182 SEXUAL SELECTION: BIRDS. [Part IL 



species which represent each other in distinct countries. 

 For with several of these representative species the adult 

 males have undergone a certain amount of change and can 

 be distinguished ; the females and tlie young being imdis- 

 tinguishable, and therefore absolutely imchanged. This 

 is the case with certain Indian chats (Thamnobia), with 

 certain honeysuckers (Nectarinia), shrikes (Tephrodornis), 

 certain kingfishers (Tanysiptera), Kallij pheasants (Gal- 

 lophasis), and tree-partridges (Arboricola). 



In some analogous cases, namely, with birds having 

 a distinct summer and winter plumage, but with the two 

 sexes nearly alike, certain closely-allied species can easily 

 be distinguished in their summer or nuptial plumage, yet 

 are undistinguishable in their winter as well as in their 

 immature plumage. This is the case with some of the 

 closely-allied Indian wag-tails or Motacilla. Mr. Swinhoe * 

 informs me that three species of Ardeola, a genus of her- 

 ons, which represent each other on separate continents, 

 are " most strikingly different " when ornamented with 

 their summer plumes, but are hardly, if at all, distinguish- 

 able during tlie winter. The young also of these three 

 species in their immature jilumage closely resemble the 

 adults in their winter dress. This case is all the more 

 interesting because with two other species of Ardeola 

 both sexes retain, during the winter and summer, nearly 

 the same plumage as that possessed by the first three 

 species diiring the winter and in their immature state ; 

 and this plumage, which is common to several distinct 

 species at different ages and seasons, probably shows us 



troduction, p. xxix. In regard to Tanysiptera, Prof. Sehlegel told Mr. 

 Blyth that he could distinguish several distinct races, solely by compar- 

 ing the adult males. 



' See also Mr. Swinhoe, in 'Ibis,' July, 1863, p. 131 ; and a previous 

 paper, with an extract from a note by Mr. Blyth, in 'Ibis,' Jan. 1861, 

 p. 52. 



