Law enforcement officials say that because the birdwatchers have equipment such as binoculars, telescopes and cameras, they have the potential to commit acts of espionage. The areas they use are sometimes close to military bases, dams and sewage plants

Because they have "sophisticated gear and [are] looking at things not normally photographed by the common citizen in this area, they may be stopped and asked a few questions," Lieutenant Jamie Rickerson, of the US coastguard service, told the Los Angeles Times.

Enthusiasts wanting to go birdwatching at the Chesapeake Bay Bridge-Tunnel in Virginia used to need an annual permit obtained over the phone, by post or in a fax. There are four islands, one of which was open to the public. To visit any of the other three, to the north, they needed a birdwatching permit, photo ID and car registration number.

But earlier this year the three northern islands were closed by Virginia's department of transportation.

Two weeks ago, after protests from birdwatchers, they reopened with greater restrictions. Now, groups of no more than 15 people can visit and they must notify the authorities in advance, pay $50 an hour to be escorted by an off-duty police officer, and submit to random searches.

"We discovered that we had areas of concerns," said Clement Pruitt, the area's chief of police. "If you can get into the ventilation buildings, you have direct access to the tunnel and can inflict serious damage to the tunnel."

The ornithologists are not convinced. "These sorts of national security issues seem to be intruding in ways one would never have expected," said Perry Plumart, director of conservation advocacy for the American Bird Conservancy. "You expected airline security; you don't expect it when you go birding. Who knew you'd have a police escort?"

Donald Dann, the president of the Bird Conservation Network, which represents a number of ornithological groups in the Chicago area, was more blunt. "Someone can lob a grenade from the street if they wanted to. I'm not sure they're achieving any great national security objective."