BIRDS' EGGS. 



69 



which is deposited in layers. Tlie 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, resemhling 

 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." 



Wliile the eggs of the same species more or less 

 closely resemble one another, tliei-e is often so great a 

 range of variation in color that, unless seen with the 



Fig. 24. — 'Egg of (a) Spotted Sandpiper, (b) Catbird, to show difference in 

 size of eggs of prsecocial and altricial bii'dsof same size. (Natural size.) 



parent, it is frequently impossil)le to identify eggs with 

 certainty. The eggs of prajcocial birds, whose young are 

 born with a covering of down and can run or swim at 

 birth, are, as a rule, |)roportionately 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 dej^end- 

 ent u])on the size of the egg^ and varies from ten days 

 ill the Ilunnningbird to forty odd in the Ostrich and, it 

 is said, some fifty in the Emu. 



