Colleen E. Moulton
Ecology of burrowing owls in southwestern Idaho: Association with
agriculture, food habits, and territorial behavior
Western
burrowing owls (Athene cunicularia hypugaea) are the only species of
raptor that show an 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.
