xviii INTRODUCTION. 



eating species having the hind portion of their stomach 

 very muscular and thick ; this is known as the gizzard, 

 and acts as a grinding mill ; the process is assisted by 

 the gravel which the bird swallows for the purpose. 

 Among insect-eating and carnivorous birds the hard 

 parts of the prey are often ejected in the form of pellets, 

 as by Owls and Fly-catchers, but some species with 

 the same dietary have such powerful digestions that 

 all such matter appears to be digested ; at any rate, 

 no castings are formed. Such is the case with Grebes 

 and Starlings, for example ; the former often eat and 

 digest their own feathers. The Lammergeier, or Bearded 

 Vulture, has so powerful a digestion, that it feeds upon 

 bones, coming to a carcase after the other vultures 

 have left it. The appetite of birds is very great 

 and their digestion rapid ; all their bodily processes, 

 in fact, are energetic, but, nevertheless, they are re- 

 markably long-lived compared with beasts, though 

 they present notable differences in this respect. The 

 flightless birds seem to enjoy less long lives than flying 

 species, and of these those attaining the greatest age 

 are the larger kinds of Parrots, Birds of Prey, and 

 Waterfowl ; there is evidence that some of these 

 may reach a century, while even small birds like the 

 Canary may reach twenty years. They often, on attain- 

 ing a great age, die almost suddenly, with little per- 

 ceptible failing of the powers. 



The senses of birds are very acute. Their hearing 

 seems to be as good as that of beasts, while their 

 sight is better than that of any other animal. Contrary 

 to a very common idea, the nocturnal species appear 

 to be able to see quite well by day, though the converse 

 is not the case ; although some birds with no special 

 adaptation to night vision, such as Ducks, are constantly 

 out at night, and a large number of the small diurnal 

 birds perform their migration at that time, apparently 

 to avoid enemies such as Hawks, Crows, and Gulls. 



