INTRODUCTION. xxix 



M. rubronigra of Bengal and Tenasserim, and M. malacca, 

 of S. India and Ceylon; and very many others. Such differ- 

 ences may be very slight indeed, and yet constant. In 

 several such cases where the sexes differ, the adult males 

 only can be distinguished, as exemplified by Thamnobia 

 Carabayensis of Northern India, and T. fulicata of S. 

 India and Ceylon ; also, by some of the long-tailed Necta- 

 rinise, and by certain of the Kalij pheasants. Or, only the 

 older males may assume a distinguishing mark, as in 

 Lanius superciliosus of the Malayan countries, as compared 

 with its representative in India. Or, perhaps, the old of 

 both sexes may, alone, be distinguishable, as instanced by the 

 amethystine Chrysococcyx xanthorhynchos of the Malayan 

 countries, and its emerald-green Indian representative. 

 Lastly, the nuptial plumage may^ alone, present a constant 

 diversit}?-, which is very great in Motacilla alboides, and 

 M. dukhunensis. 



A constant variation of colour is often associated with 

 a marked difference in the voice, as in Pratincola indica. 

 compared with that of the European P. rubicola ; and in 

 others a structural peculiarity, in one sex only, is the chief 

 point of difference, as in the remarkable case of the con- 

 formation of- the trachea in the female Rhynchtea austra- 

 lis, or painted Snipe, entirely absent in both sexes of the 

 Indian R. bengalensis. 



It may next be remarked that a great difference of 

 voice and of habits may be only indicated in the structure 

 by minute variations in the form of particular feathers ; 

 e. g,, by the crest, as in Pernis cristata and Limnaetus 

 cristatellus, compared with P. apivora and L. niveus, 

 the latter of which, moreover, assumes an ultimate phase 

 of plumage never seen in the other. In many other in- 

 stances, the distinction is best shown in the varying relative 



