4 CIRCULAR 363, U. 8S. DEPARTMENT OF AGRICULTURE 
The hibernation theory survived for more than 2,000 years and is 
still occasionally repeated by credulous persons to account for failure 
to locate definitely the winter home of the chimney swifts (Chaetura 
pelagica), which each autumn gather in immense flocks in southern 
Georgiz and northern Florida and then suddenty disappear. There 
are, however, records of occurrence during migration for a few points 
in the West Indies, Mexico, and Central America, and it is probable 
that these birds spend the winter season in the great rain-forest area 
of the Amazon Valley in Brazil, passing most of the daytime high 
in the air with other swifts that are local residents. 
Aristotle was also the originator of the theory of transmutation, 
basing it upon the fact that frequently one species will arrive from the 
north just as another species departs for more southerly latitudes. 
From this he reasoned that although it was commonly believed that 
such birds were of two different species, there really was only one, 
and that this one assumed the different plumages to correspond with 
the summer and winter seasons. 
Probably the most remarkable theory that has been advanced to 
account for migration is contained in a pamphlet mentioned by 
Clarke (4, v. 1, pp. 9-11), as published in 1703 under the title: ‘‘An 
Essay Toward the Probable Solution of this Question: Whence 
Come the Stork and the Turtle, the Crane, and the Swallow, when 
they Know and Observe the Appointed Time of their Coming.” 
It was written ‘‘By a Person of Learning and Piety’’, whose ‘“‘ prob- 
able solution’? was that migratory birds flew to the moon and there 
spent the winter. 
Some who easily accepted the disappearance of the larger birds as 
migratory travelers were unable to understand how the smaller species, 
some of them notoriously poor fliers, could make similar journeys. 
They contended that the larger species, as the storks and cranes, 
carried their smaller companions as living freight. In some of the 
Mediterranean countries it is still believed that these broad-pinioned 
birds serve as aerial transports for the hosts of small birds that con- 
gregate on the shores of the Mediterranean Sea, awaiting opportunity 
for this kind of passage to their winter homes in Africa. Similar 
beliefs are found among some tribes of North American Indians. 
ADVANTAGES OF MIGRATION 
Before presenting some of the present theories concerning the 
origin of bird migration, it seems well to consider briefly the ends 
served by this annual round trip between breeding grounds and 
winter quarters. It is apparent that the migratory habit enables 
a species to enjoy the summers of northern latitudes while avoiding 
the severity of the winters. In other words, migration makes it 
possible for some species to inhabit two different areas at seasons 
when each presents favorable conditions. In the performance of its 
reproductive duties every pair of birds requires a certain domain, 
the extent of which varies greatly in different species. Generally, 
however, this territory must be large enough to provide adquate 
food, not only for the parent birds but also for the lusty appetites 
that come into being with the hatching of the eggs. Thus, if all 
birds were to remain constantly either in tropical or in temperate 
regions, there might be intolerable overcrowding during the breeding 
season. By the spring withdrawal to regions uninhabitable earlier 
