xii PHYLUM CHORDATA 485 



hue helping to conceal the bird among herbage, the latter on 

 snow. Frequently, as in pheasants and birds of paradise, the 

 female alone is protectively coloured, while the male presents 

 the most varied and brilliant tints enhanced by crests, plumes 

 or tufts of feathers on the wings, elongated tail, etc. etc. 

 These have been variously explained as " courtship colours " 

 for attracting the female ; as due simply to the exuberant 

 vitality of the male bird, or as helping to keep the number of 

 males within proper limits by rendering them conspicuous to 

 their enemies. Such ornaments as the bars and spots on 

 the wings and tail of many gregarious birds, such as plovers, 

 fully exposed only during flight, and often widely different 

 in closely allied species, have been explained as "recog- 

 nition marks," serving to enable stragglers to distinguish 

 between a flock of their own and of some other species. 



The toothless jaws with the horny sheaths forming the 

 bill are universal in the class. But the dimensions and 

 form of the bill vary very widely in different groups of birds. 

 It may be extremely short and wide for catching moths and 

 other flying insects, as in swifts and goatsuckers ; short and 

 conical for eating fruit, as in finches ; strongly hooked for 

 tearing the bodies of animals, as in birds of prey, or for 

 rending fruits of various kinds, as in parrots ; long, conical, 

 and of great strength, as in storks ; slender and elongated, 

 as in swifts, ibises, and curlews ; broad and flattened for 

 feeding in mud, as in ducks and geese ; expanded at the 

 end, as in spoonbills ; immensely enlarged, as in hornbills 

 and toucans. It is most commonly bent downwards at the 

 tip, but may be straight or curved upwards, as in the avocet, 

 or bent to one side, as in the New Zealand crook-billed 

 plover. It is sometimes, as in the toucans, brilliantly 

 coloured, and there may also be bright coloration of the 

 cere, as in the macaws, and of naked spaces on the head, 



