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 BIRDS' EGGS. 69 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 Avith the Fig. 24.— Egg of (a) Spotted Sandpiper, (b) Catbird, to show difference in size of eggs of pmjcoeial and altricial birds of same size. (Natural size.) parent, it is frequently impossible to identify eggs with certainty. The eggs of praecocial 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- panjnng 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 ten days in the Hummingbird to forty odd in the Ostrich and, it is said, some fifty in the Emu.