(04-23) 06:16 PDT WASHINGTON, (AP) --

City folks, don't worry. Nobody expects pigeons, more common than manhole covers, will bring the deadly bird flu virus.

Pigeons are not immune from the virus. But tests indicate the birds pick it up only when they are exposed to very high doses, do not always become infected under those conditions and are carriers only briefly.

"Pigeons aren't a big worry," said Rex Sohn, a wildlife disease specialist at the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis. "But to make absolute predictions that pigeons won't be susceptible to this virus, in whatever form it arises in North America, is not something you want to say."

Government scientists looking for the first signs of the H5N1 bird flu strain in the United States are focusing on wild migratory birds, not resident birds such as pigeons, starlings and sparrows that stay close to home.

In February, a 14-year-old pigeon seller in Iraq died after coming down with bird flu-like symptoms. Authorities said three of his cousins also were hospitalized with similar symptoms.

There have been no pigeon die-offs in parts of the world experiencing H5N1 outbreaks, according to USGS wildlife disease specialist Grace McLaughlin.

Three studies since the late 1990s by the Agriculture Department's Southeast Poultry Research Laboratory in Athens, Ga., have produced "more questions than we have answers," said the center's director, David Swayne. The lab has been working on bird flu since the 1970s.

In one experiment, researchers squirted into pigeons' mouths liquid drops that contained the highly pathogenic H5N1 virus from a Hong Kong sample. The birds got about 100 to 1,000 times the concentration that wild birds would encounter in nature. "We couldn't infect the pigeons," Swayne said. "So that's good news."

In 2004, the lab did two more experiments. Using a pigeon and a crow that had both died in Thailand, researchers gave 12 pigeons similarly high doses of the bird flu virus. Seven became infected and one died. Five others did not become infected.

"What that tells us is that pigeons can be susceptible. But they're not uniformly susceptible," Swayne said. "Not like chickens or ducks — they all become infected."

Infected pigeons carried the virus about 10 days. But they were infectious for only about two days and then at levels below what it would normally take to infect a chicken.

"The experimental data is not very strong that pigeons are going to be spreading this virus around," Swayne said. "At this point they have not been implicated in spreading it to humans and to farms."