Vireos are quite difficult to photograph and identify and this is not a great photo. They are small, active birds that don’t remain in one place for long. This individual seems to be the plumbeous form of the solitary viero Vireo solitarius plumbeus, sometimes assumed to be a distinct species Vireo plumbeus. The photograph was taken in the grounds of Ecosur. The species is a winter migrant
The similar Cassin’s vireo is a resident of Baja California but winters north of the Isthmus. The identity of the birds in San Cristobal requires checking.