444 The Outline of Science 



earned; this is related in a large degree to the length of the 

 incubation period, while this in turn depends to an important 

 extent on the state of development of the young when hatched, 

 a subject which has already been discussed. 



In texture of shell, eggs vary from the brilliantly polished 

 egg of the Tinamous to the soft chalky eggs of the Cormorant, 

 from which the white outer surface can be scraped to show a 

 pale-blue layer beneath. Thickness of shell is also a variable 

 factor, apart from the mere relation to general size. 



Egg Coloration 



It is, however, the colour of eggs that have always attracted 

 most attention; some of these are exceedingly beautiful both in 

 tint and in the patterns of marking. Blues and greens are com- 

 mon especially among tree-nesting birds, while ground-nesters 

 usually show neutral brown tones which are most effective for 

 purposes of "camouflage"; some splendid red tones are char- 

 acteristic of the birds of prey. Markings may be small spots 

 or larger blotches, and they may be evenly distributed or con- 

 centrated in a particular zone; fine lines also are found in some 

 cases, witness the Buntings, and in many birds there is a plain 

 marked ground-colour. Pure-white eggs are usually found in 

 species which nest in holes, and this is perhaps of some use in 

 the dark, although the more important point is probably the 

 absence of any occasion for an attempt at "camouflage" colora- 

 tion. Coloration in many instances serves a protective purpose, 

 and, generally speaking, it is related to some extent to the nature 

 of the bird's environment. There are, curiously, no pure black 

 eggs. 



16 

 Behaviour of Birds 



More than any other creatures, birds have claimed the atten- 

 tion of those who are fond of what Fabre called "scrutinising 



