Western Burrowing Owls

Burrowing Owls

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 insect prey.

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 foxes.

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 Mexico.

In 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

INL Development Zone

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

References

Fahler, 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.

Haug, 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.

Idaho Department of Fish and Game. 2005. Idaho Comprehensive Wildlife Conservation
Strategy. Idaho Conservation Data Center, Idaho Department of Fish and Game, Boise, ID. http://fishandgame.idaho.gov/cms/tech/CDC/cwcs.cfm

Rich, T. 1986. Habitat and nest-site selection by Burrowing Owls in the sagebrush steppe of Idaho. Journal of Wildlife Management 50:548-555.