CHAP. VIII. MIGRATIONS OF BIRDS. 251 



known to endeavour to board or pass over a vessel. 

 On their passage overland, if attacked by men, they 

 will raise themselves up, utter a kind of barking sound, 

 and fly at the legs of their invaders ; and will fasten 

 so fiercely at the end of a stick, as to suffer themselves 

 to be swung about before they will quit their hold, and 

 are with great difficulty put to flight. It is said that 

 an intestine war sometimes takes place in these armies, 

 during their migration, and that the animals thus de- 

 stroy each other. 



(267.) In the class of BIRDS, migration is much 

 more general, and has, consequently, attracted greater 

 attention from zoological writers. It is not our inten- 

 tion to go into all the details of this subject, our 

 purpose being chiefly confined to one of its numerous 

 ramifications. Whatever may be the combination of 

 causes which instinctively lead birds, no less than 

 other animals, to perform long and dangerous journey s, 

 certain it is that the desire and necessity of procuring 

 a supply of suitable food, either for themselves or for 

 their expected progeny, is the chief impelling motive. 

 The annual arrival of our summer insectivorous birds, 

 no less than of the aquatic species in winter, places this 

 beyond doubt. The comparatively short migrations of 

 the frugivorous families peculiar to Tropical America 

 are guided by the same principle : they are seen in 

 particular districts or provinces at regular and stated 

 periods, arriving when certain fruits are ripe, and 

 departing so soon as the season of their food is over 

 The most remarkable migrations, both as to the num- 

 bers congregated together, and to the extent of space 

 they go through, are performed by birds belonging to 

 the natatorial and the grallatorial types of form. Hence 

 we find that among perching birds, the fissirostral tribe, 

 which represents the first of these types, contains the 

 whole family of swallows (Hirundinidci). These, 

 also, are represented by the ducks, the bee-eaters, the 

 flycatchers, and the passenger pigeons (Ectopistes Sw.). 

 Next to these are the grallatorial types, comprising the 



