Wednesday, November 30, 2011

RAPTORS IN THE CITY

One of the surprises in studies of urban wildlife in recent years is the invasion of our cities and suburbs by predatory birds such as hawks and owls. Perhaps "invasion" is too active a word, and a better word is "acceptance." More and more raptors are apparently becoming comfortable with living in the cities of the Pacific Northwest and, in fact, all across North America.

Bird feeders have become a dependable part of the urban/suburban environment, and many small birds are attracted to them. The juncos and finches and sparrows that abound at feeders attract Sharp-shinned Hawks in winter, but the hawks leave for the mountains in spring. However, their larger relatives Cooper's Hawks are year-round residents, feeding on pigeons, jays, and flickers and nesting in wooded areas that give them a sense of security. The males are conspicuous in spring, with display flights and vocalizations, so more and more of them are being detected living among us.

Merlins too are in the process of becoming city dwellers. These small falcons nest in forests near open country, where they usually capture their prey (small birds). Wintering Merlins probably moved into cities to take advantage of local concentrations of small birds, and eventually pairs remained throughout the year and nested because of that prey abundance. Their nesting efforts have been successful, and the population of urban-nesting Merlins has slowly risen.

Barred Owls have moved into the Pacific Northwest from the north, and they too appear to be comfortable in wooded parts of our cities. Preferred prey include the gray squirrels and rats that have been introduced into the region, and our bird feeders attract and concentrate these mammals as well. There are now pairs of Barred Owls, Cooper's Hawks, and Merlins nesting in wooded areas near my house in Seattle that weren't there 10 years ago.

Red-tailed Hawks do not live around dense housing developments, but large parks and open spaces furnish adequate hunting grounds for rats and squirrels, and if any trees in their territory are large enough to support their nests, they may nest in our cities. Many more of them appear in winter to hunt in open areas around parks and airports.

The Great Horned Owl is a top predator surprisingly willing to live in cities, mostly in relatively large parks. It is large enough and fierce enough to handle mid-sized mammals such as raccoons, opossums, and domestic cats that thrive in cities. Crows and pigeons furnish abundant prey populations as well. Nest sites (usually old Red-tailed Hawk nests) may be limiting, however.

Even more surprisingly, after a long period of decline during the DDT era, Bald Eagle populations have rebounded in the Northwest, and they not only visit but even nest in the cities, often in large parks near water where prey species such as waterfowl and fish are available. They need large trees for their huge stick nests.

Whenever a shift like this occurs, it is worth trying to distinguish between behavioral adaptation and genetic adaptation. Have these birds "learned" that cities are OK? Or are there genes for city living that have become favored? Are there genes for increased tameness, recognition of bird feeders as sources of bird concentrations, resistance to being mobbed by crows? Actually, cities aren't bad places to live for mobile predators such as these, as there may be local sources of abundant food and some of their own potential predators may be absent. Also, the first individuals to accept life in the cities have no competition from others of their species!

Dennis Paulson

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