Foraging Habitat and Its Effects on the Tricolored Blackbird’s Breeding Distribution and Abundance in the Sierra Nevada Foothills, California

Abstract

The grassland-dominated eastern Central Valley and Sierra Nevada foothills of California, from Placer to Stanislaus counties, supported at least 43,000–55,000 breeding Tricolored Blackbirds (Agelaius tricolor) annually from 2014 to 2018—about 30% of the statewide population. We found that within 5 km of a colony, the extent of land cover used for foraging, grassland–herbaceous, and to a lesser extent annual crops and irrigated pasture, exceeded their proportions in this region as a whole, suggesting that nesting Tricolored Blackbirds select colony sites where these habitats are more abundant. Other land-cover types were underutilized for foraging, suggesting avoidance. The probability of Tricolored Blackbird occupancy of blocks of 100 km2 was strongly associated with the extent of the selected land-cover types. The relationship between average density of the breeding population and the extent of the selected land-cover types was significant but weaker, implying that other factors are important in determining density. From 2014 to 2018, development and mining eliminated or degraded 9 of 79 colony sites and made 4 others unsuitable by reducing the extent of nearby foraging habitat, although the total breeding population in the region did not decline. We recommend that conservation measures for the Tricolored Blackbird in federally and state-approved habitat-conservation plans in Placer and Sacramento counties, which support the largest breeding populations in the central Sierra foothills but where urban development is rapid, be reevaluated on the basis of recent colony locations and recognition of the critical role of grassland, annual crops, and irrigated pasture as foraging habitat.