16 BIRDS OF OHIO. 
beginning of the Quaternary, with its revolutionary changes 
of climate from almost universal tropic to frigid conditions 
over the northern half of the northern hemisphere, drove all 
life out of what is now Ohio. We are therefore concerned 
only with the reappearance of the birds in the region. 
A glance at the accompanying map will indicate the ex- 
tent of the ice-cap over Ohio. It will be noticed that the 
south-eastern counties escaped with nothing more than flood- 
ing by water in the lowlands when the ice-cap began to 
recede by melting. It will also be clear that the whole of the 
state must have been frigid during the most of the year, 
with perhaps short periods of freedom from snow and ice 
when the sun was highest in mid-summer. Clearly the con- 
ditions were not favorable anywhere in the state then for 
breeding. Such birds as survived the changes occasioned 
by the ice invasion, the crowding into the tropics, and the 
necessary change of diet, must have remained well south 
of the ice barrier for long periods. 
As the ice-cap began to recede in consequence of another 
change of climate to warmer, the birds, under the necessity 
of securing more favorable conditions for breeding, must 
have pushed northward to the limit of the ice, only to be 
forced south with the approach of winter. With the recur- 
rence of summer and the further recession of the ice these an- 
nual migrations would extend farther and farther north- 
ward, and the instinct for regular migrations be formed. 
This being true, it is clear that the reoccupation of Ohio 
must have been from a southerly direction. But since the 
mountains lying eastward and southward now form a con- 
siderable barrier to the migrations of the birds, it is likely 
that they did to an even greater extent then, since time has 
served to lessen their height. Remembering, also, that 
birds follow large features of topography in their annual 
migrations, like river valleys and coast lines, it will seem 
more than likely that the Ohio tributary of the great Missis- 
sippi river served as a highway for the birds living at the 
close of the Glacial epoch as it does to-day. We are there- 
fore safe in assuming that the first invasion of the region 
