Although Costa’s Hummingbird (Calypte costae) is a common species of the arid Southwest, its status remains poorly understood or poorly documented in much of its range. This is due in part to the female and young of this species being frequently inseparable in the field by sight from those of Black-chinned (Archilochus alexandri), Ruby-throated (Archilochus colubris), and, to a certain extent, Anna’s (Calypte anna) hummingbirds. The problem is further compounded by recent range expansions and by apparent resurgence into some areas of historic occurrence where, until recently, there had been few records since the late 1800s. Aids to identification recently published by Baltosser (1987) and the references cited therein provide sufficient information to identify virtually any bird in the hand and even many seen at close distances. In addition, the call of Costa’s Hummingbird is diagnostic (Stiles 1971) and being learned by many observers. In time, therefore, some of the inherent mystery associated with this and similar species may dissipate.