Most of the range of olive warblers is in the coniferous forests of Mexico, but they do breed in AZ and NM. This is considered a non-migratory species, but New Mexican populations leave the state from November through February. Plumage of males is mostly grey with a tawny orange head and breast. In females and juveniles, the orange is replaced by yellow and the facial black mask is more diffuse. This is a medium sized bird but the species shows clinal variation in size; northern populations are larger than those in the south. Likewise, northern birds are less brightly colored than those from the south. Males sing throughout the year, delivering a clear whistling song from the top of the tree canopy.