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As this colour feature (speculum) occurs in
several species of related ducks, whatever the advantage (and almost certainly
it relates to social signalling for the ducks and nothing to do with predators),
the advantage was most likely established in some ancestral duck and not in the
currently existing Pacific Black Duck, which maintains
it for probably the same reason. Although the colour is not the same in all the
species. Green colour in birds is generally created by combination of structural
blue and yellow pigment. A look at variation in domestic budgerigars
demonstrates the point.
Philip
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