HIKDS' EGGS. 



GO 



w liicli is deposited in layers. The final layer varies greatly 

 in iij)[)earance, and may l)e a rongh, chalky deposit, as in 

 Coruiurants and others, or thin and highly polished, as in 

 AV^oodpeckers. 



The colors of cprtr> are due to ])ignients, resenihling 

 bile ])ignients, deposited by ducts while the eg<^ 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."" 



AVliile 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.— Eirsr of (a) Spotted Sandpiper, {h) Catbird, to show difference in 

 size of eggs of prjecocial and altricial birds of same size. (Natural size.) 



parent, it is frequently impossible to identify eggs with 

 certainty. The eggs of praicocial birds, whose young are 

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

 l)irth, are, as a iiile, proportionately larger than the eggs 

 of altricial birds, who.se young are born in a much less 

 advanced condition. This is illustrated by the accom- 

 panying figure of the eggs of the Spotte 1 Sandpiper and 

 the Catbird. 



The ])ei"i<)d of incubation is aj)par('ntly closely depend- 

 ent upon the size of the t'g^r^ and varies from ten days 

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

 is said, some fifty in the Emu. 



