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- 
nosited 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 tints 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 iess 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 Variations. — 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 1m- 
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 
