(MercuryNews) - Firefighters are battling two small wildfires fueled by gusty winds in Northern California wine country.

State fire officials say the Yellow Fire in Sonoma County north of Calistoga began around 2 a.m. Wednesday and has burned less than half a square mile. The Silverado Fire near Yountville in Napa County started a couple of hours earlier and has burned less than a tenth of a square mile.

State fire spokesman Daniel Berlant says neither fire is threatening structures. The causes are under investigation.

The Napa fire is 40 percent contained, with full containment expected later Wednesday morning. There is no time estimate on containment of the Sonoma County blaze.

A fire in the Central Valley county of Madera, meanwhile, that has burned less than half a square mile is 50 percent contained