The Audubon Societies 



243 



has once begun, the same bird will permit of a great many liberties, and its 

 attachment for its nest increases as incubation progresses, reaching a maximum 

 at the time of hatching. A conspicuous egg, therefore, is in reality a benefit 

 to the species building a conspicuous nest. 



It has been stated that hole-nesting birds, Hke Woodpeckers and Kingfishers, 

 lay pure white eggs, but there are certain exceptions to this rule which give 

 added strength to it and also to that accounting for the degeneration of the 

 pigment in birds' eggs. The exceptions are birds like the Wrens, Nuthatches, 

 Chickadees, and Bluebirds which build nests at the bottom of the cavities and 



NEST OF SONG SPARROW SHOWING VARIATION IN THE COLORATION 



OF THE EGGS 



lay spotted or blue eggs. The fact that they build unnecessary nests at the 

 bottom of the holes shows that they have come to a hole-nesting habit com- 

 paratively recently, for they must have first learned to build their nests in 

 the open. The fact that they still do build nests and lay colored eggs indicates 

 only the slowness of the course of evolution. It is interesting to note in pass- 

 ing, however, that the eggs of the Bluebird are the palest of all the Thrushes', 

 disclosing the trend of evolution in changing what was once doubtless a pro- 

 tectively colored egg to a pure white egg. 



It would not be feasible here to discuss the identification of birds' eggs by 

 their color or size, for many kinds can be distinguished only by specialists. 

 The only safe way for an amateur to identify most eggs is by identifying the 



