Mass die-off of hundreds of pigeons alarm biologists in California

© Krysta Rogers

California's only native pigeon is the band-tailed pigeon, which spends springs and summer in the Bay Area and other parts of Northern California

An alarming increase in the number of dead and dying band-tailed pigeons along the California coast has prompted wildlife biologists to ask the public for help documenting the apparent decline of the only native pigeon left in the state.

At least 1,000 of the pigeons, which winter in Central and Southern California, have been found dead in Santa Clara and Santa Barbara counties since December, the apparent victims of a parasite spread by the common rock pigeon, said Krysta Rogers, an environmental scientist with the California Department of Fish and Wildlife.

The sudden increase in mortality is disturbing, Rogers said, because the closest living relative to the extinct passenger pigeon has been struggling for decades to recover from rampant hunting, habitat loss and other environmental problems.

"The potential death of a thousand pigeons is very concerning, especially since they have a relatively low reproductive rate. A pair produces about one chick per year," Rogers said. "When there is really high mortality like this, it can take the population years to recover. In addition to that, these mortality events with band-tailed pigeons have been reported with increasing frequency over the past 10 years."

Band-tailed pigeons are the West Coast version of the passenger pigeon, which was once the most abundant bird in North America until it was hunted to extinction. Millions of band-tailed pigeons used to inhabit California, but they too were hunted for food throughout the 19th century, and much of their habitat was destroyed. They were eventually protected by the , which was passed largely out of guilt over the wholesale slaughter of many bird species, including the killing off of the passenger pigeon.

Band-tailed pigeons, which are not listed as endangered, prefer redwood and pine forests in higher elevations along the Central Coast and Sierra Nevada mountain ranges. The pigeons spend their winters in oak and conifer forests between the Bay Area, Santa Barbara County and the San Bernardino Mountains before migrating in late winter or early spring to the northernmost regions of Northern California, Oregon, Washington and British Columbia.

About 500 dead band-tailed pigeons have been found over the past two months in the Saratoga and Los Gatos areas of Santa Clara County. An additional 500 carcasses were found in the Solvang, Los Olivos and Santa Ynez areas of Santa Barbara County. The state Department of Fish and Wildlife Investigations Laboratory determined the cause of death to be avian trichomonosis or, more specifically,

It is believed that non-native rock pigeons, the species commonly seen in urban areas, including San Francisco, are spreading the parasite. Rock pigeons were introduced to North America from Europe.

"Researchers believe that this trichomonosis parasite evolved with these rock pigeons in Europe, and they are immune to it but can carry it," Rogers said. "Right now, it's really hard to estimate mortality because the reports I am getting are all in locations where people live. So if there are deaths in remote locations,we're not getting reports. That's the challenging thing about this."

The parasite was first reported in band-tailed pigeons in the 1940s, but Rogers said it has become more common over the past 10 years. It lives in the mouth and throat of infected birds, causing lesions in the mouth or esophagus that eventually block the passage of food. Infected birds die from starvation or suffocation.

"Band-tailed pigeons are highly susceptible to it," she said. "When band-tailed pigeons get infected by it, they die."

Rogers is researching the disease to determine exactly how it is being spread, whether from direct contact or whether other species are also carriers. It is believed that water sources, like bird feeders and stagnant pools, may play a role. She said the death toll from the parasite seems to be worse during dry winters.

"These events seem to be more common in winters with less precipitation, so I do suspect there is some weather component in these mortality events," Rogers said. "When you have large flocks and there is a disease like this circulating, and you have stagnant pools and puddles and not much flowing water, the parasite can become more concentrated in that small amount of water and the disease is going to spread more easily."

Rogers urged residents to be on the lookout for band-tailed pigeons this winter and to report sick or dead birds.

"It's very complex, but that's part of the research, to test these ideas out and get some real proof that these things are happening," Rogers said. "My job is to determine whether these mortality events are contributing to the decline, why they occur and how many birds die when they do occur."

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