12 BIRDS' EGGS 



The period of incubation depends upon the size of the egg, 

 and varies from twelve days in the smaller Passeres to fifty-six in 

 the Emeu. 



The Shell. — The egg-shell is composed largely of carbonate 

 of lime, which is deposited in layers on the surface of the egg in 

 the lower, or uterine, dilation of the oviduct, the process re- 

 quiring from twelve to twenty hours. The final layers vary 

 greatly in character, and may be simply a rough, chalky deposit, 

 easily scraped off, showing the harder layer below, as in Gannets 

 and Flamingoes, or thin and highly polished, as in Woodpeckers 

 and Tinamous. The structure of the surface, whether pitted or 

 smooth, depends upon the action of the walls of the uterus, from 

 which it receives an impression. 



Colors of Eggs. — The colors of eggs are due to pigments de- 

 ■oosited by ducts while the egg is receiving its shell. One or all 

 the layers of the shell may be pigmented, and variation in color 

 may be caused by a super-imposed stratum of carbonate of lime, 

 producing lilac tmts and "clouded" or "shell" markings. It is 

 supposed that eggs are colored for the purpose of rendering them 

 less conspicuous, and, as a rule, birds which nest in holes lay white 

 eggs. 



Shape of Eggs. — Birds' eggs are generally ovate in form, but 

 there are many variations from this shape. The conical eggs of 

 Snipes and Plovers are placed in the nest point downward or in- 

 ward, and they fit together so closely that they can easily be 

 covered by the comparatively small-bodied parent. The pyri- 

 form egg of Murres, when moved slowly, describes a circle about 

 its own point, and is therefore less liable to roll off the rocky cliffs 

 on which it is laid, than one more oval in shape would be. Con- 

 trary to the generally prevalent idea, the egg passes down the 

 oviduct and is deposited large end first. 



Individual \ 'ariations. — Although, generally speaking, eggs 

 of the same species resemble one another, there is so great a range 

 of variation in both color and pattern, that it frequently is im- 

 possible to identify eggs unless the parent is known. This varia- 

 tion is due largely to the physiological condition of the bird. 

 Fully adult, vigorous individuals of birds which lay colored eggs, 

 secrete a larger amount of pigment than their younger or weaker 



