Ravens are raiding great egret nests in the canyons alongside Bolinas Lagoon, gradually shrinking the population of the snow-white migrants.
"The ravens have been hanging around the colony here for the past four years, eating the unguarded eggs," said Helen Pratt, a volunteer bird watcher at Audubon Canyon Ranch since 1967.
"People have also seen the ravens eating the three- to four-week-old egrets," she said. "We still don't know if the young are killed in their nests before the ravens fly them somewhere, or afterwards."
The great egrets, the main draw at the ranch, have lost 30 percent of their young in 1996. Only 48 percent of the nests became nurseries, producing an average of 1.57 fledglings each.
And fewer couples arrived here to breed in March - eight fewer than last year's count of 86. The number of visiting egrets is expected to decline again next year.
While the ravens have been accused of kidnapping egrets in recent years, other predators committed similar crimes in the past.
"Great horned owls leave just a pile of feathers beneath a perch," Pratt said. "We also had trouble with climbing raccoons. Now we have metal bands wrapped around the tree trunks to prevent them from reaching the nests."
Pratt recalled that a golden eagle appeared several years ago and was caught stealing chicks. "But he left the egrets alone and picked on the herons because their newborn are smaller."
"I saw a raven eat two heron chicks a couple of years ago," said Sarah Tappen, Research Coordinator of Cypress Grove Preserve in Marshall and data manager for the Northbay Heron and Egret Project.
"He just came in, took one by the neck and took off with it. Then he brought a raven buddy to the nest to take the second one."
Tappen hasn't heard of any egret-eating activity at the one nest monitored by volunteers at Blake's Landing on Tomales Bay. "In fact, this year there are two great egret chicks in the nest, whereas last year there was none."
All told, the great egret population seems "pretty healthy" in the seven colonies studied in Marin county.
And of the 350 heron and egret nests monitored on the islands of San Francisco Bay and in Solano, Marin, Sonoma, and Napa counties, Tappen said, "I've received no alarming reports about ravens. Mostly, I hear of the egrets being harassed by redtail hawks. Overall, the egrets seem to be doing well."
But Tappen added that assessing nest failure is "an inexact science." It's hard to catch a predator redhanded. "You have to observe a nest regularly and for several hours to get all the action."
Which predator should estuarine birds fear the most?
"Humans are pretty destructive," Tappen said. "They can wipe out a whole bunch of birds each time they cut down or top off a nesting tree."
To Pratt's mind, "oil spills and pesticide use - especially DDT" have historically done the most harm."