Birds of the Philippine Islands. 259 



the fact tliat the underparts iu true S. calvus remain black 

 at all seasons, only the flanks being grey, or he would not 

 have united it with his S. lowi from Sibutu. This allied 

 form, of which only the type is known, has the sides of the 

 chest and the whole of the underparts grey, with only a line 

 of blackish feathers down the middle of the belly, and 

 appears to me to bo perfectly distinct from S. calvus, which 

 is found in the adjacent Tawi Tawi group and northwards 

 throughout the Philippines. I think Dr. Sharpe was right in 

 the first instance when he described the Sibutu bird as distinct. 



I have been much puzzled over the diflPerences in plumage 

 in the true Sarcops calvus (that is, the species with the 

 underparts black). I have a large series of birds before me 

 which are now arranged, geographically, and in this way only 

 have I been able to obtain a satisfactory solution to the 

 problem of plumage. Some specimens have the upper parts 

 entirely silvery grey : that is to say, the neck, mantle, back, 

 rump, and upper tail-coverts ; others have the entire mantle 

 and upper back smoky black, in strong contrast to the silvery 

 grey of the neck and lower back, &c. In studying the series 

 before me I have asked myself the following questions : — 



Can the difference of plumage be seasonal ? No, for both 

 entirely grey and black-backed forms are to be met with at 

 all seasons. 



Can the difference in plumage be sexual ? No, for I have 

 specimens belonging to both sexes iu both styles of plumage, 

 the sex having been ascertained by reliable collectors. 



Can the difference in plumage be due to age, the grey- 

 backed forms being the adult and the brown-backed the 

 young, as has been already suggested ? No, for the brown- 

 backed forms occur in every mouth of the year and have no 

 appearance of being young birds. 



Can the difterence be geographical ? Yes, it is. The 

 grey-backed forms come from North Luzon, Mindoro, 

 Marinduque, Sulu, and Bongao. 



The black-backed forms come from Catanduanes, South 

 Luzon (Albay district), Leyte, Cebu, and Negros, also 

 Mindanao (extreme north and south) and Basilan. 



