LEAST BELL’S VIREO NESTS IN THE SIERRA SAN PEDRO MÁRTIR, BAJA CALIFORNIA

Abstract

Surveys for the Least Bell’s Vireo (Vireo bellii pusillus) during 18 field trips to four localities near the west base of the Sierra San Pedro Mártir in Baja California, Mexico, yielded detection of 42 individual vireos and 41 nests, 15 in various stages of use and 26 old. Arroyo willow (Salix lasiolepis) was the plant most frequently found supporting these nests, and the big sagebrush (Artemisia tridentata) was second, even though it has not been reported previously as a site of Least Bell’s Vireo nests. Other plants newly identified as supporting nests of this vireo were the single-whorl burrobrush (Ambrosia monogyra), hoary coffeeberry (Frangula californica tomentella), and holly-leaf cherry (Prunus ilicifolia). Rancho Meling, where we found the most vireos, has long been known as a site for the species, but ranchos El Potrero and La Costilla represent newly identified sites. Only one of the 15 active nests had been parasitized by the Brown-headed Cowbird (Molothrus ater).