Downtown Richmond's two rare falcons have disappeared.

The popular pair of peregrine falcons might be dead, or they could have moved to some isolated spot, experts say.

"We just can't see any evidence of them at all, and it's right at egg-laying time," said Ray Fernald, manager of nongame programs for the state Department of Game and Inland Fisheries.

The crow-sized birds were last seen calling and circling over their 21st-story nest at Riverfront Plaza on March 6.

The birds have served as ambassadors for conservation, said Shawn Padgett, a falcon specialist with the College of William and Mary's Center for Conservation Biology. "[They] keep people's attention on the fact that wildlife still needs human help to succeed."

If one falcon had died -- as one did near Hopewell just last month -- the mate should be sticking by the nest, said Bryan Watts, director of the conservation center.

"It's just bizarre because you just never see that, a pair just up and leaving at this time of year," Watts said.

"They are either somewhere else, or they are both dead. That is the story. . . . As time goes by, and they're not being seen, it doesn't sound good."

If the birds did move, they probably are nesting within 4 miles of downtown, because that's the area they are familiar with, Watts said.

Falcon experts are asking people who see the birds to call or e-mail them.

The birds began nesting in Richmond in 2003. They produced 15 chicks and a legion of fans who watched them live and on a Web site. The whereabouts of the surviving chicks are unknown. Of the four born last year, two were relocated to the mountains.

"You'd be surprised at the number of phone calls and e-mails we get from people who work downtown who are very happy to be able to look out the window and see something that most people don't get to see," said Padgett.

A double death would be bizarre, but it's possible the birds ate a pigeon or starling that had been poisoned, Padgett said.

Because peregrine falcons are so rare -- there are only about 20 nests in Virginia -- scientists are trying to help them build their numbers. Pesticides nearly wiped them out in the 1950s and '60s.

Last year, the Richmond pair nested on a walkway on the westernmost of Riverfront Plaza's twin towers at 901 E. Byrd St. That's where they were last seen.

The first three years, they nested on the 17th-story balcony of the nearby First National Bank Building.