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The RRAS is the Southern Nevada chapter of
the National Audubon Society.
The mission of Red Rock Audubon Society is to
protect, restore, and improve natural ecosystems, focusing on birds and
other wildlife, and to educate the public about our unique Nevada
environment.
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Text provided by
Christiana Manville, US FWS . |
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| PROJECT
RATIONALE AND OBJECTIVES In Nevada the burrowing owl (Athene cunicularia) is a state protected bird and Bureau of Land Management Sensitive Species. Like most migratory birds, it is protected under the Migratory Bird Treaty Act of 1918, which makes it unlawful to kill or injure migratory birds, eggs, or occupied nests during the breeding season. Unfortunately there are no laws that directly protect habitat for this species. Habitat loss is occurring at a rapid rate in the Las Vegas Valley as the Las Vegas metropolitan area continues to grow. Urban expansion in this valley occurs in a pattern that leaves many undeveloped smaller parcels within the urban area. Many residents have become attached to burrowing owls that they have watched rear young for several years in adjoining vacant lots. After waterfowl calls, the U.S. Fish and Wildlife Service office in Las Vegas receives more calls relating to burrowing owls than any other urban wildlife issue. The public often becomes concerned when they see construction equipment starting to work in parcels where “their” owls live. In the Nevada Partners in Flight Bird Conservation Plan, burrowing owls are listed as a Priority Bird Species in the Mojave Region. The plan lists protecting and maintaining suitable burrowing habitats as a conservation strategy for burrowing owls. Actions listed under this strategy include: 1) working with developers in urban and suburban areas to preserve open space; and 2) mitigating for loss of owl nest sites by constructing artificial burrows. Before too much conservation effort is spent on burrowing owl conservation in these urban areas, we believe it necessary to determine if owls in urban areas in the Mojave Desert are increasing or decreasing in number. If owls do not produce enough young to replace themselves and their mates over a lifetime, then populations in an area will decline. If owls produce more young than what is necessary to replace themselves and their mates, then the local population may increase if adults and juveniles have high survival rates. Owls in an urban environment may have more food sources than in the surrounding desert, but are probably exposed to new threats such as collisions with vehicles, predation by domestic dogs and cats, and exposure to pesticides and trash. Owls in urban areas may also provide valuable opportunities for environmental education. The goals of the Urban Burrowing Owl Monitoring Project are: 1) to map the location of burrows used by breeding burrowing owls in the Las Vegas Valley; and 2) to monitor these burrows for at least three years during the breeding season to estimate reproductive rate of these owls. |
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| BURROWING
OWL NATURAL HISTORY AND IDENTIFICATION Observing Burrowing Owls Burrowing owls are unmistakable, small, long-legged, ground-inhabiting owls. Although often considered to be diurnal, burrowing owls are mostly nocturnal or crepuscular—active at dawn and dusk—in their foraging habits. They are generally considered to be diurnal because they frequently perch conspicuously during daylight hours, especially early morning and late afternoon, at or near the entrance to their burrow or on a nearby low perch. You will be observing burrowing owls during this period near sunrise and sunset when they are often perched next to their nest burrow in the breeding season. Despite the fact that one or both owls will usually be on guard near the entrance to their nest, the birds may not be easy to find. Keep your search image in mind: a small, round grayish-brown head blending into the dirt. Burrowing owls will invariably see you before you see them and, being curious birds, will continue to watch you to see what you are doing. As a result, their eyes and the tops of their heads will usually remain visible as you scan an area. Burrowing owls prefer very open country with sparse vegetation or short grass. In the Mojave Desert burrowing owls use burrows dug by other animals, including desert tortoise (half moon shape), kit fox (round), and ground squirrel (round). The area immediately around the mouth of an active burrow is often bare of vegetation. Often the burrow is next to a bush. Burrowing owls eat mostly insects in the summer and small mammals (mice and kangaroo rats), reptiles, and small birds the rest of the year. An active burrowing owl nest will invariably contain a large amount of white droppings near its mouth and, generally, numerous pellets (inedible portions of prey, usually comprised of insect remains, but also often with fur and bones from small mammals). A few bones from small mammals will often be scattered around the nest burrow as well. In addition, burrowing owls generally decorate the entrance to their nest burrow with all manner of strange objects, including pieces of cloth, paper and other litter, as well as dried dog, cow, and horse droppings. |
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| The Breeding
Cycle In southern Nevada, the burrowing owl breeding season is March through August. Burrowing owls tend to return to the same territory year after year and may use several burrows in that territory. There is usually one nest burrow and several other burrows (satellite burrows) that are used in one nesting season. Some of our breeding owls spend the whole year here, while others are only here during the breeding season. The males establish territories and prepare the burrows for use. Courtship Period Displays near the nest burrow include mutual billing and preening of the head and facial areas, the male presenting food to the female, and the male singing the primary song, a 2-note call described as “coo coooo”. The male may also perform display flights consisting of a circular motion or an ascending/descending and hovering motion. Unlike other birds of prey, both sexes are similar in size with the male burrowing owl averaging slightly larger than the female. The size difference in burrowing owls often appears exaggerated, as the male tends to fluff up his feathers and raise himself to his full height in display, whereas the female will often crouch low and compress her head feathers. Of course, this behavioral difference between the sexes is only a general tendency, and either sex can act either way, depending on the circumstances. Another difference between the male and female is the relative ‘paleness’ of the plumage of the male, especially the head and the sides and flanks. Male burrowing owls are usually paler than their mates. While this paleness may be due in part to relatively larger white spots and white bars and narrower and paler dark bars on the feathers (especially on the sides and flanks), it may also be due to the bleaching of the feathers by the sun. This occurs because the male owl spends much of his daylight hours in the open sun at or near the nest burrow, while the female spends much more of her daylight hours in the darkness of the nest burrow incubating her eggs and brooding her young chicks. As a result the basic background color of the head of a male burrowing owl is a pale grayish-brown, while the background color of the female’s head is a medium plain brown. Of course both sexes have small white spots on the head feathers. These white spots may be larger and more extensive in males, further adding to their pale-headed appearance. The difference in head color is often quite noticeable when the birds are sitting together. However, as the season progresses and females begin to spend more time sitting in the sun, the difference may become less noticeable. Another difference between the sexes is their voices. Both sexes utter a distinctive alarm call that consists of a chattering series of “chack” notes. These tend to be louder and longer in males than in females, especially in males trying to distract an observer from the location of the nest burrow. A second distinctive vocalization is the primary song, a two-note “coo-coooo” that seems to be given only by males, generally during the early morning and late afternoon hours, or at night. Males give the primary song as a territorial defense, and during pair bonding and pair formation vocalization. So both mated and unmated males give the primary song. Birds observed giving this call should be observed carefully to see if a female is present. Incubation Phase The female lays an average clutch size of five to nine eggs in the nest burrow. Usually one egg per day is laid. Incubation by the female takes 28 to 30 days. During this time, the female is mostly underground except when the male feeds the female at the burrow entrance. Re-nesting may occur if the first nest is destroyed early in the breeding season. Nestling Phase The young first emerge from the nest burrow when they are around two weeks old. The juveniles wait for adults to bring them food at the burrow entrance. At three weeks of age, juveniles can hop, flap their wings, run, and preen themselves. At four weeks of age juveniles take short flights. At six weeks, juveniles fly well but remain near the nest burrow. Nestlings are able to move to adjacent burrows almost as soon as they emerge from the nest burrow. These satellite burrows need to be identified so that when/if the brood separates into multiple burrows they can all be counted. Fledgling Phase Juvenile owls fledge when they are around 44 days old, meaning that their flight feathers are developed enough to leave the nest but they are still dependent on parental care. Young may remain at or nearby the nest burrow for an additional 21 to 28 days. The juveniles change plumage at 45 to 70 days old (pre-basic molt) after which they look indistinguishable from the adults. The juveniles begin chasing live insects when they are 7 to 8 weeks old. They will join adult burrowing owls on foraging flights at dusk. Distinguishing Between Adult and Juvenile Burrowing Owls Adults Adults have a spotted dark brown and buffy breast, a brown back, head and wings with whitish spots, a white throat and white eyebrows, and yellow eyes. These owls have long legs and no ear tuffs. They are approximately 9.5 inches long from bill tip to tail tip. Juveniles The most distinguishing characteristic of juvenile burrowing owls is their unmarked lower chest and belly. Juveniles also have a buffy wing patch (also visible in flight) and unspotted dark brown head. |
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Table 1. Plumage characteristics and behaviors of juvenile burrowing owls. |
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ALL MEMBERSHIP MEETINGS AND FIELD TRIPS ARE OPEN TO THE PUBLIC - COME JOIN US! Sign up to receive important local news and events emails! Call or write us if you have any questions, want to volunteer, or need information: RRAS Information (702-390-9890) Red Rock Audubon Society, P.O. Box 96691, Las Vegas, NV 89193 |
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©
2007-2008 Red Rock Audubon Society. All rights reserved. All photographs are the property of the photographers and may not be downloaded or reproduced without written permission. Website Design: M. J. Kammerer Updated: 08/17/2008 |
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