Day 197: Roger Tory Peterson describes Purple Finch (Carpodacus purpureus) as "a sparrow dipped in raspberry juice." While that aptly describes the appearance of the male, the female is a rather blandly coloured bird whose streaky breast and flanks make her hard to distinguish from House Finch where the two occur together (female Purple has an eye stripe, female House has narrow white wing bars; male House has "shingles," i.e., a streaky breast similar to that of the female, a feature the male Purple lacks).
Sexual dichromatism (a difference in colour) is common in birds. Less common is true sexual dimorphism in which there are other differences in characteristics and size. It has been theorized that the female's lack of vivid colouration provides her with greater security from predators, but why would the males be brightly coloured? It is known that birds can see a much wider range of hues than are recognizable by humans, and a Scientific American article suggests that the female judges the male's health and vitality by the strength of his colour, holding up the Purple Finch as the paradigm. If that's true, my yard should be over-run by little Purple Finches in a few weeks!
I've just finished more pages in Darwin's Descent of Man than I ever cared to read about color variations between male and female birds and no real conclusion as to why--one of those mysteries we can happily live with.
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