AILSA CHANG, HOST:
Linda Horvath found the first dead bird on her back deck outside Sacramento, Calif. A few weeks went by, and there was another one, then another.
LINDA HORVATH: So I was bothered by that and, you know, troubled to wonder what was happening to these birds.
AUDIE CORNISH, HOST:
The California Department of Fish and Wildlife was getting these inquiries from hundreds of residents, and so its scientists investigated, scientists like Krysta Rogers.
KRYSTA ROGERS: So the disease is called salmonellosis, and we see it periodically in some winters.
CORNISH: In other words, salmonella poisoning.
ROGERS: And it affects a small, migratory songbird in California that's mostly pine siskins, which are a small finch.
CORNISH: Finches that migrate south from Canada.
(SOUNDBITE OF BIRD CALLS)
WESLEY HOCHACHKA: It's fairly slender. It's got a little bit of a conical bill, and it's a streaked light and dark gray. But when it spreads its wings out, you can see a yellow line running the length of the wing.
CHANG: Wesley Hochachka is senior research associate at the Cornell Lab of Ornithology. He says birds, particularly pine siskins, develop salmonella poisoning through tainted food or water or through other infected birds. It makes them appear lethargic.
HOCHACHKA: You just see them sitting on a perch quietly, eyes closed, incredibly puffed out and not reacting to the world around them, to the point that you could go up to the feeder and literally pluck a bird off its perch.
CHANG: This year, California reports more than double the number of dead birds it saw during its last big outbreak. And the outbreaks are happening throughout the country.
CORNISH: California's Krysta Rogers says one reason for the sick birds might be related to the human pandemic.
ROGERS: During the pandemic, maybe more people got bird feeders because they weren't going anywhere and they wanted to see wildlife in their yards.
CORNISH: In fact, the Audubon Society surveyed bird supply stores and found sales were up 30% to 80%.
HOCHACHKA: Bird feeders are becoming superspreader sites.
CHANG: Wesley Hochachka says people should consider removing their bird feeders for a few days and cleaning them with bleach and getting rid of nearby feces. That advice makes Linda Horvath feel a lot better.
HORVATH: So much better. It was just so sad to see these little birds dying and, you know, feeling like you're trying to help them by putting out the feeder and then doing something that would cause them harm.
CORNISH: Meanwhile, cases are falling in the U.S. As it gets warmer, the pine siskins are already heading back north.
(SOUNDBITE OF BOYGENIUS SONG, "SOUVENIR") Transcript provided by NPR, Copyright NPR.