As one proceeds from N. Temperate regions towards the Tropics, the proportion of non-passerines in the avifauna increases. Number of individuals per species for a given area decreases towards the tropics for passerines but not for non-passerines. At all latitudes non-passerines are less abundant than passerines. This is interpreted as support for the notion that phylogenetically older non-passerine species are insufficiently plastic in their niche requirements to colonize temperate areas, tropical niches being smaller and less subject to change.