Species, Age and Sex Identification of Ducks Using Wing Plumage
Separation of Red-breasted and Common Mergansers
The white greater secondary coverts on common mergansers hide the black bases of the secondaries and adult males normally show only one black bar, i.e., the bases of the greater secondary coverts. Females and immatures of both sexes normally show none. Adult females and a few immatures have a different type of bar due to black tips on the greater coverts rather than exposed bases of the secondaries. On red-breasted mergansers of all ages and sexes, the white greater secondary coverts do not cover the black bases of the secondaries. Thus, the wings of adult males show two black bars and the wings of adult females and immatures of both sexes show one black bar anterior to the speculum. On red-breasted mergansers, the most distal tertial is partially white on adult females and both sexes of immatures. On common mergansers of the same age and sex groups, this feather is dark gray.
Sex of approximately 97% of immature common mergansers can be determined from wing notch-length measurements provided primary growth is complete. In addition, most immature males have several more distal middle and lesser coverts that are a lighter shade of gray than the surrounding coverts. This light patch of feathers is subject to considerable variation in shade, but it does not occur on immature female wings.
Red-breasted mergansers do not show the same degree of difference in wing length between sexes as common mergansers. Among immatures, the degree of overlap is great enough to make measurements of little use for determining sex. Also, because immature males do not have pale coverts similar to those of common mergansers, their sex cannot be determined with accuracy.
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