To elucidate the causal factors responsible for diversity gradients in avian communities, avian populations were studied for 12 months in seven lowland tropical areas in the Republic of Panama and compared with populations in structurally similar habitats in Illinois. Resident (breeding) species made up 38-49% of the species on temperate areas but only 20-36% of the species in similar tropical habitats. There were significantly more species of irregular occurrence in mature tropical habitats. Tropical grassland avifaunas were no higher in diversity (number of species and information measures of diversity) than those of temperate grassland areas, but shrub and forest habitats in the tropics had higher diversity as measured by either diversity measure. The number of species was proportionately higher than the information measure because of the relatively small effect of the many rare species on the information measure. Determination of the biomass and energy relations for the bird populations showed that higher total population size in tropical areas is correlated with smaller bird size and reduced individual energy requirements. When only breeding seasons are compared, the reduced energy requirements are a result of the higher temperatures in tropical areas. Twenty-five to fifty per cent of the increased number of tropical breeding species when compared to similar temperate habitats results from the addition of a new food source, i.e., fruits. Additional species are primarily insectivores. The increase in insectivores seems to be due to additional subdivision of the resources and exploitation of a new kind of insect food, especially by species that depend on relatively large insects. Since energy requirements of tropical and temperate avifaunas are about the same, increased productivity is not related causally to the increased tropical diversity. The stratal distribution of species in several tropical (lowland and montane) and temperate avifaunas indicates that avian communities may subdivide the vegetation profile similarly throughout the world despite sharp differences in the number of species in the various areas. Bark species are about equally numerous in Panama and Illinois forest, but ground, low, medium, and high strata contain increased numbers of species in the tropics. The shift away from classical defended territories in mature tropical habitats seems to be correlated with patchy distribution of food resources in frugivores and ant-following species and reduced food abundances in other species. As a result of this food distribution, the frequency of flocking increased in shrub and forest habitats. In grassland habitats, which experience greater seasonal changes in precipitation, the spacing systems of birds are more similar to those in all temperate habitats studied. Degree of territorial defense against conspecifics and amount of interspecific flocking is inversely related to the distribution of food resources; more patchy or unpredictable food distributions, or both, result in fewer species defending exclusive territories. The major factor related to the increased diversity of tropical avifaunas seems to be the relative stability of food resources resulting from environments that are more or less @'equitable@' throughout the year. Both within- and between-habitat increases in diversity of avian communities are discussed. The stability-time hypothesis is discussed in light of available data on organization of avian communities.
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