396 THE OWLS 



in the socket as to be practically incapable of notatory movement, 

 and hence the whole head has to be turned in the direction of the 

 object to be scanned. In colour, as in the Accipitres, they are either 

 yellow e.g. longeared and eagle owls or of a dark hazel, so as to 

 appear black e.g. barn-owl, tawny-owl. The nictating membrane, or 

 curtain, which is incessantly drawn over the eyeball, is well developed, 

 as it is in the Accipitres. But the Owls differ from other birds in that 

 the upper lid, as in ourselves, closes the eye, instead of the lower. 



The young of the Owls, as with the Accipitres, are nidicolous that 

 is to say, they remain long in the nest helpless, and must be fed by 

 the parents. They are also daring, as with young Accipitres. But 

 while it is now known that the young owls develop two generations of 

 nestling down, as much cannot yet be certainly said of the young of 

 Accipitres. These two generations are known as the protoptyle and 

 mesoptyle generations. 1 In the barn-owls, for example, the protoptyle 

 down is vestigial, and the mesoptyle degenerate, while in the eagle 

 and tawny owls the mesoptyle down is extremely well developed, being 

 semi-plumous in structure, and transversely barred, a pattern rare, 

 indeed, in such plumage outside the Owls. The mesoptyle generation 

 of down lasts in the tawny -owl for about eight weeks, when it is 

 replaced by the contour feathers, the quill and tail feathers having 

 meanwhile appeared. In Accipitres the nestling down is composed 

 both of down which is later replaced by down in the adult, and is 

 thus called pre-plumulce, 2 and down which is later replaced by 

 contour feathers, and hence is called pre-pennce. In the Owls, on 

 the other hand, pre-pennae alone are present, the adult owl having 

 no down feathers. Further, we may remark, in the Accipitres the 

 oil-gland is tufted, but not so in the Owls. 



1 Pycraft, British Birds, vol. i. p. 162. 



1 Pycraft, " Morphology of the Owls," Trans. Linn. Soc., 1898. 



