The western burrowing
owl (Athene cunicularia) is one of the smallest species of
owls, measuring about 9-11 inches tall and weighing about four
ounces. It has no ear tufts, yellow eyes, a short tail and
distinctively long legs. The stilt-like legs are very long for an
owl, but are well designed for running around on the ground after
Unlike most owls,
burrowing owls nest in and inhabit the abandoned
underground burrows of other animals, such as the ground squirrel or
badger. The problem with being a ground-dwelling bird is the
increased threat of predation. While most birds avoid predation by
nesting in trees, the burrowing owl has to deal with numerous
threats. Nesting in areas of short vegetation helps the burrowing
owl to see possible ground predators that are approaching the nest.
Keeping the nest away from trees and tall structures helps the
burrowing owl avoid being prey to a larger bird that could be
perching outside the nest. Some of the main predators of the
burrowing owl are larger raptors, weasels, badgers, snakes, coyotes, bobcats, and
The burrowing owl is
known for its habit of bobbing up and down to ward off intruders.
When threatened, the owl fluffs its feathers to appear larger, drops
and rotates its wings forward and begins to bob up and down in front
of the intruder. Both the male and female will chase and strike at
intruders that approach the burrow site. To deter approaching
predators, young burrowing owls can utter a call that mimics the
rattling sound of a rattlesnake within the burrow, causing wary
predators to keep their distance from its burrow.
While most owl species
are nocturnal feeders, the burrowing owl is mainly crepuscular,
feeding in the early morning and late evening hours, They will hunt
throughout the day and night, however, especially when they have
young to feed. Burrowing owls are opportunistic feeders, mostly
eating small mammals such as moles and mice during late spring and
early summer. Later they switch to insects, especially grasshoppers
and beetles. They also prey on birds, amphibians and reptiles.
In the winter, burrowing
owls migrate to California, Arizona, New Mexico, Texas, and parts of
the U.S., the burrowing owl is identified as a "candidate" species
by the U.S. Fish and Wildlife Service.
In Idaho, the
burrowing owl’s conservation status is considered to be imperiled and it is on the BLM’s Watch List as a species that may
warrant designation as a species of special concern (Idaho
Department of Fish and Game 2005).
Burrowing Owls on the INL
The burrowing owl is a
breeding resident on the INL and has been observed on site from as
early as mid-March (Fahler 1998) through as late as August (Gleason
1978). Although burrowing owls hunt for small mammals at night, they
are typically diurnal and crepuscular during the breeding season (Haug
et al. 1993). Eggs are laid and incubated during May. Chicks emerge
from their burrows during the first week of June and begin to fledge
in July (Gleason 1978, Fahler 1998).
Due to the burrowing
owl’s fossorial nature, it is closely associated with burrowing
mammals (Haug et all, 1993). Previous studies on the INL found that
burrowing owls will typically utilized badger digs as nesting sites
(Gleason 1978, Fahler 1998). However, Gleason (1978) located nests
in lava outcrops as well. Burrowing owls avoid dense stands of
sagebrush and typically select open sites within 100 meters of
sagebrush for nesting (Rich 1986). The majority of owls and nest
sites located within the INL have been in grazed and ungrazed
crested wheatgrass and native grass areas (Hansen 1994, Fahler
1998). Additionally, Fahler (1998) found burrowing owl nests in
roadside ditches, burned areas, and near INL facilities.
2006 Burrowing Owl Survey Results
Surveys to determine the
distribution, abundance, and nesting locations of burrowing owls
within the development zone of the INL were conducted during the
first two weeks of May 2006. Call-broadcast surveys were used to
locate nesting burrows by soliciting a response (visual or aural)
from burrowing owls with taped recordings of their primary call. Ten
survey routes along T-roads were selected to provide adequate
distribution across all habitat types throughout the proposed
development zone. Survey points were located 0.8-1.0 km apart along
survey routes depending on visibility and topography. Surveys were
conducted on calm days for approximately four hours following sunrise
and two hours prior to sunset. Call-broadcast surveys were 6-minutes
in length and consisted of a 3-minute observation segment followed
by a 3-minute call-broadcast segment. The observation segment
entailed listening for burrowing owl calls and scanning the
landscape with binoculars. The 3-minute call-broadcast segment
consisted of 30 sec of calls then 30 sec of silence, with this
pattern repeated 3 times. The survey area was observed during and
after the call-broadcast for owl activity and behavioral responses.
This procedure was repeated at each of the survey points.
Burrowing owls were
observed at ten different locations and in all habitat types. Nine
were located within the development zone, and one was located south
of the development zone. Five detections were the result of
call-broadcast survey efforts, and five were from incidental
observations. Occupied sites were revisited two to five times during
May through June to determine the fate of each nest. In three
instances, a single burrowing owl was detected at a burrow once but
never observed at that location again. Another burrow was occupied
by a single adult with no sign of a second adult or chicks. The
remaining known active sites were occupied by confirmed pairs. Of
these, one nest was abandoned; one nest produced one confirmed chick;
one nest produced four confirmed chicks; one nest produced five confirmed
chicks; and the outcome of two nest sites is unknown.
Call-broadcast survey efforts for the 2007 breeding season will
begin during the last half of April and extend through mid-May.
Additional survey routes will be added including surveys along fire
breaks via ATV (funding and access permitting). More resources will
be allocated to an increased effort in determining habitat use and
requirements, diets (through pellet analysis), and nesting success.
- Kristy Howe and Alana Jensen
N.A. 1998. Owls of the Idaho National Engineering and Environmental
Laboratory. M.S. Thesis, South Dakota State University. 97p.
Gleason, R. S. 1978. Aspects of the breeding biology of burrowing
owls in southeastern Idaho. M.S. Thesis, University of Idaho,
Moscow, Idaho, USA.
Hansen, R. W. 1994. Raptor use of the Idaho National Engineering
Laboratory. M.S. Thesis. South Dakota State University, Brookings,
South Dakota, USA.
E .A., B. A. Millsap, and M. S. Martell 1993. Burrowing owl (Speotyto
cunicularia). In The Birds of North America, No. 61. A. Poole
and F. Gill, eds. Philidephia: The Academy of Natural Sciences;
Washington D.C.: The American Ornithologist’s Union.
Department of Fish and Game. 2005. Idaho Comprehensive Wildlife
Strategy. Idaho Conservation Data Center, Idaho Department of Fish
and Game, Boise, ID.
T. 1986. Habitat and nest-site selection by Burrowing Owls in the
sagebrush steppe of Idaho. Journal of Wildlife Management