A previous paper (Scott and Grant 1969) discussed the plumages of the Common Tern Sterna hirundo and the Roseate Tern S. dougallii that are equivalent to the so-called 'portlandica' phase of the Arctic Tern S. paradisaea. Field differences between Common and Arctic Terns in this plumage were described and sketches of two different 'portlandica' Common Terns were included (plate 23a). The possible caus...

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