Energy limitation of hummingbird populations in tropical and temperate communities.

Regular censuses were conducted at both a temperate alpine and a tropical lowland site to determine seasonal changes in the composition of hummingbird communities and the availability of their food. From these data we calculated the total daily energy demand by the hummingbirds (Daily Energy Expenditure; DEE) and the daily energy supply available from floral nectar (Daily Energy Production; DEP) for each community census. Despite differences in habitat type and hummingbird community structure between these two sites, the hummingbird populations were often at or near carrying capacity. On average, all of the daily nectar production was cropped by the birds. We suggest that the supply/demand economics of coevolved mutualisms favour the evolution of complete resource use.