Don't worry: Sexual dimorphism is G-rated. It means that males and females of a species look different. Dimorphism means "two forms." Cardinals are a great example: Adult male cardinals are bright red; females are a warm brown. Similarly, in house finches and purple finches: Males are various shades of streaky red, raspberry, or orange, while females are streaky brown. Male bluebirds are brighter than their mates, too. Male downy and hairy woodpeckers have red on the back of their head, while their female counterparts do not. Not all birds exhibit sexual dimorphism; the sexes of wrens, jays, chickadees, and titmice, for example, might be obvious to each other, but are not distinguishable to humans.
In some birds that visit your feeders, sexual dimorphism is subtle. Male and female nuthatches, for example, aren’t identical. Males are brighter; females duller. The summertime plumage of both male and female American goldfinches is quite distinctive, but in winter, both grow into dull feathers. Still, the male is dull yellowish, while the female is gray. Look closely at robins: Males are a brighter orange on the breast, and the black head contrasts with a gray back. The head and back of female robins are uniformly gray. Just for the record, in a few species of birds, most notably belted kingfisher and the phalaropes—unlikely to visit bird feeders—the females are more colorful than the males!
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