Although no human cases have been reported, authorities say this is a strong reminder that West Nile virus is being actively transmitted across the region.
"Our concern is that it's early, and it's abundant. Four species of birds are involved -- and spread throughout the county from Brea to San Clemente," said Michael Hearst from the Orange County Vector Control District.
The virus is usually transmitted by mosquitoes.
In 2004, 66 people were infected with West Nile virus in Orange County, four of them died.