Feeding Ecology of Hummingbirds in the Highlands of the Chisos Mountains, Texas

-The feeding habits of three species of hummingbirds were examined during the summer of 1975 in the highlands of the Chisos Mountains, Big Bend National Park, Texas. The Blue-throated Hummingbird (Lampornis clemenciae), the Broad-tailed Hummingbird (Selasphorus platycercus) and the Rufous Hummingbird (S. rufus) were common in the study area, the first occupying the mesic Cypress-Pine-Oak Association and the other two species inhabiting the Pinyon-Juniper-Oak Woodland. Blue-throated Hummingbirds were almost exclusively insectivorous prior to the flowering of mountain sage (Salvia regla), after which their diet began to include its nectar. Males fed outside their territories in the morning at nectar sources in the juniper woodland. Selasphorus hummingbirds overlapped extensively in diet. S. platycercus was common prior to the full flowering of sage, and migrating S. rufus entered the area later, when the sage was in bloom. Males of both species typically established territories around large patches of the sage and open areas while females were relegated to smaller Salvia stands in the woodlands. Such a pattern is apparently maintained by the aggressiveness of the dominant males. Recent examinations of tropical nectarivorous guilds (Feinsinger 1976, Feinsinger and Colwell 1978) have shown that such communities are organized about the distribution of nectar resources. Niche differences among tropical hummingbirds may result from aggression (Wolf 1970, Lyon 1976, Wolf et al. 1976), different foraging patterns (Linhart 1973, Feinsinger 1976), microhabitat selection (Stiles and Wolf 1970), or the exploitation of specific resources by highly specialized species (Snow and Snow 1972). Far fewer hummingbird species inhabit temperate areas than the tropics, and most species are migratory. North American hummingbirds differ from one another ecologically in the same ways as their tropical counterparts. These differences include habitat selection (Pitelka 1951, Lyon 1973, Yeaton and Laughrin 1976), spatial distribution of individuals through territoriality (Pitelka 1942, Cody 1968), and variations in foraging patterns (Feinsinger and Chaplin 1975, Yeaton and Laughrin 1976). We examined the feeding ecology of three interacting hummingbird species with emphasis on their habitat partitioning and timing. We hypothesized that these characteristics would be important to the organization of a nectarivore guild where nectar sources are uncommon. Such conditions appear to occur in the highlands of the Chisos Mountains during the summer. STUDY AREA AND METHODS Data were obtained in the Chisos Mountains (29'15', 103'18') of Big Bend National Park, Brewster Co., Texas during the summer of 1975. The study area was located in Boot Canyon (2,072 m) and centered in two highland plant associations: the Cypress-Pine-Oak Association and the Pinyon-Juniper-Oak Woodland (Wauer 1973). We also examined open areas located within Boot Canyon because they contained patches of nectar-producing flowers and were often visited by the hummingbirds. The Cypress-Pine-Oak Association (henceforth "cypress woodland") occurs in the most mesic sections of the Chisos highlands, Pine and Boot canyons, where annual rainfall may exceed 500 mm (Wauer 1973). In the study area, this association occupies a narrow band along Boot Creek and is less diverse than the other woodland association. Arizona cypress (Cupressus Arizonica), pifion pine (Pinus cembroides) and Graves oak (Quercus gravesii) predominate. The Pinyon-JuniperOak Woodland (henceforth "juniper woodland") is the most common plant association in the Chisos Mountains and in our study area. Typically, it contains pifion pine, emory oak (Q. emoryi), Graves oak and Mexican drooping juniper (Juniperus flaccida), which can be found on north-facing slopes from 1,650 m to 2,300 m. Within Boot Canyon, the juniper woodland adjoins the cypress woodland. Plant species diversity is greatest in the few open or exposed areas along upper Boot Creek, where herbaceous growth is abundant. Nectar sources were most plentiful in these areas.