GENERAL HABITS 211 



phases of plumage, broadly described as a light 

 one and a dark one. This peculiarity is not 

 in any way a sexual one, and very often a male 

 and female of each type of plumage may be 

 noticed paired. Whether the Hooded Crow 

 and the Carrion Crow are two types of a 

 single dimorphic species seems not to be quite 

 so well determined, ornithologists differing in 

 opinion, some asserting that the birds are spe- 

 cifically distinct, although interbreeding. The 

 peculiar variety of the Ringed or Bridled Guil- 

 lemot may also be another instance of colour 

 dimorphism. Of structural dimorphism inde- 

 pendent of sex we may mention the large and 

 small billed form of the Dunlin, and lastly the 

 large and small race of the Wheatear. 



