It's getting cooler outside and grows dark earlier, but West Nile virus remains active in Sonoma County, as evidenced this week by two birds found dead from the disease in Santa Rosa and Rohnert Park, the Marin/Sonoma Mosquito & Vector Control District said.

A sentinel chicken in Cloverdale, part of a flock established to monitor presence of the disease, also tested positive for antibodies to the potentially fatal virus, officials said.

The spread of West Nile slows during the cooler months, but some of the more than 20 species of mosquito are particularly active when it's cold, the vector control agency said.

Residents are urged to water-proof their yards to prevent standing water from winter-season rains in everything from buckets to planters to kiddie pools and tires. Anything that can hold water for five days can contribute to mosquito production.

More information on water-proofing is available at