The Mountain Bluebird (Sialia currucoides) is a winter migrant in San Diego County and does not breed here. The Mountain Bluebird is highly irregular in Southern California, usually localized and uncommon. But in rare invasion years it is seen in flocks of hundreds from November to mid March. Large grasslands and patches of bare dirt are the birds' principal habitats in San Diego County. The Ramona grasslands, Santa Ysabel Valley, Lake Cuyamaca, and Borrego Valley are all favored areas.

The Mountain Bluebird is quite localized in San Diego County, as grasslands large enough to attract it are few and scattered. The species' numbers are so irregular, however, that the birds may be rare or absent even at favored sites in some years. In 2000-01, however, San Diego County received perhaps the greatest invasion of Mountain Bluebirds ever recorded. Daily counts in 3 mile square areas ranged from 150 to 200 in several of the birds' wintering sites.

Mountain Bluebirds are slightly larger than Western Bluebirds. They have longer and thinner bills and their wings are longer. Mountain Bluebirds also tend to hover when hunting, much like an American Kestrel. The sky blue male Mountain Bluebird is easily distinguished from male Western Bluebirds by having blue rather than red on the breast. And the female is distinguished from the western females by her grayish rather than reddish-brown breast.