Nesting habitat of Warbling Vireos across an elevational gradient in the southern Sierra Nevada

Populations of Warbling Vireos (Vireo gilvus) are declining in California, apparently due to low reproductive success. From 1989-2002, I studied the nest-site selection and reproductive success of Warbling Vireos across an elevational gradient in the southern Sierra Nevada. Warbling Vireos regularly nested in upland coniferous forests with few or no deciduous trees, and tree species used by nesting vireos included five species of conifers and four species of deciduous trees. Overall, hardwoods were used more than expected based on their availability, but 69% of all nests were in conifers. Hardwood trees were found only in low and mid-elevation ponderosa pine (Pinus ponderosa) and mixed-conifer sites. In low-elevation ponderosa pine habitat, 87% of nests were in hardwoods, with 67% in California black oaks (Quercus kelloggii), a species that typically occupies upland sites. In mixed-conifer sites where reproductive success was high, 65% of nests were in incense cedar (Calocedrus decurrens) and California black oak was the next most commonly used species. Because fire suppression has likely increased numbers of shade-tolerant tree species like incense cedar, shade-intolerant species like black oaks may have been more important as a nest substrate for vireos in the past. Only conifers were used as nesting substrates at higher elevations. Nest success was greater for Warbling Vireos that nested in tall trees in areas with high basal area. My results suggest that Warbling Vireos in the Sierra Nevada would benefit from management activities that encourage retention and recruitment of California black oaks at lower elevations, and development of stands with large trees, dense foliage, and semi-open canopy throughout their elevation range.