424 ZOOLOGY 



SECT. 



pigment becoming broken up as it passes through the super- 

 ficial layer of the feathers in its passage to the eye : no blue or 

 violet pigments occur in feathers, and green pigments are very rare. 

 The beautiful metallic tints of many birds are entirely the 

 result of structure, owing their existence to a thin, transparent, 

 superficial layer, which acts as a prism : in such feathers the 

 colour changes according to the relative position of the Bird 

 and of the eye of the observer with regard to the source of 

 light. 



There is also infinite variety in the general coloration of Birds. 

 In many the colouring is distinctly protective, harmonising with the 

 environment, and even changing with the latter as in the Ptarmi- 

 gan, which is greyish-brown in summer, white in winter, the 

 former 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, &c., &c. 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 

 "recognition marks," serving to enable stragglers to distinguish 

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



Skeleton. The vast majority of Birds have saddle-shaped or 

 heteroccelous cervical and thoracic vertebrae, but the thoracic verte- 

 bras are opisthoccelous in the Impennes (Penguins), the Gavias 

 (Gulls), and the Limicolas (Plovers, &c.), while in the Ichthyornithes 

 alone they are biconcave. The spaces between adjacent centra 

 are traversed by a meniscus with a suspensory ligament, as in the 

 Pigeon (p. 386). The number of vertebrae is very variable, especi- 

 ally in the cervical region, where it rises to twenty-five in the 

 Swan and sinks to nine in some Song-birds. There is very com- 

 monly more or less fusion of the thoracic vertebras, and the 

 formation of a syn-sacrum by the concrescence of the posterior 

 thoracic, lumbar, sacral, and anterior caudal vertebras, is universal. 

 The posterior cervical and anterior thoracic vertebras commonly bear 

 strong hypapophyses or inferior processes for the origin of the great 

 flexor muscles of the neck. The number of sacral vertebras 

 varies from one to five. A pygostyle, formed by the fusion of more 

 or fewer of the caudal vertebras, is of general occurrence, but is 

 small and insignificant, or absent, in the Ratitas. 



The ribs are always double-headed : the sternal ribs are ossified, 

 not merely calcified, and are united with the vertebral ribs by 



