Costa's Hummingbird occupies hot deserts and other xeric habitats west of the Continental Divide and south of the Great Basin: “the dry desert hummingbird par excellence ” ( Phillips et al. 1964a : 62). Even though Sonoran desert scrub is the heart of its range, it is not exclusively a desert species; it leaves the deserts of Arizona and southern California in summer. The restriction of Costa's Hummingbird to the Far West is due probably to a Pacific slope origin and to Sonoran Desert climate, which permits flowering in winter and spring.
Although partial to xeric habitats, Costa's Hummingbird is no less attracted to nectar than are other hummingbirds. It forages for small arthropods in typical hummingbird fashion—by fly-catching. Nectar supplies vary greatly during the 6 to 9 months each year that this species inhabits desert scrub in Arizona and California. Individuals are common only when nectar plants are flowering well, yet rarely are they common enough during peak flowering to harvest a majority of chuparosa (Justicia californica) or ocotillo (Fouquieria splendens) nectar.
Migratory behavior is geographically variable and only partly understood. In Arizona and southern California deserts, abundance of this species peaks in March and April, when most breeding occurs; by late June most birds have dispersed, possibly to habitats near the Pacific Coast.
California chaparral and sage scrub host Costa's Hummingbirds from March (before the desert exodus) to September; most birds then withdraw, perhaps mainly to Mexico. Little is known of migration in Mexico, except that certain regions are primarily winter quarters.
As in other Nearctic hummingbirds, males perform spectacular courtship dives, but Costa's Hummingbirds give a distinctive whistled vocalization while perched and diving. Although displays are the way many observers first see the species, no one has seen an entire successful courtship sequence (ending in copulation). There is no evidence of pair cooperation after mating, which is typical of most hummingbird species. Timing of breeding varies with latitude, habitat, and nectar supply. Females nesting in desert scrub sometimes rear 2 successive broods in 4 months, but such attempts appear to be rare. Rearing of 2 broods per year in different habitats (first desert, then chaparral) has been hypothesized but is difficult to prove.
The status of this species remains poorly known in many portions of its range. In some areas it appears to be expanding its range, in others there is a resurgence into historical areas of occurrence. Regarded as sedentary until recently, the species has proved remarkable in dispersal ability, with sporadic or vagrant occurrences north to Alaska, east to Kansas, and south at least to the Mexican state of Jalisco.
The basic biology of this species remains poorly understood. Females and young are difficult to distinguish from those of Black-chinned (Archilochus alexandri) and Anna's hummingbirds (Calypte anna). This fact, coupled with recent range expansions, has clouded our understanding. Confusion dates back to the time Costa's Hummingbird became known to science (collected 1837 in Magdalena Bay, Baja California; described and named 1839 by Jules Bourcier in honor of Louis Marie Pantaleon Costa, Marquis de Beauregard; Palmer 1918 ). Among the most significant works on Costa's Hummingbird are Woods 1927 , Stiles 1973 , George George 1987c , George 1987b , Baltosser 1989a , and Scott 1989 —life history, ecology, distribution, and status; Stiles 1971a and Baltosser 1987 —identification; Lasiewski Lasiewski 1963 , Lasiewski 1964 , Brice and Grau 1991 , and Powers 1991 —physiology; and Wells et al. 1978 —hybridization and behavior.