xvi. 5 COLOURS OF BIRDS 437 



The actual colour patterns vary with the habits of the bird. Con- 

 cealing (cryptic) coloration is very common; even the brighter colours 

 may serve this purpose, by breaking up the outline of the bird when at 

 rest or in motion. Most birds are dark above and white below. The 

 feathers often show mottled or speckled patterns rather than a homo- 

 geneous colour. Finches and other birds living in the sunlit upper 

 branches show bright yellow, yellow-green, and blue colours, either 

 singly or combined. Birds living in thickets, such as the thrush and 

 blackbird, are usually duller brown or black. An example of disruptive 

 coloration that is easy to observe is the white patch on the throat of a 

 thrush. If the nest is approached while the bird is sitting the head is 

 held rigidly still with the beak upwards ; the white mark on the neck 

 breaks the outline and instead of an obvious bird's head there appear 

 only the meaningless shapes of the sides of the jaws. In most species 

 coloration is a compromise between concealment and conspicuous- 

 ness. Sometimes selection has acted so that the female is cryptic, the 

 male conspicuous (e.g. ducks). In hole-nesting shelducks both sexes 

 are conspicuous. In other birds bright colours are concealed most of 

 the time (e.g. the robin's red breast is underneath, many waders have 

 conspicuous colours under their wings). 



Some colour patterns seem to make the bird conspicuous and may 

 be a warning of a distasteful quality. The black and white pattern 

 shown by the magpie may be an example of such sematic coloration; 

 certainly this bird is seldom preyed upon, no doubt partly because of 

 its large size. The conspicuous black of rooks and starlings may be 

 connected with their social life, making it desirable that the birds 

 should easily follow each other, the group being protected by the com- 

 bined receptors of its many members and the quick response of all 

 to escape movements by any one. 



The protective functions of the colour often give place in one or 

 both sexes to garments used for communication between individuals, 

 for such purposes as pair formation, aggression between males, nest 

 site selection, or rearing the young. 



5. The skeleton of the bird. Sacral and sternal girders 



The arrangement of the whole locomotor apparatus is based on the 

 plan of the bipedal archosaurian reptiles, modified and simplified for 

 the purposes of flight and balancing and walking on two legs. The 

 bones are very light and often of tubular form, but sometimes with 

 internal strutting well suited to the stresses they must bear (Fig. 252). 

 Many of the bones contain extensions of the air-sacs; even the wing 



