170 THE ZOOLOGIST. 



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* 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.f 



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 oenanihe and Ruticilla phosnicurus, 

 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 satis- 

 factory explanation. 



III. All British birds which build covered nests have both 

 sexes alike, and are sombre in colouring. They are small 



* Newton's ' Dictionary of Birds,' p. 101 ; Wallace's ' Darwinism,' 

 pp. 217-226. 



f Newton's ' Dictionary of Birds,' p. 100. 



