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From a eucalyptus high above Church Street, a crow watches a songbird return to its nest with a fat bug for its young. When the mother bird flies off in search of more food, the crow swoops down on the unguarded nest and attacks the babies with its knifelike beak. A cloud of songbirds rises to try to drive the crow away, but it quickly devours the young. Leaving only a pile of feathers and with a raucous cry, the crow returns to its treetop aerie and looks for its next meal.

This scene played out numerous times this summer, leading some local residents to wonder: Has a band of thuggish crows invaded Noe Valley?

Well, sort of, say the experts. But that's nothing unusual.

Crows--and the pitiful evidence of their crimes--are now a common sight in cities from Seattle to San Diego. At home in urban parks and back yards, they find plentiful food, abundant roosting areas, and few natural enemies. The number of crows and ravens sighted in San Francisco and Oakland in the National Audubon Society's annual Christmas bird count jumped 74 percent to 756 in the past decade, while the San Diego County towns of Oceanside and Escondido recorded fourfold increases between 1995 and 2005.

Opportunistic omnivores, crows scavenge grain and roadkill, and prey on insects, rabbits, raccoons, frogs, and snakes, as well as small birds and eggs. With their keen intelligence and superior size--the typical wingspan is 40 inches--crows are often able to overcome the diversionary tactics and "mobbing'' defenses of smaller birds. It's not for nothing that a flock of these ink-black birds is called "a murder of crows.''

And as dismaying as it is to see songbirds and other creatures meet untimely ends, it is nature's way.

"Most baby birds never make it out of the nest, baby crows included, because of predation, disease, competition for food, bad luck, etc.,'' says John Matzluff, an authority on corvids (ravens, crows, magpies, and jays) at the University of Washington and the co-author of In the Company of Crows.

"This is nature, where what seems cruel is just the way it works,'' he continues. "Predators need to eat, which is a bummer if you are prey.''

However, says Matzluff, songbirds may soon get a reprieve. West Nile virus has hit crows hard, killing up to 40 percent of the population in some areas. It could be years before their numbers rebound and the circle of life takes a new turn.

In the meantime, we should keep "some wildness in our yards,'' suggests Jack Dumbacher, curator and chair of the Department of Ornithology and Mammalogy at the California Academy of Sciences. He notes that cutting grass, trimming hedges, and pruning trees make it difficult for small birds to hide their nests from voracious predators.

"And,'' he adds, ''we can keep our cats indoors.''