The hummingbird in these photos is clearly different from any species found in Arizona. The red base of the bill suggests Broad-billed, a species widespread in southern Arizona and a rare but fairly regular visitor to Boyce Thompson Arboretum. However, its size, the amount of blue on its crown, and the large areas of pure white on its underparts rule out this possibility. Violet-crowned Hummingbirds are even more extensively white below than this bird in all plumages, but a close relative, the Azure-crowned Hummingbird (Red-billed Azurecrown) of eastern Mexico and Central America, has similarly marked underparts. This species is a potential stray to the Rio Grande Valley of southern Texas.

Azure-crowned is not only unlikely from a geographic standpoint, but the field marks are inconsistent with this ID. There's too much red at the base of the upper mandible (maxilla), the tail is dark and deeply notched, and the undertail coverts appear pure white rather than dull olive gray with broad pale margins.

When a bird shows characteristics inconsistent with any known species, hybridization is one logical explanation. The size, overall shape, extensively snow-white underparts, and dull back color point to strongly to Violet-crowned parentage, while the color and shape of the tail suggest Broad-billed as the other half of this hybrid combination.

Violet-crowned × Broad-billed is a comparatively rare hybrid combination among North American hummingbirds, having been recorded only twice before: March 1887 at Nacosari, Sonora and July 1905 at a mining camp in the Huachuca Mountains. In 1891, William Brewster took the original specimen to Great Britain and showed it to ornithologist Osbert Salvin, who identified it as belonging to a species unknown to science. Brewster described the new species and named it Salvin's Hummingbird (Uranomitra salvini). In 1934, Ludlow Griscom reevaluated "Salvin's Hummingbird" and determined it to be a hybrid Violet-crowned × Broad-billed. More recent authors have either agreed with this assessment or maintained that it is simply a plumage variation of the northern subspecies of Violet-crowned. It is unlikely that the bird illustrated above is merely an aberrant Violet-crowned, but only detailed examination is likely to settle this controversy.

Thanks to Jack Holloway and Rich Ditch for use of their photos and Troy Corman for contributing background information for this page.