BIRDS' EGGS. 



69 



which is deposited in laj'ers. The final layer varies greatly 

 in appearance, and may bo a rough, chalky deposit, as in 

 Cormorants and others, or thin and highly polislied, as in 

 Woodpeckers. 



The colors of eggs ai'e 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 lie pig- 

 mented, and variations in the tints of the same pigment 

 may be caused by an added layer of carbonate of lime, 

 pnidncing 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, nnless seen with the 



FiQ. 24. — Egg of (a) Spotted S.imlpipcr, (b) Catbirc-l, ti.i shoiv ditferenoe in 

 yize of eggs of pi'.Tvooial and altrieial liirds of sunii' .size. ( Natural .size.) 



parent, it is fretpiently impossible to identify eggs with 

 certainty. The eggs of priBcocial birds, whose young ai-e 

 born with a covering of down and can rnn 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 illu,strated 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 ten days 

 in the Hummingbird to iorty odd in the Ostrich and, it 

 ifi said, some fifty in the Emu. 



