The Audubon Societies 239 



their nesting cavities but they build no nests within for their eggs. Birds that 

 had been in the habit of burying their eggs, however, and now had to lay them 

 on the surface of the ground where they could be incubated, had other problems 

 to meet. There were the floods, the cold, wet ground, the numerous terrestrial 

 enemies, all threatening to destroy the eggs. It is easy to imagine, therefore, 

 that those individuals that learned to raise their nests off of the ground 

 were the ones that persisted until the habit was formed. The first nests were 

 doubtless very crude and the beautiful structures with which we are familiar 

 are no doubt the result of a gradual evolution as already indicated. 



We have stated that nests are ordinarily built by the female birds though 

 the male often makes a pretense at helping. The time required depends a good 



A PAIR OF COWBIRDS 

 They have no home-life, but are parasitic upon other birds 



deal upon the time at the disposal of the birds, but, with ordinary birds, like 

 Robins or Blackbirds, it is about six days. Three days are spent on the outside 

 and a like time on the interior. The same bird, however, if the first nest is 

 destroyed while the eggs are being laid, might build an entirely new nest in a 

 single day. A pair of Phcebes, on the other hand, under observation this 

 spring, began repairing an old nest fully a month before any eggs were laid. 

 Usually the nest is completed the day before the first egg is laid. 



Incubation does not ordinarily begin until egg-laying is completed, so that 

 all of the eggs will hatch at about the same time. Otherwise the first young 

 to hatch would have an unfair advantage over the others in the nest. Occa- 

 sionally one finds Owls or Bitterns beginning to incubate before all of the eggs 

 have been laid, but they are, perhaps, less regular about egg-laying than most 



