Bird nests aren't rare, but they're hard to find in a metropolis.
That was the problem volunteer bird-watchers faced when they fanned out across 4,083 square miles of mountains, woodlands, beaches, parks, tree-lined neighborhoods and landscaped freeways to gather information for Los Angeles County's first breeding bird atlas.
Last week, in a back room of the Natural History Museum of Los Angeles County, ornithologist Kimball Garrett and avid birder Larry Allen were putting final touches on the compendium of maps and field notes showing what actually goes on in urban trees and shrubs each spring.
Overall, "the good news is that there is still a lot of diversity throughout the county, despite the massive loss of habitat due to urbanization," Allen said. "Unfortunately, many of the areas we found breeding birds on are already being bulldozed, particularly in the Santa Clarita and Antelope valleys."
The aim of the atlas is to put a spotlight on the county's avian diversity and provide a baseline of information for future researchers.
"Twenty years from now," Garrett said, "people will be able to use this atlas to determine how things have changed."
Launched in 1995, the project divided the county into 410 10-square-mile blocks, then dispatched 200 volunteers to comb each for verifiable signs of breeding behavior such as nest-building.
"Volunteers were instructed to carefully avoid disturbing nests or birds," Garrett said.
After 10,000 hours of fieldwork between 1995 and 2000, the volunteers confirmed 230 breeding species.
Some blocks were more productive than others. Only nine species were found in the sparsest, which framed torrid desert terrain in the county's extreme northeast corner. The most fecund was at the base of Santa Anita Canyon, where 97 breeding species were found.
The effort answered questions that have nagged the region's birders for decades.
"What is the most common breeding bird in Los Angeles County?" asked Garrett, the museum's ornithology collections manager. "House sparrows? Not even close.
"Surprisingly, red-tailed hawks are far more widespread throughout the county," he said. "They've learned to take advantage of freeway and railroad rights-of-way for hunting small mammals."
But the most widespread breeding bird of all was the mourning dove, followed by the house finch, the red-tailed hawk and the common raven. The house sparrow was No. 10.
Given that the fieldwork ended six years ago, Allen conceded that it has taken longer than anticipated to complete the atlas. "It's been a major undertaking," he said. "There are two authors, Garrett and myself, and we've had to fit the work around the rest of our lives."
Breeding bird atlases are relatively new to the world of bird-watching. The first noted the breeding species of Great Britain in the late 1960s. A decade later, the endeavor had spread to New England states. The first California atlases covered Marin and Monterey counties in the late 1980s.
Garrett and Allen expect to publish their effort, sponsored by the museum and the L.A. chapter of the Audubon Society, next year.
"It'll be full of surprises," Garrett said.
Certain once-abundant native species such as loggerhead shrikes and snowy plovers have nearly vanished amid successive building booms, Garrett said, while some rare species -- mountain bluebirds and brewer's sparrows, for instance -- were found nesting in mountains and foothills for the first time.
Black skimmers, striking black-and-white birds with brilliant red beaks, have settled into the sandy fringes of Long Beach Harbor's new Pier 400. When the first skimmers started arriving in the 1990s, naturalists thought they were stragglers.
The great-tailed grackle, a large black bird with a long keel-shaped tail and a purple sheen on its head, showed up in many places, including the Sepulveda Basin and Echo Park. Once confined to the Colorado River Valley, grackles have followed the development of farms and water impoundments west, Garrett said.
Another new arrival was Allen's hummingbird, a small iridescent reddish-brown bird with vibrant patches of green and an orange-red throat. The hummer, the range of which was restricted to local islands until the 1950s, now resides in the Los Angeles Basin, as well as in the San Gabriel and San Fernando valleys.
Among the county's 29 nonnative breeding species, few interlopers have been as successful as a dozen varieties of parakeets and red-crowned and lilac-crowned parrots, flocks of which now screech and squawk from Malibu and Northridge to Long Beach and the San Gabriel Valley.
Many of the county's breeding birds tended to congregate in preserves, parks, river bottoms and other relatively undisturbed parcels of natural habitat surrounded by urban development.
On a foray to South El Monte's Legg Lake, Allen pointed out that dozens of great blue herons and double-crested cormorants have begun building enormous new nests in the limbs of 100-foot-tall eucalyptus trees at the water's edge.
As squadrons of herons with 4-foot wingspans swooped into the trees with cargoes of twigs in their beaks, he said: "When I first started bird-watching in the 1980s, great blue herons were not nesting at all in Los Angeles County. Now, there are several hundred, probably because of improvements in air and water quality."
Peering through binoculars at a pair constructing a lofty nest about 2 feet wide and deep, he said: "These birds will probably lay eggs later this month. The eggs will hatch sometime in March.
"And I think that's wonderful," he said. "It means something is going right with the environment."