Pigeon carcasses are piling up in parts of the Peninsula due to a seasonal outbreak of avian trichomonosis. It's caused by a primarily water-borne parasite that is infecting band-tailed pigeons flocking to the Pacific coast in search of acorns this winter.

California Department of Fish and Wildlife Senior Environmental Scientist Krysta Rogers said that as of March 2, 257 pigeon deaths have been reported in seven Bay Area counties, including San Mateo and Santa Clara, but that the number is only an estimate.

"It is challenging to get a precise number of cases because not everyone reports dead birds, or they may report once rather than each time an individual bird is found," Rogers said. "Additionally, some reports do not specify a number and some use words like 'several' or 'many' rather than providing an absolute number."

She added that reports tend to be biased towards more densely populated areas, "whereas dead birds in less populated areas may go unreported."

In Woodside last month, for example, one resident went online to wildlife.ca.gov to file a mortality report after he found his dog playing with a dead pigeon in his backyard. He then discovered a second dead pigeon in his yard, and spotted yet another one on a nearby road.

He compared notes with a neighbor who had discovered three dead pigeons on their street and a fourth one on a trail in Huddart Park, but did not report them.

One possible explanation for the recent concentration of pigeon deaths in Woodside is the presence of both oak trees, which produce acorns, and livestock.

"The parasite can spread rapidly between pigeons using shared water sources like bird baths, fountains, water troughs, and other relatively small water sources, where the parasites can become more concentrated,' Rogers said.

The signs of infection are lesions that choke off the airways and block feeding, leaving the birds weak and unable to fly. To prevent more birds from getting sick, it's recommended to remove small water sources when pigeons are flocking in the area.

"Other bird species may be at risk of developing avian trichomonosis, including mourning doves, various songbirds, and avian predators that prey on infected species," Rogers said.

However, she said pigeons, mourning doves and many songbirds are at a lower risk of catching avian flu because "we see most cases of avian influenza among waterfowl, other waterbirds, and avian predators and scavengers that feed on waterfowl."