These are Hooded Mergansers. Why do some birds have such a dramatic difference in the appearance of the males and females?
Answer:
Dramatic plumage can be an indicator of the health of the bird. Birds that have parasites or diseases have inferior feathers, and are not as likely to be chosen as a mate. This can also help the males avoid fights, letting them show that they are strong and healthy, without having to risk injury.
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