POLYMORPHISM IN THE WHITE-THROATED SPARROW, ZONOTRICHIA ALBICOLLIS (GMELIN)

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Abstract
A sample of 286 museum and 199 live specimens of white-throated sparrows were examined for morphic differences. Two morphic types in both sexes are described according to the color of the median crown stripe. These are the white-striped morph and the tan-striped morph. Regardless of sex, white-striped morphs have more black on the lateral crown areas, less streaking on a wider and grayer chest band, less intense black on the malar markings of the white throat patch and brighter yellow on the superciliary stripe. The proportions of white-striped males and tan-striped females increased from Newfoundland to the Northwest region. White-striped morphs mate selectively with tan-striped morphs. Evidence was presented to show that these morphic types are genetically determined, and once attained, do not change with age. Mechanisms governing polymorphism are discussed in an attempt to explain the existence of the morphic types in the white-throated sparrow.