Metabolic Rates, Resources, and the Occurrence of Nonpasserines in Terrestrial Avian Communities

Abstract

The possible ecological importance of the difference in metabolic rates of passerine and nonpasserine birds is discussed in an attempt to explain 3 general patterns of passerine and nonpasserine species abundance. These patterns are: there are relatively more nonpasserine species in tropical than temperate regions; tropical nonpasserine species are usually larger than passerines of similar trophic status; tropical islands have relatively more nonpasserines than the tropical mainland. A possible explanation for these patterns is discussed that considers metabolic rates, activity levels and resource availability. It is suggested that passerines possess both higher metabolic rates and higher levels of activity, factors which give these species a competitive edge in feeding on abundant resources in highly competitive situations. On rarer resources (which in the tropics are usually the larger resources), the metabolically more conservative nonpasserines can support larger populations on a given amount of resource and expend less energy during the time spent looking for these rarer resources. This argument supports the observed trend toward large-sized nonpasserines. Because many of these large tropical resources disappear with increasing latitude, it also explains the decreasing importance of nonpasserine species in temperate avifaunas. The argument is extended to insular situations where the maintenance of adequate densities through stress periods in crucial to the survival of a species. Because of their more conservative metabolic demands, nonpasserines can achieve higher densities on a given amount of resource and should be favored over time. With increasing insular stresses, we should expect the increased dominance of nonpasserine species, a pattern which is supported by the data. Closely related nonpasserines can be more similar in size than closely related passerines. The increased densities of land-bridge relict species on Trinidad and Tobago are presented as further support for the increased dominance of nonpasserine species on islands.