Walk the shore of Lake Chabot and you'll likely see something that few have ever seen there before - a pair of bald eagles swooping to grab fish, building a nest and teaching their fledgling to fly.

After decades of absence, nesting pairs of the strikingly beautiful bird began returning to the Bay Area in 1996. And the first-ever sighting in the Castro Valley area is more evidence of the growing health of California's bald eagle population, researchers say.

There are at least six Bay Area locations where bald eagles have built nests, from San Pablo Reservoir to Crystal Springs Reservoir.

"It feels like bald eagles are coming back, and it is real exciting," said Allen Fish, director of the Golden Gate Raptor Observatory. "It is a post DDT-world."

Before 1996, it had probably been 100 years since birds nested in the Bay Area, said Doug Bell, wildlife program manager for the East Bay Regional Park District.

"It is just a beautiful thing to see part of the natural environment reestablish itself," Bell said.

Nearly wiped out

The bird population was assaulted for most of the 20th century by urban sprawl, pollution and intentional poisoning, Bell said. In the second half of the century, the pesticide DDT nearly knocked out the bird's entire population in the lower 48 states.

But things have turned around and, little by little, the population of the national bird has crept back. Bald eagles made it off the national list of endangered species in 2007 but remain on California's list, Bell said.

Today, there are more than 200 nesting pairs in California.

"It is a minor miracle when I think back to the years when there were very few breeding raptors in the area," Bell said. "When I think of the '60s and '70s, the situation was very dismal and very bleak. You almost have to pinch yourself now."

The pair at Lake Chabot arrived in February and quickly began building a nest in a secluded spot near the Columbine Trail. After a few months, observers realized the pair had an egg.

"Each step of the way, we were worried about the next step," Bell said. " 'OK, they are building a nest, can they lay an egg? OK, they did that. Are they fertile?' "

The process can be treacherous. A bald eagle couple at Crystal Springs Reservoir lost their nest earlier this year when a tree branch cracked and sent everything crashing to the ground.

Mary Malec, an Oakland resident who has been observing the Lake Chabot birds about eight hours a week, said that for a time she wasn't sure the young couple was mature enough to raise an eaglet.

"She seemed not to want to incubate," Malec said of the young, 4-year-old mother. "The male had to keep chasing her back to the nest. At her age, she may not have had the right level of hormones to have the instincts to want to do it."

But the mother eventually learned and the couple shared incubation duties until April 22 - Earth Day - when the female chick hatched.

First flight

In early July, the young eagle stepped out of the nest for the first time and took a whirl around Lake Chabot. The chick can now be seen swooping around the lake, learning to hunt on the all-you-can-eat bonanza that is the human-stocked trout pond. Its parents keep an eye on it while they orbit overhead, Malec said.

The fledgling must get fat before winter or it will die, Fish said. Between 60 to 80 percent of bald eagles die in their first year.

In a few months, the chick will leave Lake Chabot and set off for the delta, where it can feast on duck, or head north for the salmon runs, Fish said. In four to five years, it could find a mate and return to the Bay Area to set up its own nest.

By then, if thing keep going the way they have been, bald eagle nesting could expand to include the South Bay wetlands, Bell said.

"I think it is all just a great indication that the parks and the people in the area are doing something right for a change," Malec said.