By dissection alone can the sexes be distinguished 

 acquired by the young at a very early age. 



and the plumage of the adult bird is 



All the upper and under surface, a rich dark brown : crown of the [head a bluish grey, gradually 

 deepening into brown as the back is approached ; tail rather darker than the back and wings ; immediately before 

 and behind the eye a triangular mark of black, the anterior part of which is the largest and very sharply defined ; 

 bill, black ; feet, reddish-brown ; claws, black. 



Habitats: Besides the Houtman's Abrolhos, these birds breed in prodigious numbers in Torres 

 Straits, particularly at Bramble Key and Raine's Islet, but are not found to the southward of the 

 latter place on the eastern coast : among the guano islands of the South Pacific, and from thence to 

 Christmas Island, situated about one hundred and twenty miles to the north of the Equator, the 

 species is also met with, though in somewhat limited numbers. 



