Each year, just as their counterparts do in San Juan Capistrano, hundreds of swallows converge along a predetermined spot near the Delta. Making their nests under the lips of the railroad bridges that cross the Old and Middle rivers in Brentwood, each year the swallows arrive like clockwork.

And each year Greg Johnson welcomes them back.

This time however, was different. While the 400 or so birds did indeed make their annual pilgrimage, their visit seemed especially fleeting. According to Johnson, the swallows were here one day, gone the next; weeks before their usual departure time.

Where they went remains a mystery.

"These birds have been coming here for years," said Johnson, who lives at nearby Cruiser Haven Marina. "And every night for about half an hour they swarm that one area. There are hundreds of them dipping and diving, but a few weeks ago they just disappeared, and I wondered what had happened to them."

So he did a little detective work. Following what Johnson termed a "work day" by the Burlington Northern/Southern Pacific Railroad - an exercise that involves a crew in a truck crawling along the tracks cleaning and clearing debris from the area - Johnson said the birds and the nests were inexplicably gone the next day.

"I think I know what happened, but I don't have any proof," said Johnson. "I called and notified the California Fish and Game Department as well as the National Wild Life Conservancy, but no one has gotten back to me. Swallows are on the endangered species list. I wish I knew what had happened."

Officials at the Department of Fish and Game said they have no knowledge of that particular group of birds, or of the particular area described by Johnson.

And so the mystery remains, at least until next year.