burrowing owls may nest near irrigated agriculture because of increased diversity or availability of prey.
To investigate the diet composition of burrowing owls nesting in the Snake River Birds of Prey National Conservation Area of southwestern Idaho, I collected and analyzed regurgitated pellets and documented prey remains. I used this information to accomplish two objectives: 1) describe the breeding season food habits of Burrowing owls in the study area, and 2) compare food habits of owls nesting in agricultural and non-agricultural habitats. Although Burrowing owls consumed a greater percentage of arthropod prey items than vertebrate prey items, vertebrate species comprised a greater percent biomass. Coleopteran, Orthopteran, and Arachnid species were common invertebrates found in regurgitated pellets, but they were not well represented in prey remains. Microtus montanus, Perognathus parvus, and Peromyscus maniculatus were common vertebrate species in both pellets and prey remains. Microtus montanus, which were not present in pellets in non-agricultural areas, represented the greatest percent biomass of pellets in agricultural areas. Perognathus parvus, which occurred in pellets in both habitats, represented the greatest percent biomass in non-agricultural areas. Pellets of owls nesting in agricultural areas had greater species richness, but pellets from nests in non-agricultural areas had greater species evenness and broader food-niche breadths. These results suggest that diets of Burrowing owls within the NCA can vary by habitat, and such variation may reflect differences in prey availability between agricultural and non-agricultural habitats.
Finally, to investigate the potential expression of territorial behavior of burrowing owls, I conducted playback experiments on owls nesting in southwestern Idaho during the 2001 and 2002 breeding seasons. My objectives were to 1) determine if Burrowing owls actively defend their nesting site from conspecifics and 2) if so, determine the extent of their territorial boundaries. Eighty-eight percent of male burrowing owls (n = 42) responded to the broadcasting of conspecific primary calls. All responsive males uttered primary calls, while other common behaviors were approaching the broadcast speaker, white-and-tall stances, and bobbing. Females responded less frequently than males, but one female whose mate was missing, and presumably dead, exhibited an intense response to the playback trial. There were no differences in 1) number of primary calls uttered, 2) number of white-and-tall stances performed, or 3) number of bobs of focal males between three broadcast distances: 0 m, 50 m, and 100 m from the active nest burrows. However, focal owls approached the broadcast speaker more closely at broadcast distances of 0 m and 50 m than at 100 m. These findings suggest that 1) owls actively defend their nesting site from conspecifics, 2) they defend an area larger than that immediately surrounding the nest burrow, and 3) although they will continue to vocalize at distances of at least 100 m, they may not physically approach an intruder at this distance as frequently as shorter distances. Therefore, burrowing owls appear to defend a territory that encompasses some, but not all, of the foraging area used during nesting.