278 With Rod and Gun in New England 



The beginning of a southward migration is induced by two causes. 

 First and most important is the lack of food. In insect-eating species this 

 lack will occur through increasing cold in the autumn. But this will not 

 explain why such strictly insectivorous birds as the chimney-swift, some of 

 the swallows, and some of the warblers migrate even in August or early 

 in September, when insects are still abundant. Strange as it may first 

 appear, I answer that the increasing length of the night causes these birds 

 to migrate thus early. 



In order to explain this we have to understand a little about the 

 physical condition of birds. In most of these animals the blood circulates 

 very rapidly, and digestion and all other functions are carried on much 

 more rapidly than in mammals. They have to feed more often than mam- 

 mals in order to acquire the supply of nutriment which they need. Insect 

 food is assimilated more rapidly than is a vegetable diet, consequently 

 swallows and swifts are almost constantly upon the wing in search of food. 

 When the days are long and the nights short, they have a longer time to 

 feed ; but when the reverse is the case, and the nights become more than 

 ten hours long, then the truly insectivorous birds cannot obtain food 

 enough to last them through the long night. This is especially true when 

 we consider that in order to withstand the fatigue of migration, birds must 

 be in good condition before they start, and long nights would draw upon 

 the accumulated supply of fat which has to be used upon the southward 

 flight. 



A difficulty in this theory of migration which must be answered, is the 

 fact that some species are not wholly migratory ; examples are the jays and 

 crows, the titmouse, grouse, etc. The answer to this will be that as fast as 

 species become fitted, through gradual adaptation to environment, to live in 

 any locality, they lose the migratory instinct, as they have no inducement 

 to leave a given locality. Such birds will as a rule be found to have be- 

 come modified to a great extent to meet the conditions under which they live. 

 Some, as the grouse, have acquired crops, an enlargement of the gullet, in 

 which to store food for long nights and through storms. The intestinal 

 cceca have become greatly enlarged, so that a greater amount of nutriment 

 is obtained from a smaller amount of food than can be assimilated by birds 

 with smaller cceca. 



But perhaps I have said enough to convey my idea of the origin of the 

 phenomenon of migration among birds, and that further discussion will 

 prove too technical to interest the general reader. Should there be any 

 who read this chapter who have a further interest in the matter, I will refer 

 them to an article upon migration in my Contributions to Science, Vol. III. 



I will now proceed to a subject which is of vital interest to all sports- 

 men — the migration of water and shore birds. It is a rather singular fact 



