34 BIRDS. 



present, springs from the under side of the stem of the main feather at the junction 

 of the quill (calamus) with the shaft (rhachis) close by the umbilieus. Though 

 occasionally (as in the Emeus and Cassowaries) almost as large as the main feather, 

 it is generally very much smaller. It is never developed on the large wing and 

 tail feathers, and is entirely wanting in all the feathers of some groups of birds. 

 In the one-wattle Cassowary the accessory plume is as long as the main feather, 

 and in the Emeu it is nearly so. In the feathers on the back of the Argus 

 Pheasant the after-shaft is about one-third the length of the full feather (Nat. 

 Hist. Museum). 



Peculiarities of the arrangement and structure of Feathers. 



In the Grey Jungle-Fowl [Q. sonnerati) the ends of the shafts in the upper 

 wing coverts show flattened horny expansions resembling sealing-wax, and in the 

 American "Wax-wing (A. cedrorum) the shafts of the cubital, or secondary flight 

 feathers, are expanded in vermilion-coloured horny terminations. 



The Great Crested Grebe (P. cristatus) has a double occipital crest, and the 

 Cock of the Eock (R. crocea) carries a red fanlike crest, which extends from the 

 crown past the eye to the gape. 



The Australian Darter (P. novae-hollandice) shows a ribbed structure in the 

 vanes of its tail, and the Curl-crested Toucan (P. heauharnaisi) has peculiar 

 expansions and curved prolongations of the shafts of the crown feathers. 



In the Common Peacock (P. cristatus) the terminal portion of one of the tail 

 coverts shows the beautifully coloured ocellus, or eye, surrounded above by the 

 "transparent zone," an effect produced by the absence of the barbules on a limited 

 portion of each of the barbs. 



EXTERNAL VARIATION IN THE TWO SEXES 

 AND AT DIFFERENT SEASONS. 



All individuals of many species of birds are, when full grown, closely alike 

 externally, both male and female, and at all times of the year. 



In some species the two sexes are more or less marked by different plumage at 

 all times. 



In others the two sexes are alike for part of the year, but differ in the pairing 

 season. 



ADAPTATION OF COLOUR TO 

 SURROUNDING CONDITIONS. 



The genus Lagopus (Willow-Grouse and Ptarmigan) in summer more or less 

 resemble in colour the rocks and plants among which they live, while in winter 

 they change to white, like the snow which covers the ground around them. 



Such complete changes only occur in latitudes and localities where the differ- 

 ences between the general external conditions in the different seasons are extreme, 

 where the snow completely disappears in summer and remains continuously on 

 the ground during the greater part of the winter. The change is in direct 

 adaptation to the surroundings, the advantage gained being concealment from 

 their enemies or their prey, as the case may be. 



