BIRDS' EGGS. 59 



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 



FIG. 24. Egg of (a) Spotted Sandpiper, (5) Catbird, to show difference in 

 size of eggs of pnecocial 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- 

 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. 



