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Tucson Audubon Society Reprinted from the May-June, 2001 Vermilion Flycatcher, newsletter of the Tucson Audubon Society. Raptor
Nest Sites in Urban Areas Urbanization generally creates a gradient of environments with highly developed land on one end of the continuum and moderately developed or underdeveloped land on the other. For example, towns and cities frequently have on their outskirts a mixture of native vegetation, agricultural fields, and a few private residences or other buildings. In contrast, centers of urban environments often are almost wholly covered by buildings and pavement. Vegetation, if present, is dominated by exotic species. The gradient of environments created by urbanization provides an array of resources (e.g., food, cover, nest sites, water) required by birds for nesting. Not surprisingly, species composition of breeding birds changes markedly along this gradient. Hawks, owls, and falcons also should respond to the gradient of resources created by urbanization, but information about the distribution of raptor species in urban settings is lacking for two reasons. First, standard survey methods for birds do not effectively estimate raptor abundance; raptors are, therefore, often excluded from bird counts in urban environments. Second, most reports of raptors nesting in urban environments focus on a single species and many describe only a few nests. Tucson is a rich environment for nesting hawks and owls, and supports breeding Harris’s Hawks, Red-tailed Hawks, Cooper’s Hawks, American Kestrels, Great Horned Owls, Barn Owls, Screech Owls, Burrowing Owls, Elf Owls, and a few Cactus Ferruginous Pygmy-Owls. Locations of nests of many of these species in Tucson have been recorded since about 1993. Described below are where the nest sites of Harris’s Hawks, Red-tailed Hawks, Cooper’s Hawks, Great Horned Owls, and Burrowing Owls fall along the urban gradient. All five species, except perhaps Great Horned Owls, generally nest in identifiable places along the urban gradient. The gradient for these species is characterized at its most developed end by high-density residential neighborhoods and parks and golf courses, and at its opposite end by low-density housing and undeveloped land. Cooper’s Hawks nested in areas near the most urbanized end of the gradient, Harris’s Hawks and Great Horned Owls in moderately developed areas, and Burrowing Owls and Red-tailed Hawks in places with the least development. Selection of nest sites along the urban gradient by hawks and owls appears to be influenced by the availability of nest structures, the abundance of favored prey species, and the structure of vegetation as it relates to hunting behavior. Cooper’s Hawks and Harris’s Hawks in Tucson build their nests most frequently in Aleppo pines and eucalyptus trees. These trees are exotic to the southwestern United States, and those used as nest sites are taller than the native trees or cacti in Tucson. Nest trees in urban settings also are taller than the trees or cacti used as nest sites in undeveloped desert outside the urban environment. Tall trees may provide the necessary level of security needed by nesting hawks to tolerate human activities in urban settings. However, tall, exotic trees occur in Tucson in a variety of environments, and thus only partly explain the distribution of urban-nesting raptors. A relatively constant supply of water and green vegetation in Tucson, compared to undeveloped desert, probably increases the abundance of prey species for raptors and reduces fluctuations in prey populations during periods of drought. Availability of water and abundant prey explain, at least in part, why hawks and owls nest in urban environments at all, and why some species (e.g., Cooper’s Hawks) nest in higher densities in the city than in exurban areas. Distribution of hawks and owls along the urban gradient likely is determined largely by species-specific prey preferences and hunting strategies. For example, Red-tailed Hawks, although generalists in their selection of prey, hunt in open areas. It could be that they nest most frequently along the outskirts of Tucson because undeveloped land with open vegetation is most common there. Harris’s Hawks in the United States hunt in areas with dense ground cover and rely primarily on ground-dwelling prey, such as cottontail rabbits and Gambel’s Quail. Urbanization can lead to the reduction of ground-dwelling birds, due partly to predation by domestic cats. However, in areas where native species, such as Opuntia cacti, are used as ornamental ground cover, some ground-dwelling birds and mammals persist and even flourish after development. Both rabbits and quail are common in Tucson in areas with desert landscaping or native desert vegetation. Harris’s Hawks probably nest most frequently in suburbs with relatively low housing density because their favored prey are present in these areas and their hunting strategies are effective in the desert vegetation that exists around and between houses. Cooper’s Hawks in Tucson prey primarily on birds, especially Inca Doves and Mourning Doves. We speculate that Cooper’s Hawks nest in highly urbanized areas for two primary reasons. First, they are physically adapted to hunt in forests and shrublands and thus are capable of capturing birds in structurally complex, urban environments. Second, developed areas generally support higher bird abundance, especially doves, than undeveloped areas. Great Horned Owls in Tucson appear to be partly dependent upon diurnal raptors for nest sites, and they occupy nests built by Harris’s Hawks, Red-tailed Hawks, and Cooper’s Hawks. We suspect that the tendency of Great Horned Owls in Tucson to nest near the urbanized end of the gradient is a result of the availability of nest structures and not selection for conditions around the nest. Cooper’s Hawks and Harris’s Hawks are more abundant in Tucson than Red-tailed Hawks; thus, stick nests are most abundant in areas with moderate to high development. We note, however, that Great Horned Owls in Tucson also nest in naturally formed cups at the base of fronds in palm trees, and on buildings and other structures. The flexibility of Great Horned Owls in selecting nest sites and their catholic diet probably allows them to nest almost anywhere along the urban gradient. Burrowing Owls in the western United States depend on burrows dug by mammals (e.g., ground squirrels), or holes created in other ways (e.g., by erosion) as nest sites. They usually nest in burrows in areas where vegetation is low or nonexistent, ostensibly so that they can see and avoid predators. In Tucson, it appears that even expanses of structurally simple, native vegetation, such as creosote bush, do not provide the visibility they require. Burrowing Owls in Tucson, therefore, nest most frequently in areas where natural vegetation has been removed and grass or bare ground is maintained. These areas in Tucson are most common around airports, along river corridors where flood plains are periodically scraped, in vacant lots, and in new subdivisions where vegetation is removed before development. The conditions which trigger different species of hawks and owls to nest in urban environments potentially could be duplicated and expanded in existing developments, or created in future developments, if hawks and owls are desired components of urban avifaunas. However, nesting by raptors in towns and cities does not mean that urban environments provide high quality habitats for them. Developed areas present environmental challenges to raptors, such as electrocution, disease, poison, collisions with vehicles and windows, and human disturbance that may preclude self-sustaining populations. It is incumbent on biologists and city planners to understand the dynamics of populations of urban raptors, their species-specific habitat requirements, and the problems they face in urban settings before extensive habitat management efforts are undertaken on their behalf. Bird questions? Check Birding | General questions? Contact: Tucson Audubon Society | Webmaster: Email This page was updated on 02/09/06 |