NESTING CHRONOLOGY, DENSITY AND HABITAT USE OF BLACK-CHINNED HUMMINGBIRDS ALONG THE COLORADO RIVER, ARIZONA

--Black-chinned Hummingbird (Archilochus alexandri) nests (n = 119) were located by means of systematic searches of two different riparian habitats along the Colorado River in Grand Canyon, Arizona, from April through July 1982-1987. Most nests (94%) were located in the introduced riparian shrub tamarisk (Tamarix ramosissima). Active nests were present from 22 March through 10 July, with >80% of all nests active during the peak of nesting in May. Black-chinned Hummingbird nests were located only in tamarisk-dominated habitats, where nesting density ranged from 0 to 3.5 active nests/ha, and no nests were found in native habitats dominated by honey mesquite (Prosopis glandulosa). Nests were found only in riparian habitat patches that were >0.5 ha in size, exhibited > 69% vegetative cover and had a mean canopy height of >-1.4 m. Maintenance of this Black-chinned Hummingbird population depends on the continued existence of tamarisk-dominated habitat with these minimal patch-size characteristics. CRONOLOG•A DE ANIDAMIENTO, DENSIDAD Y UTILIZACI•N DE HABITAT POR PARTE DE ARCHILOCHUS ALEXANDRIA LO LARGO DEL RiO COLORADO, ARIZONA Sinopsis.--De abril a julio de 1982 a 1987, se 1ocalizaron nidos del zumbador Archilochus alexandri, a trtlves de una bfisqueda sistemtltica, en dos habitats riparios diferentes, a 1o largo del Rio Colorado, en el Gran Cafi6n de Arizona. La gran mayoria de los nidos (94%) fueron 1ocalizados en el arbusto introducido Tamarix ramosissima. Se encontraron nidos activos desde el 22 de marzo hasta el 10 de julio, con -> del 80% de los nidos durante el pico de la 6poca reproductiva en mayo. Los nidos del zumbador, fueron encontrados finicamente en habitat dominados por el arbusto ex6tico, en donde la densidad de nidos vari6 de 0 a 3.5 nidos activos/ha. No se encontraron nidos del ave, en habitats dominados por vegetaci6n ativa como Prosopsis glandulosa. Los nidos fueron encontrados olo en parches de habitat ripario cuyo tamafio era -> a 0.5 ha, con una cobertura de vegetaci6n -> a 69% y que tenlan una altura promedio del docel a 1.4 m. E1 mantener a esta poblaci6n de zumbadores dependertl de la continua existencia de habitat dominado por Tamarix ramosissima y parches con las caracteristicas mlnimas previamente mencionadas. The Black-chinned Hummingbird (Archilochus alexandri) is a fairly common summer resident of the southwestern United States, where it Current address: P.O. Box 3741, Tucson, Arizona 85722 USA.