Unseen by many a Pleasanton resident, Shadow Cliffs Regional Recreation Area is home to a burgeoning civilization: a waterbird colony. A large one, in fact.

And along a high, dusty trail that offers a keen vantage point of the critters' island habitat, there's a chance you will encounter Bill Rose, squinting through a telescope, binoculars strapped around his neck and floppy hat perched atop his head -- counting the birds and nests of the four avian species that have taken up a communal residence at the park and lake just off Stanley Boulevard, on the border of Pleasanton and Livermore.

Rose's official title is "citizen scientist," given to him by the San Francisco Bay Bird Observatory (SFBBO), though he also volunteers with local chapters of the National Audubon Society and participates in other ornithological activities.

But it's his citizen scientist role that brings the 83-year-old to Shadow Cliffs a few times a month, to monitor the waterbird colony and collect data for SFBBO -- data, he says, that serves as a vital check on the environmental health of the Bay.

"When you put all the data together, it's a gauge on how healthy the Bay Area is," said Rose, who has officially monitored the colony since 1998. "You talk about the canary in the coal mine. This is the canary in the Bay Area.

"If these populations start dropping overall, then we've got problems."

Unofficially, Rose has been watching the colony since the opening of Shadow Cliffs -- a former gravel quarry-turned-park -- in the early 1970s. Born and raised in Sacramento, he moved to Pleasanton in 1968 with his now-deceased wife and three boys to work as a research scientist in the food processing industry, following time in the military and in government work.

His interest in birds stems from childhood hunting expeditions and earning merit badges for the Eagle Scouts.

"I just like the outdoors, I like to camp, hunt and fish," he said. "I'm interested in anything nature, not only birds. I have about 6,000 slides of wildflowers. I look at fungi, mushrooms."

At Shadow Cliffs, his eyes are constantly on alert for signs of motion and his slow, methodical speech is punctuated by bird sightings ("tern," "hummingbird," "mud hen").

"That used to drive my wife crazy, driving up and down I-5," Rose said. "'Oh, look at the meadowlark.' 'Where, what, who?'"

The Shadow Cliffs colony resides on an isolated pond at the park, separated from the more recreational body of water operated by East Bay Regional Park District. (The birds' pool is owned by Zone 7 Water Agency.) The birds used to all collect on the pond's central island, Rose recalls, but after some of the cottonwoods there started to die off, a group branched off to foliage on a nearby bank.

There are four species spread among 70 nests currently at the colony: great blue heron, double-crested cormorant, great egret and snowy egret. A few distinct characteristics help differentiate the waterbirds from one another, Rose said.

"Black, cormorant. Gray, great blue heron. White, great egret, snowy egret. Great egret is big, snowy egret is small. Great egret has got a yellow-orange bill. Snowy egret has got a black bill."

Their communal residence at the Shadow Cliffs colony is due to similar environmental needs. All four species are attracted to the tall cottonwoods and protected body of water. Overall, Rose said, they live together fairly harmoniously, though he catches some squabbling going on, from pecking wars to nest-stealing herons.

"The herons are great robbers," he said. "And sometimes I think they conspire. There'll be a heron fighting with somebody on this side of the nest, and his partner's on the other side stealing the sticks."

Not all birds form colonies -- in fact, only about 13% do, according to experts. Sometimes, Rose will lead morning bird walks at Shadow Cliffs for the Audubon Society, during which time he can see over 50 bird species before breakfast, but most of these are loners.

Bird-monitoring is much easier in the winter when the trees are stripped of their foliage, he says; now the leafy greens hide the critters. (Though the shiny "whitewash" grazing some leaves, created by bird droppings, can be a good clue of a tree's inhabitants.)

As per SFBBO requirements, Rose and a few other nearby citizen scientists need to monitor the colony once every three days, counting birds and making observations for the "focal nests" Rose has selected. The process is still not as exact as Rose would like.

"Being a scientist, trying to put as many zeroes behind the decimal point as possible, it gets to me that I can't be more accurate," he said.

But the rise of "citizen scientists" has proved a great boon to the ornithology world, he said, in terms of collecting data on creatures whose airborne lifestyles make tracking difficult.

Ornithologists used to "poo-poo" the data people like Rose collected, he said.

"Now they're starting to accept it. And they've found a lot of things that they didn't know about. Some of these species migrate east and west as much as they do north and south," he said.

Max Tarjan manages the SFBBO colonial waterbird program, which is "pretty much two of us in the office," she said.

"The birds are really spread out throughout the Bay Area, so we're usually covering more than 50 sites in a season," she said. "So getting that help from volunteers to actually be on the ground to get to all of those areas throughout the Bay Area is super helpful."

Dolores Bengtson, former director of Pleasanton's parks and recreation program, came aboard the bird-monitoring train herself after encountering Rose in an observation session, while he was also interacting with casual park visitors.

"I was thoroughly impressed with the positive impact Bill was having over time in educating the public about the need to conserve birds and their habitats by understanding how human activity impacts their surroundings," Bengtson said.

"Habitat loss impacts all creatures. If we understand how human activity impacts birds, perhaps we can understand how we can develop strategies to protect birds as well as other creatures," she said. "Monitoring bird colonies in the Bay Area with its resulting data base provides a valuable investigative tool to assess the effectiveness of human efforts directed toward protecting habitats."

Bengtson now helps Rose monitor the cormorants on the island, in addition to a colony in Sunol for the SFBBO.

Currently, the colony is teeming with youngsters, born throughout the spring, though many are preparing to leave their respective nests. (Rose's general guidelines stipulate "30 days in the egg and 30 days to fledge," or fly.) Last summer, Rose said, he saw newly hatched birds all the way through July.

The nests, which birds start to construct in December, might have two to five chicks in them at once, according to Rose, distinguished by comical tufts of hair and their "clucking" noises, especially during feeding time as they vie with one another to snatch food from their parents' throats.

Next weekend, on May 27 at 9 a.m., Rose will lead an observation walk at Shadow Cliffs Regional Recreation Area, a chance for the public to observe the colony themselves. All are welcome to the "Birds in Your Neighborhood" event, hosted by SFBBO in conjunction with EBRPD, and Rose will set up viewing scopes.

"Its main purpose, as far as I'm concerned, is to get people aware, knowledgeable about it," Rose said, adding that connecting people to the bird colony might make them more supportive of wildlife preservation initiatives.

"The more people we can get involved and aware, the better," he said. "We talk to several people, there's a couple people I recognize all the time. And they come out here and they'll see me, and they'll say, 'How's our cormorants doing?' Our cormorants. They take possession."