Latitudinal Gradients in Colors and Patterns of Passerine Birds

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Abstract
Whether tropical birds in general are more colorful than their non tropical relatives, using all 784 native passerine species of North and Middle America, was examined. The entire membership of major taxa in tropical and non-tropical regions using objectively quantifiable components of colorfulness was compared. Many statistically significant latitudinal gradients were found, but none of them offers much support for the concept of tropical colorfulness. In terms of both the porportion of the bird''s surface covered by bright color and the number of different bright colors, the passerine faunas of all the regions are remarkably similar. Dull species predominate everywhere. Significant sexual dichromatism and the presence of a molt to a duller non-breeding plumage both become more common with latitude. Strong iridescence is more frequent in the tropics, and an explanation is offered.

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