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 



