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1. As one proceeds from North Temperate regions towards the Tropics, the proportion of non-passerines in the avifauna increases. 2. The number of individuals per species for a given area decreases towards the tropics for the passerines but not for the non-passerines. 3. At all latitudes, however, non-passerines are less abundant than passerines. This is interpreted as support for the notion that the phylogenetically older non-passerines species are insufficiently plastic in their niche requirements to colonize temperate areas, tropical niches being smaller and less subject to change.
The American Naturalist © 1960 The University of Chicago Press