6 CIRCULAR 363, U. S. DEPARTMENT OF AGRICULTURE 
cap gradually retreated, each spring the birds endeavored to return 
to their ancestral homes in the North, only to be again driven south 
at the approach of winter. As the size of the ice-covered area dimin- 
ished, the journeys made became ever longer, until eventually the 
habits of migration were fixed to accord with the climatic conditions 
of the present age. 
Thus, this theory supposes that today migratory birds follow the 
path of a great racial movement ‘that took place in a distant past, 
associated with advances and recessions of the ice. The actions of 
the birds themselves lend some support to this theory, as every bird 
student has noted the feverish impatience with which certain species 
push northward in spring, sometimes advancing so rapidly upon the 
heels of winter as to perish in great numbers when overtaken by late 
storms. Itis probable, however, that at that season the reproductive 
impulse urges the birds on to their northern breeding grounds. 
SOUTHERN ANCESTRAL HOME THEORY 
The opposing theory is simpler in some respects and supposes that 
the ancestral home of all birds was in the Tropics and that as all bird 
life tends to overpopulation there was a constant effort to seek breed- 
ing grounds where competition would be less keen. Species that 
strove for more northern latitudes would be kept in check by the ice 
and forced to return southward with the recurrence of winter condi- 
tions. As the ice retreated, vast areas of virgin country became 
successively suitable for summer occupancy, but the winter habitat 
remained the home to which the birds returned after the nesting 
season. It is a fact that some species spend very little time on their 
breeding grounds; the orchard oriole ([cterus spurius), for example, 
spends only 24 months in its summer home, arriving in southern 
Pennsylvania about the first week in May and leaving by the middle 
of July. 
Both theories assume that migration is an ingrained habit, but 
neither is supported by positive biological data. Both have been 
criticized also on geological grounds, and neither can be accepted 
without qualification. It is apparent, however, that whether the 
ancestral home of any species was at the northern or at the southern 
limits of its present range, or even in some intermediate region, the 
search for conditions favorable for breeding in summer and for feeding 
in winter has been a principal factor underlying the origin of migration. 
THEORY OF PHOTOPERIODISM 
A modern view, based on studies of living behavior, favors the 
theory of photoperiodism, propounded by recent investigators as the 
cause of the annually induced movements of the birds. This theory 
holds as its major premise that quantity of light and length of day are 
the stimulating causes of migration. Its proponents urge that migra- 
tion is a phenomenon far *oo regular to be created anew each season 
merely under stress of circumstances, such as need for food; and that 
it begins before the necessity for a change in latitude becomes at all 
pressing. Swallows, nighthawks (Chordciles minor), shore birds, and 
others may start their southward movement while the summer food 
supply in the North is at peak abundance; while robins (Turdus 
migratorius), bluebirds (Siaha sialis), and others may leave an abun- 
