Loss of oak woodlands to vineyard development in California is a growing concern to conservationists. Analyzing breeding performance of birds that nest in and around vineyards versus those that nest in nearby native habitat can provide information on the suitability of vineyard environments to birds. We placed predator-protected nest boxes in vineyard and oak-savannah habitats and monitored nest-box occupancy, nesting success, and life history characteristics of Western Bluebirds (Sialia mexicana) that used the boxes. Western Bluebirds were common occupants in both habitats, occupying >50% of available nest boxes. Analysis using program MARK revealed that nest survival was not associated with habitat type; however, clutch size was greater and nests were initiated earlier in vineyard than in oak-savannah habitat. Our results suggest that when naturally occurring nest sites are limiting, vineyards could be converted to good breeding habitat for Western Bluebirds with the addition of nest boxes. Nest boxes, however, should not be viewed as a remedy for the chronic problem of habitat loss and degradation.