THE BIRD DEPARTMENT 



355 



necessity for keeping the body heat at a fairly constant 

 temperature that the need of incubation arose. The heat 

 of the sun or the decaying vegetation proved too in- 

 constant to satisfy this requirement of the embryo and 

 so it became necessary that the heat should be supplied 

 by the body of the parent bird. The birds then ceased 

 burying their eggs and began laying them in places 

 where they would comfortably sit upon them. Those that 

 had been laying them in holes in trees perhaps con- 

 tinued to do so and their descendents still do today so 

 that we find all of the woodpeckers, the rollers, the bar- 

 bets, the toucans, the hornbills and the trogons still 

 nesting in holes in trees. The saw dust or soft wood at 

 the bottom of the cavity has served as a nest and, even 

 today, no nest is built at the bottom of the hole by these 

 birds. The same is true of the kingfishers and bee- 

 eaters and some of their allies which make burrows in 

 the banks of streams. Birds like the chicadees, nut- 

 hatches, wrens and bluebirds that build well-formed 

 nests at the bottom of the cavities have probably come 

 back or reverted to a hole-nesting habit and are de- 

 scended from birds that nested among the branches of 

 trees. 



The descendants of those birds which scratched holes 

 in the ground and buried their eggs, when the time came 

 that it was necessary to incubate, met with more difficul- 

 ties than the hole-nesters. There were storms and 

 floods to combat, the coldness of the ground, and nu- 



EGGS OF THE WHIP-POOR-WILL 



This bird builds no nest whatsoever, laying its eggs on the leaves of the 

 forest floor with apparently little concern for location. 



merous terrestrial enemies. The various ways in which 

 the different species of birds adapted themselves has given 

 rise to the many types of nests which we find today 

 serving as receptacles for holding the eggs and young 

 and giving protection to them and to the incubating bird. 

 A consideration of the resulting structures proves a most 

 interesting subject for thought and conjecture for among 



our present day birds we find every gradation from the 

 ancient reptilian habit of burying the eggs to the more 

 elaborate affairs we first mentioned. 



Among the brush turkeys and mound birds of Aus- 

 tralia and the South Sea Islands, for example, we still 

 find birds which come down from the forests to the ocean 

 beaches at the breeding season and scratch holes in the 

 warm sand in which they deposit their eggs and after 

 covering them, leave them entirely to the heat of the 

 sun to develop; just as their relatives, the great sea 

 turtles, migrate from the sea to the same beaches to 

 deposit their eggs. The Egyptian plover and the os- 

 trich are likewise said to cover their eggs with sand and 

 leave them for long intervals to the heat of the sun. 

 The regular practice of the ostrich, however, is to incu- 

 bate much like other birds, the female taking her turn 

 during the day and the male at night. The Australian 



SOMEWHAT OF A CARPF.NTER 

 Nest and eggs of the catbird, built of sticks, but well lined with rootlets. 



mound birds scratch together huge mounds of earth, 

 leaves, and decaying vegetation of all kinds in which to 

 lay their eggs. A curious fact in connection with the 

 young of these birds is that they are said to hatch in a 

 much more advanced stage than any other birds, acquir- 

 ing feathers while still in the egg and being able to care 

 for themselves and even to fly almost as soon as they 

 have dug their way out of the mounds, and like young 

 reptiles, it is probable that they never see their parents. 

 The number of birds which have advanced but a step 

 further and, although they practice true incubation, build 

 no nest at all or at least very crude affairs on the ground, 

 is still quite large and varied in its relationships. Among 

 our familiar birds the nighthawk and whip-poor-will 

 make no pretense at a nest, laying their eggs upon the 

 bare gravel or upon the leaves of the forest floor with 

 apparently little concern for the location. The same 

 habit of building no nest is true of many of the sea birds 



