10,000 Band-tailed pigeons die over the winter in California, but cause remains unexplained

Band-tailed pigeon

An unexplained epidemic ravaging band-tailed pigeons in California has resulted in more than 10,000 of the bird's deaths this winter.

Dead birds have been found in 21 counties in California, including in the Sacramento region, marking one of the largest die-offs recorded. It's particularly troubling because the species reproduces at a lower rate than similar birds.

Researchers are at a loss to explain the cause, but they suspect that the multiyear drought and changes in acorn supply may be to blame.

The band-tailed pigeon is a common, medium-size, gray-feathered bird known for a white crescent on its neck and a long tail that sports a pale band at its tip. The bird is a relative of the passenger pigeon - a species once as common as the band-tailed pigeon, but now extinct.

The band-tailed pigeon population has been known to ebb and flow. However, seven of the last 10 years have seen an unusually high number of deaths, said Krysta Rogers, avian scientist with the state's Department of Fish and Wildlife.

Two years ago, Rogers began to investigate the historical record to establish mortality events over time and try to detect a pattern. She found the distant and recent past has not been kind to band-tailed pigeon populations.

Records date back to 1941, showing that the bird's population has been declining, Rogers said.

"I found that mortality had been reported at least since 1945 in California," Rogers said. "Up until probably the last 10 or 15 years those mortality events were fairly sporadic - you could go three years, sometimes five or 10 years without one."

In the winter of 2011-12, there were eight events that led to approximately 1,500 band-tailed pigeon deaths, according to data reported by UC Davis project scientist Yvette Girard , Rogers and others in the journal Infection,

The culprit is a disease called trichomonosis, which is spread when the parasite is readily passed between bird species wherever they congregate in large numbers. But researchers don't know why the parasite is spreading so quickly.

One theory blames climate change and the current multiyear drought, said Rogers. The situation has led to more birds flocking to scarce water sources.

In the Sacramento Valley, rice farmers have let their normally flooded fields lie fallow because of reduced water allocations. That has caused a similar situation where avian botulism is spreading among birds that congregate in whatever remaining wetland can be found along the Pacific Flyway.

"Band-tailed pigeons like to drink water daily and they will fly great distance for that," said Rogers. "When you have less natural sources available, this brings a lot of birds into potentially close contact."

The availability of the acorn, the bird's favorite food, is also a factor.

Researchers believe that less acorn availability overall will bring many birds to one area as opposed to many.

"Preliminary results indicate that wherever you have high concentrations of food like trees that bear acorns you're more likely to have mortality events in those locations," said Rogers.

The decline in band-tailed pigeon populations over time may be related to a decline in oak habitat or acorn scarcity, said Walter Koenig, bird population specialist at the Cornell Lab of Ornithology. In the last 60 years, the oaks that supply acorns for band-tailed pigeons have steadily disappeared.

Some of those declines have been natural, like sudden oak death, identified in 2000, which killed millions of oaks in California. Forest fires also have hurt acorn supply and bird habitat. The 2013 Rim fire burned 257,314 acres in Tuolumne and Mariposa counties, which is band-tailed pigeon habitat.

Other oak declines are man-made. An estimated 18,000 acres of oak woodland was lost each year due to conversion to subdivisions, roads or vineyards between the 1990s and early 2000s, according to the U.S. Forest Service.

In California, band-tailed pigeons are typically found in the coastal mountains and foothills to the west and in the Sierra foothills to the east. But this winter, acorns were not available at higher elevations, and this may have forced the birds to find food at lower elevations, Rogers said.

As a result, the band-tailed pigeon has been seen in unlikely places, especially on the Central Valley floor. Bird deaths have occurred in places like Granite Bay and the pigeon has been spotted where it was once rare - along the American River Parkway, in Oroville and within Yolo County.

Some avid birders, like Sacramento resident Chris Conard, are seeing the bird close to home for the first time.

"I've spotted a flock of 200 birds around Folsom and also in the Rancho Murieta area," Conard said, calling it "unprecedented."

"I do not see the birds every year. I've seen one or two, here and there, and maybe a flock of 10 or 15," he said.

Rogers said that mortality numbers are in the rough estimate stage and being culled from birders, game wardens, wildlife biologists and the public, as well as from her field research and work being done at UC Davis.

"Birds are dying in locations where people are not encountering them," she said. "So, we're likely underestimating mortality."

Researchers must still determine if drought or climate change is spurring mortality events. They are also examining habitat loss as well as the band-tailed pigeon's low reproductive rate.

The band-tailed pigeon reproduces, on average, one chick per year. Similar species, like the mourning dove, can produce from two to six chicks a year.

"When you have high mortality it can take the population years to get back up to its prior levels," Rogers said. "If we keep having these mortality events year after year, there is concern it could cause a significant decline in their population."

Also troubling is that band-tailed pigeon die-offs can spill over into other bird species, said Girard.

"There is a risk of parasite spillover from band-tailed pigeons to raptor species that regularly prey on doves and pigeons," Girard said.

There have been no incidences of the parasite infecting humans, she said.