268 THE BIRDS 



is divided into two parts ; the walls of the lower portion or " gizzard " 

 are extremely muscular and usually very rough. In birds that 

 feed on grain, etc., the walls of the gizzard are provided with a 

 pair of thick, rough pads which, in conjunction with a quantity 

 of small stones swallowed by the bird, form a very complete 

 apparatus for crushing and grinding tip the hardest food. 

 Flesh-eating birds and those which live on fruit or insects have 

 the gizzard but little developed. 



The migration of birds is one of the most fascinating of all 

 phases of bird life. The coming and going of the feathered legions 

 has been observed and commented on by naturalists and philo- 

 sophers of all ages, and yet, to-day, we really know but little about 

 it. The actual forces that determine the movements of migrating 

 birds still remain something of a mystery, and of the exact 

 routes the travellers follow we have no very definite knowledge. 

 To-day the flocking together of immense numbers of birds, and 

 their orderly, concerted movements as they wing their way over 

 land and sea in their long periodical journeys, may be witnessed 

 as in years long gone by. " The hawk that stretches her wings 

 towards the south is as familiar to the latest Nile-boat traveller 

 or dweller in the Bosphorus as of old to the author of the Book of 

 Job. The autumnal thronging of the myriads of water-fowl by 

 the rivers of Asia is witnessed by the modern sportsman as it was 

 of old by Homer. Anacreon welcomed the returning swallows 

 in numbers which his imitators of the colder North, to whom the 

 associations connected with it are doubly strong, have tried in vain 

 to excel. The Indians of the fur countries in forming their rude 

 calendar name the recurring moons after the birds of passage, 

 whose arrival is coincident with their changes. But there is no 

 need to multiply instances. The flow and ebb of the feathered 

 tide has been sung by poets and discussed by philosophers, has 

 given rise to proverbs and entered into popular superstition, and 

 yet we must say of it still that our ignorance is immense." 1 



For the sake of convenience birds are generally divided into 

 two classes " resident " birds and " migratory " birds ; but 

 actually it is not possible to draw a hard and fast line between 

 them, for to a certain extent almost all birds are migratory 

 that is to say that although certain species, such as robins, thrushes, 



1 Professor Newton. 



