238 Bird - Lore 



Dove, and the Cuckoos. Crows and Catbirds have advanced a step further, 

 for while they still use sticks they build deeply hollowed nests and line them 

 with softer materials. Nests of the Yellow Warbler, Redstart, and Goldfinch, 

 made entirely of soft materials, doubtless represent a still higher stage in the 

 evolution of nests that culminates in the beautifully woven structures of the 

 Vireos, Orioles, and Weaver Birds. Such is the present status of birds' nests 

 and doubtless it indicates the various steps through which the more compli- 

 cated nests have passed. If we would understand the real origin of nest- 

 building, however, we must go back to the earliest birds when they first arose 

 from their reptilian ancestors. 



Doubtless their habits of egg-laying at that time were about the same as 

 those of reptiles today. Turtles bury their eggs in the sand; lizards hide them 

 in holes in stumps or decaying logs; snakes bury theirs in decaying vegetation; 

 and alligators build nests of the same material in which they hide their eggs, 

 and are the only reptiles which are said to take an interest in the welfare of 

 the young later on. But, as in all other reptiles, the eggs are hatched by the 

 heat of the sun or from the decaying material. Now it must be remembered 

 that reptiles are 'cold-blooded' creatures and are not affected by great changes 

 in their bodily temperature. A turtle basking in the sun may have a blood 

 temperature nearly boiling while the temperature of the same animal hiber- 

 nating in the mud may be near the freezing point. As its temperature drops, 

 it becomes more sluggish, but its health is not affected. The warm-blooded 

 birds and mammals, on the other hand, can endure but a very slight change 

 from the normal temperature of their blood without ill effect. What is true 

 of the grown bird is equally true of the embryo developing within the egg. 

 Its temperature must be m.aintained or it will not develop and will soon die. 

 There are a few birds, such as the Megapodes of the Australian region, which 

 still rely upon the ancestral method of burying their eggs in the sand or in 

 piles of decaying vegetation, but they lay their eggs at a time when the tem- 

 perature is remarkably uniform in the places which they select. All other 

 birds have to depend upon supplying the heat from their own bodies; that is, 

 they have to incubate their eggs. The longest stride in the change from the 

 reptile to the bird and the one. which affected their habits even more than the 

 development of wings and feathers, was this advance from a changeable to a 

 constant temperature, from 'cold-blooded' to 'warm-blooded.' We have not 

 the space, nor is it appropriate here, to go into all the differences which this 

 change brought about, but we can point out that the need for incubating the 

 eggs which followed gave rise to the nest-building habit. Birds that had been 

 in the habit of nesting in holes in banks or in trees where they could remain 

 with their eggs with no great inconvenience, were doubtless less affected. 

 They did not have to learn how to build nests, except in so far as they had to 

 learn to dig their own excavations instead of accepting natural cavities. Such 

 is the habit of the Woodpeckers and the Kingfishers today. They excavate 



