which is deposited in layers. The final layer varies greatly 
in appearance, and may be a rough, chalky deposit, as in 
Cormorants and others, or thin and highly polished, as in 
Woodpeckers. 
The colors of eggs are due to pigments, resembling 
bile pigments, deposited by ducts while the egg is in the 
oviduct. One or more of the layers of shell may be pig- 
mented, and variations in the tints of the same pigment 
may be caused by an added layer of carbonate of lime, 
producing the so-called “ clouded ” or “ shell markings.” 
While the eggs of the same species more or less 
closely resemble one another, there is often so great a 
range of variation in color that, unless seen with the 
Fro. 2. 
—Egg of (a) Spotted Sandpiper, (b) Catbird, to show difference in 
aaents and al bi 
precocial of same size. (Natural size.) 
parent, it is frequently impossible to identify eggs with 
certainty. The eggs of precocial birds, whose young are 
born with a covering of down and can run or swim at 
birth, are, as a rule, proportionately larger than the eggs 
of altricial birds, whose young are born in a much less 
advanced condition. This is illustrated by the accom- 
panying figure of the eggs of the Spotted Sandpiper and 
the Catbird. 
The period of incubation is apparently closely depend- 
ent upon the size of the egg, and varies from twelve days 
in some Passerine Birds to forty odd in the Ostrich and, 
it is said, some fifty in the Emu. 
