contrast with the grey or black of their upper parts; as, for instance, Terns, Auks, Gulls, Divers, Grebes.
Here it must be pointed out that (1) parts of plumage which are never displayed are dull-coloured; (2) parts which are out of sight when the birds are at rest, but which appear during flight, or under excitement, are often ornamented with beautiful colours or patterns. Examples may be found among Pheasants and Sandpipers. Conspicuous marks exposed during flight possibly act as danger signals.[1] Probably all such showy colours and patterns are made use of in courtship and in battle.
The larger Gulls take three or four years before they attain to mature plumage. The plumage of quite young Gulls is sombre. The stages through which they pass before arriving at maturity are supposed to be recapitulations of former states of colouring.[2]
II. Birds which nest in holes.
As a rule, both sexes of such birds have conspicuous plumage; as Woodpecker, Kingfisher, Sheldrake; and, among foreign birds, Parrot, Toucan. But small birds, such as Tits, Nuthatch, are much less brightly coloured than larger ones.
There are some noticeable exceptions to this rule. In the case of the Wheatears and Redstarts, the hens are sombre in colour, and the cocks much more striking looking. According to my own experience of Saxicola œnanthe and Ruticilla phœnicurus, they place their nests quite out of sight. I have very little acquaintance with the other members of these genera, but, so far as I can gather, the nests of some species are usually quite hidden, whereas those of others may be as much open to view as are many nests of the Pied Wagtail. Is it possible that with our common Wheatear and Redstart the sombre hues of the hens' plumage date back to a time when the nest was always more exposed to view?
The Wryneck and many of the Petrels are also exceptions, as both sexes are clad in dull-coloured garb. I know of no satisfactory explanation.
III. All British birds which build covered nests have both sexes alike, and are sombre in colouring. They are small
