330 A HISTORY OF BIRDS 



the different tints of the two species in each island are exactly 

 the same. Thus in Bouru both are exactly brown ; in Ceram 

 they are both washed with yellow ochre ; in Timor the under- 

 surface is pale and the throat nearly white." So close is the 

 resemblance between the two, that even expert Ornithologists 

 have been deceived. The Oriole of Bouru was actually de- 

 scribed and figured as a honey-sucker! 



But what has been cited of late years as one of the most re- 

 markable of all these instances is that of a Bulbul {Tylas) 

 which mimics a Shrike of the Genus Xenopyrostris. These are 

 natives of Madagascar. The Bulbul, a perfectly defenceless 

 species, mimics the aggressive Shrike. I am sorry, however, to 

 have to rob this instance of much of its importance, for a care- 

 ful study of the skeleton of Tylas shows that it is not a Bulbul 

 at all, but a species of Shrike, though not of the same family as 

 the Xenopyrostris. Thus, in this case the resemblance may be 

 rather an accidental one. The mimicry of the Friar-bird and 

 Oriole may stand for the present at any rate, but none of the 

 Orioles seem to have been examined by an anatomist. 



We must pass now to a brief survey of the phenomena of 

 aggressive mimicry; that is to say, of mimicry wherein an ag- 

 gressive species assumes the livery of a harmless species in 

 order that it may thereby the more easily approach its prey 

 without exciting suspicion. 



A case in point has been cited by Wallace on the authority 

 of Mr. Osbert Salvin, an Ornithologist who, during his lifetime, 

 had few rivals. This was furnished by a bird-eating Sparrow- 

 hawk {Acdpiter pileatus), -which, in the neighbourhood of Rio 

 Janeiro, assumes a very close likeness to an insect-eating Hawk 

 (Harpagus diodon). The latter naturally excites no fear among 

 other birds, and consequently, its rapacious mimic is enabled to 

 approach and seize its prey with ease. "The curious point, 

 however, is," remarks Wallace, "that the Accipiter has a much 

 wider range than the Harpagus, and in the regions where the 

 insect-eating species is not found- it no longer resembles it . . . 

 thus indicating that the red-brown colour [which is the char- 

 acter common to the two when found together] is kept true by 

 its being useful to the Accipiter to be mistaken for the insect- 

 eating species, which birds have learnt not to be afraid of" 



One of the fierce Hawk Eagles {Spizcetus lanceolatus) of the 



