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Burrowing Owls

Burrowing Owls 1Small, but cute as a button, burrowing owls are an extremely interesting species and as big a crowd favorite as any species in the Mid-Columbia.

 

Athene cunicularia

 

Burrowing owls are a ground-nesting owl, the only one that nests exclusively underground. The barn owl will sometimes nest in a burrow, but prefers elevated sites. The snowy owl and short-eared owl nest on the ground, but on the surface.

There are two subspecies of burrowing owls in the United StatesStates—Western Burrowing Owl (Athene cunicularia hypugaea) and Florida Burrowing Owl (Athene cunicularia floridana).

Burrowing owls have a maximum life span of 8.5 years.

The adults molt about the time the young fledge, and the molt is completed in August.

Badgers, weasels and skunks prey on owl eggs, and they may also prey on young chicks still in the burrow. Domestic cats, dogs, hawks, falcons and larger owls prey on the adults and young chicks. Badgers may occasionally take an adult in the burrow. Other mortality has been documented from starvation due to not enough prey availability.

 

Appearance

 

Coloration: Light brown, mottled with white spots. The feet white to beige. The eyes are bright yellow.

Body: Long-legged (relative to body size). Tarsi (feet) are feathered.

Wingspan: 20 - 24 inches.

Length: 9 - 11 inches. These are small owls, about the size of an American robin! Of course, the owl looks bigger due to the bulk of its feathers.

Tail: 2.8 - 3.4 inches.

Average Weight: Male – 5.7 ounces (0.35 pounds!); Female – 5.4 oz. (0.33 pounds!). Or 1/3 of a pound! Most hamburger patties weigh as much.)

Males and females are the same size, unlike most owl species. Males and females appear similar in plumage, although the male may become lighter in color due to sun bleaching from standing outside the burrow, while the female remains darker in color due to being inside the burrow when nesting.

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Behavior

 

Burrowing owls are most active at dawn and dusk, that is, they are crepuscular (most active during twilight periods). However, they are also often seen roosting at the burrow entrance during the day.

Burrowing owls eat invertebrates (in this area, lots of darkling beetles, grasshoppers and Mormon crickets) and small vertebrates, such as small rodents, horned larks and lizards.

Most vocalizations are given near the nest burrow. The who, who call of a burrowing owl is associated with territory defense and breeding and is often given by adult males to attract a female to a promising burrow. They also make other sounds, which are described as chucks, chattering and screams. These sounds are usually accompanied by an up and down bobbing of the head. When alarmed, young birds will give a hissing call that sounds like a rattlesnake.

Most western burrowing owls are migratory, wintering in California and occasionally as far south as Mexico. However, a few owls overwinter locally.

Locally banded birds have been sighted in the winter in the Willamette Valley of Oregon, around San Francisco Bay and in the San Diego area.

 

Habitat

 

Burrowing owls need flat, open terrain with soft soil, short grass and sparsely distributed vegetation or exposed ground.

Burrowing owls are found in association with burrowing mammals. Burrowing owls select burrows in areas with other burrows, often close to roads (although roads increase mortality due to collisions and may be an artifact of how data is collected), or in ravines, surrounded by bare ground or short grass and with high perches nearby.

At night, the owls may use up to a square mile (640 acres). However, their home range during daylight hours is generally confined to ~16 acres near the burrow, likely for safety.

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Burrowing Owls 3

 

Burrows

 

Contrary to their name, burrowing owls do not dig their own burrows. Instead they rely on those created by badgers, ground squirrels, marmots, coyotes and skunks. Without these other species, the owls wouldn't have nesting sites and wouldn't successfully reproduce!).

Successful nest burrows have fairly wide openings, are not readily visible from the surrounding landscape, are not visible from roads, have tunnel branches and have substantial grass cover around the entrance area—the less bare ground, the better.

Burrowing owls can also make use of artificial burrows. Frequently these consist of plastic barrels cut in half and buried with tunnels of PVC pipe leading to the 'nest.' However, many conditions must be met to make the new home successful. For example, the pipe usually need a 90 degree bend to block the sun and needs the bottom cut off—the owls need to feel the earth beneath their feet! The nest needs an accessible top opening for yearly cleaning, the site must be well drained, and the best burrow openings usually face north. It’s not easy to recreate nature.

Burrowing owls are loyal to their nesting sites—28-34% of females and 41-56% of males return each year to the same nesting burrow.

Burrows are often lined with cow or horse manure. There are several theories as to why. It may be to mask the owl's scent, it may be involved in attracting mates, or it may be a method to attract insect prey closer to the nest site. This last theory is supported by scientific evidence as there tends to be more prey around lined burrows. Or it may be a combination of reasons; no one knows for sure.

Nest burrow entrances are often adorned with highly visible objects—bones, grass clumps, coyote dung, etc.

Burrowing owls will make use of satellite burrows—nearby burrows used by the adults and young owls for protection from predators and weather and for caching (storing) food.

 

Reproduction

 

Burrowing owls form monogamous pair bonds during the nesting season. However, they will re-mate for each season. That is, they rarely have the same pair mates for multiple seasons.

Nesting season begins in late March to early April.

Burrowing owls lay one clutch of 6-11 eggs, with the usual number being 7-9. Eggs are round, smooth, white, about 1.25 inches long and make up 7% of the female’s body weight. The female owl incubates the eggs for 28-30 days, while the male feeds her during this time.

Owlets open their eyes on their 5th day, lose their egg tooth on the 9th day, stand on the 12th day, and emerge from the burrow on their 12th day. By week six—approximately 40-45 days—the owlets are flying well.

The female owl does all the brooding, while the male does all the hunting for the female and young early on. The female will begin hunting again as the young become less dependent.

In central Washington, the nesting success is 64%. In southern Washington, the rate drops to 55%. Nest success is defined the percentage of nesting attempts that have at least one chick that makes it to 45 days old, a pretty low standard.

In central Washington, the density of nests is 1.74 nests/square mile, and in southern Washington it's only 0.72 nests/square mile, so large areas are needed for sustaining owl populations.

In central Washington, the annual fecundity—chicks that make it to the fledgling stage—is two fledglings per nesting attempt. In southern Washington, the fecundity rises to three. Therefore, in a clutch of 6-11 eggs, generally only 2-3 young make it to fledgling.

One of the biggest factors affecting reproductive success—outside of burrow and habitat availability—not surprisingly, is food. Female owls supplemented with food lay more and larger eggs; food availability has a direct effect on the clutch size, although more research is needed.

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Burrowing Owls 1

 

Conservation and Management

 

Burrowing owl populations have declined significantly in most of their historical range due to habitat loss and persecution of burrowing 'fossorial' animals through poisoning and trapping!

Degradation of owl habitat comes primarily from development and intensive agriculture, which results in the direct loss of burrows and foraging habitat. It also creates conditions that lead to increased predation of owls.

The U.S. Fish and Wildlife considers burrowing owls a "Species of Special Concern." A species of concern is a species which the agency believes might be in need of concentrated conservation actions, which can range from periodic monitoring of populations and habitats to a need for listing as a threatened or endangered species under the Endangered Species Act. Species of concern receive no legal protection, and the 'designation' does not mean that the species will eventually be proposed for listing as threatened or endangered. Species of special concern are those of even greater concern.

The Washington Department of Fish and Wildlife is considering listing burrowing owls within the state as threatened or endangered.

Management for burrowing owls includes protection of burrowing mammals, installation of artificial burrows, and management of vegetation through fire or grazing—except that grazing animals can trample burrows!

Adding perches, habitat management, and pesticide restrictions have not been tested for effectiveness.

The old artificial nest boxes had very low occupancy rates in the area; <7% of installed artificial burrows were used as nest sites by owls. Only 3% of installed burrows had successful nests, versus about 60% nest success in natural burrows during the same season. This limited the conservation strategy of using artificial burrows to mitigate loss of habitat and natural burrows. However, the newer designs have had a 39% success rate, much closer to what natural burrows achieve.

In central Washington, the density of nests is 1.74 nests/square mile, and in southern Washington it's only 0.72 nests/square mile. These very low densities show that large protected areas are required for conservation of nesting owls!

Page Photo Credits — Burrowing Owls on Pages 1 and 4 - Dave Goeke, Burrowing Owls on Page 3 - Jane Abel
Last Updated: Dec 03, 2013
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