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The taxonomic relationships of the Yellow-breasted Chat (Icteria virens) have been uncertain since its discovery more than 200 years ago. Although usually considered to be a New World wood warbler (Parulini) it possesses structural and behavioral characteristics that seem aberrant in comparison with the typical members of that group. The relationships of Icteria were investigated by comparing its single-copy DNA sequences with those of other New World nine-primaried oscines and representatives of other oscine families, using the technique of DNA-DNA hybridization. The data indicate that Icteria is a paruline warbler and it should continue to be included within that group. The study of Icteria provided the basis for an examination of the suggestion by several authors that the proteins of birds and, by extension, their DNAs, evolve more slowly than do those of other animals. Evidence is presented indicating that the alleged differences are due, at least in part, to differences in the human perception of the boundaries of taxonomic categories in birds versus most other organisms. Birds are taxonomically oversplit at all supraspecific levels, but small, nocturnal mammals and other groups are probably overlumped at all levels. The lack of equivalence between the taxonomic categories of birds and those of other animals results in an erroneous evaluation of their rates of macromolecular evolution. DNA hybridization data indicate that the vireos (Vireoninae) are not closely related to the wood warblers, or to other New World nine-primaried oscines. We have shown elsewhere that the vireos are members of a large, varied "corvine assemblage."