THE UNCHARTED SKY 133 



at night or frost nips the toes with which he grips 

 his perch. But there are very few birds who wait 

 for the threat of autumn, fewer still that linger 

 until the approach of winter. Long before that 

 they are gone, many of them have left us as soon 

 as the nestlings were able to fly and feed them- 

 selves. 



"One of the great conditions incident to migra- 

 tion is that of food. It is a condition, not a cause. 

 It is not that food attracts, but rather that lack of 

 food prevents. The Wild Ducks do not leave the 

 Arctic and come to North Carolina because of any 

 patriotic love for the state, but because the lakes 

 and the seas are frozen in their summer home. 

 Fish-eating ducks cannot dive for fish and dabblers 

 cannot peck through the ice for the water-weeds 

 on which they feed. 



*'It is true that at nesting time birds need to eat 

 more than they do at any other time, and, since 

 they have nestlings to feed — the amount a young 

 bird will eat is almost unbelievable — the food 

 must be easy to get at. The average number of 

 visits the smaller birds make to their nests for the 

 purpose of feeding the young varies from about 

 fifteen to twenty-five visits an hour. Eeckoning 

 this at twenty visits an hour for the fourteen 



