The Xantus's Hummingbird (Hylocharis xantusii) was first collected by Hungarian naturalist John Xántus de Vesey in 1859 and described by George Lawrence ( Lawrence 1860 ), who named it in honor of Xántus. It is a stocky, medium-sized hummingbird with a white postocular-stripe contrasting strongly with a broad, blackish auricular mask; cinnamon belly; mostly rufous tail; and a straight bill. Lamb ( Lamb 1925a : 90) described this hummingbird as having a "gentle disposition."
Xantus's Hummingbird is endemic to cen-tral and southern Baja California, Mexico, but vagrants have been reported in southern California and British Columbia. Although it is an endemic species with a restricted range, it is fairly common within its range. It can be found in open montane forest, clearings, brushy hillsides, canyons, second growth, gardens, orchards, and arid montane scrub, often near fresh water.
Little is known about the biology and ecology of the Xantus's Hummingbird. A few early studies described the appearance and plumage of the species from various seasons and localities ( Bryant 1889 , Brewster 1902b , Ridgway 1916 ); Grinnell ( Grinnell 1928b ) summarized the distributional records of the time. Lamb's ( Lamb 1925a ) study provided key information on aspects of behavior, nest-site location, diet, habitat preferences, and especially the breeding of the species. More recently, Arriaga et al. ( Arriaga et al. 1990 ) examined the ecological relationship between Xantus's and an ericaceous madrone tree (Arbutus peninsularis) that grows on hillsides and canyons of the pine-oak forest and is endemic to Baja California.
The migratory tendencies of the Xantus's Hummingbird are not well known; breeding has been documented in different months for different latitudes. Although nests of the species have been found and a few have even been briefly monitored, descriptions of the actual nest cycle and of the young are lacking. As with other hummingbirds, breeding activities probably vary with the availability of nectar resources, but this has not been directly examined in Xantus's Hummingbird. There are some descriptions of the plants it uses for nec-tar sources, including an endemic madrone that appears to require the hummingbird for pollination ( Arriaga et al. 1990 ). No detailed studies are available, however, measuring mutualisms or coevolutionary interactions between Xantus's Hummingbird and any of its potential nectar resources. Much remains to be learned about the biology of this striking species.