Hummingbirds in high alpine habitats of the tropical Mexican mountains: new elevational records and ecological considerations

摘要

Most hummingbird species are distributed in tropical lowlands and mountains with few species reaching the alpine environments of high mountain peaks. Field surveys were conducted in nine alpine sites across four of the highest volcanoes in central Mexico for three years to document hummingbird occurrence, habitat use, and some aspects of behavior. We also searched through historical museum collections and online databases to document early records of hummingbirds in Mexican alpine habitats. In addition, we conducted a search through historical herbarium collections of ornithophilous plant species in alpine habitats, and a literature search on glacier retreats in Mexico to establish past and present elevational limits of alpine vegetation. We recorded the presence of four hummingbird species using floral resources across the nine study sites: Cynanthus latirostris (Broad-billed Hummingbird), Hylocharis leucotis (White-eared Hummingbird), Lampornis clemenciae (Blue-throated Hummingbird), and Selasphorus platycercus (Broad-tailed Hummingbird). The latter two species were recorded frequently throughout the growing season (May–October), whereas C. latirostris was observed on one day in July, and H. leucotis on two days in April and May. Three of the four species were observed foraging from Castilleja tolucensis, an alpine Mexican endemic herb, at 4428 m above sea level, and all four species foraged from Penstemon gentianoides at the lower alpine sites 3900–4100. At the highest elevations, we observed hummingbirds hovering and perching on rocks while feeding from flowers at ground level. Both male and female of L. clemenciae and S. platycercus were present at patches of P. gentianoides; however, with S. platycercus, males accounted for more than 80% of the individuals observed and more than 95% of individuals observed at the highest elevations. Additionally, a survey of historical records from museum collections and bird sightings from e-databases revealed only three early records of hummingbirds in alpine sites, with only one observed earlier than 1987. This suggests that hummingbirds were not recorded earlier in alpine habitats because of a lack of sampling. Nevertheless, evidence from glacial retreats and plant distributions along an elevational gradient also indicate that hummingbirds have expanded their elevational range as new habitats have opened up for colonization associated with global climate change. The results reported here are the first records of the four hummingbird species in the Mexican alpine grassland and one of the highest altitudinal records for hummingbirds in North America. We discuss the importance and conservation implications of alpine ecosystems in Mexico as habitats for hummingbirds.