Breeding Status of the Gray Vireo on the Baja California Peninsula

Abstract

Most of the breeding range of the Gray Vireo (Vireo vicinior) lies within the southwestern United States, where the population is sparse, patchy, and declining. But the species also breeds in Baja California, Mexico, where its status has not been assessed. To rectify this, in 2021 and 2022 we surveyed four mountain ranges where the Gray Vireo is known or might be expected. In the northernmost, the Sierra Juárez, we located 43 territories—an abundance strikingly greater than just across the border in Upper California. Territories were in both treeless chaparral dominated by chamise (Adenostoma fasciculatum) and redshank (A. sparsifolium), and in the extensive pinyon woodland. In the Sierra San Pedro Mártir, all 71 territories located were in Adenostoma-dominated chaparral. Extrapolation of the densities observed in these two ranges over the extent of suitable habitat implies the population of the Gray Vireo in Baja California should be over 10,000 individuals. Confirmed by audio recording, one sighting from the Sierra de Ulloa overlooking Ensenada suggests a still wider distribution in Baja California. Isolated stands of chaparral grow south of the Gray Vireo’s previously reported breeding range, on the Sierra La Asamblea. But our reconnaissance of it revealed no Gray Vireos. Survey of the Sierra San Francisco in the center of the peninsula, 3–9 April, revealed only 3 Gray Vireos, all in winter habitat containing Bursera microphylla. Therefore two molting specimens collected in the Sierra San Francisco in October 1997 imply that some individuals molt in the winter range, not a southward extension of the breeding range. Despite Baja California representing only a small part of the Gray Vireo’s breeding range spatially, it contributes disproportionately to the species’ population and therefore conservation.