It's a classic case of the winter travel bug —?only this time, those falling ill from foreign germs are birds, not people.

This winter, a little yellow and brown bird called the pine siskin — a close relative of the goldfinch — has migrated in large numbers to Sonoma County from farther north, driven by a shortage of its favorite food, pine cones.

According to Veronica Bowers, founder and director of a rescue center in Sebastopol focused specifically on songbirds, the diminutive birds are congregating in large numbers at feeders and some appear to be getting exposed to a form of salmonella to which they are particularly susceptible. The birds are rapidly becoming sick and infecting other songbirds, mainly the goldfinch.

The outbreak won't been confirmed as salmonella until tests return from a lab, but Bowers said a similar outbreak occurred in 2008 when large numbers of pine siskins arrived in the area due to another food shortage up north.

For the past few weeks, Bowers' Native Songbird Care and Conservation has received numerous calls about sick or dead birds. Bowers estimates that between 3 and 10 birds have been brought in to the clinic each week, many of them from Petaluma and Rohnert Park. Most of those birds have died within 24 to 48 hours of arriving at the center.

Petaluma resident and bird lover Anastasia Schuster, whose bird feeder draws as many as 50 birds at a time to her yard, said she has found 10 dead birds — a goldfinch and nine pine siskins — over the course of a week. She first started seeing sick birds around Dec. 5, though birds in neighboring yards do not appear to be infected.

"I'm really concerned about it," she said.

Bowers is asking residents in Petaluma and around the county to help deter the disease, which she says can rapidly become an epidemic. Specifically, she advises immediately removing a bird feeder from a yard as soon as a sick or dead bird is identified. That's because feeders encourage birds to be in much closer quarters than they would in nature, making it easy for them to catch diseases from each other.

Feeders should be disinfected on a weekly basis, she says, but recommends not returning the feeders to the yard until one month after the last sick or dead bird is seen.

Bird baths should also be cleaned regularly.

Bowers also recommends wearing gloves and a mask when cleaning feeders, as people with compromised immune systems are potentially vulnerable to the disease if they come in contact with large amounts of the birds' fecal matter. Cats and dogs preying on the sick finches could also be susceptible, she says.

Sick birds commonly appear lethargic and, contrary to expectations, a little larger than usual, as their feathers are puffed up. They may also have red or irritated eyes.

Schuster has already decided to follow Bowers' suggestion to temporarily remove her feeders and bird baths.

"It's so sad," she said. "I take such pride in the birds that come to my feeders; they are a true joy, I love just watching them. Alas, for their sake, I'll forgo the pleasure of having them so plentiful in my yard and hope this situation passes soon."