The Premonition Yc3.rS cl§0 Brchni SttCniptcd tO 3.CCOUnt fOT 
Theory the fall migration by assuming that birds have 
premonitions of severe weather, or in other words that they are 
endowed in some mysterious way with a meteorological sense. 
This theory, which at first thought seems entirely fanciful, in 
reality contains a large element of probability but not exactly in 
the way that Brehm intended. Birds with their large lungs, pneu¬ 
matic bones and numerous internal air sacs, are, to a remarkable 
degree, living barometers, responding with great delicacy to 
changes in barometric pressure. The uneasy behavior of robins 
and the repeated calls of cuckoos before a storm are familiar il¬ 
lustrations of this fact. That birds can anticipate winter, however, 
and as a result make*an effort to avoid its disastrous effects, is 
A 
beyond demonstration and seems quite unlikely. 
The Short Day Another alternative has been suggested. 
Theory namely, that toward the fall of the year the days 
become too short for the bird to complete its daily task of feeding 
When the enormous activity of birds is brought to mind and one 
remembers how rarely a resting bird is seen, particularly among 
seed and insect eaters, the hardship resulting from shortened work¬ 
ing hours can be readily appreciated. The migration south, how¬ 
ever, begins before the days are perceptibly shorter and so this 
theory suffers, as does many another, because of a few obtrusive 
incontrovertible facts! 
The Food Supply Still Other theorists have assumed that the 
Theory factor of greatest importance in causing the fall 
migration is a diminished food supply but here again it must be 
admitted that a large per cent, of inigrating species leave for the 
south in the very height of the seed and insect harvest. It may be 
pointed out, however, that upon the ground of food supply, nat¬ 
ural selection would promptly eliminate those who did not go south 
and would tend at the same time to favor the perpetuation of those 
who varied in the direction of southern migratory habits, what¬ 
ever the cause of those variations might be. 
Theories to Ac- Turning now to the spring migration, the 
SprinVMilr^ion factor of food Supply ‘seems to be of much less 
immediate importance since in many cases birds, as for instance 
the water fowl, push their way out of a land of plenty into a re¬ 
gion of scarcity. 
The Instinct That it is a bird’s instinct to go north in the 
Theory Spring is no better an explanation of the origin 
of micrration than it is of how a bird finds its way during migra- 
tion. 
