TEH DETBlOPMEIfT OP BIEDS. 229 



sometimes so fine that the surface of the egg is quite glossy. 

 Those of the Tinamous look like glazed porcelain. Some other 

 Birds, however, as the (jrebes and the Pelicans, lay eggs 

 covered with a chalky film, often thick and with calcareous 

 protuberances. The eggs of the Stork are more or less gra- 

 nulated or pitted on the surface ; and those of the Ostrich of 

 South Africa much more so, though, strange to say, the eggs 

 of those of North Africa have a smooth unpunctured surface*. 

 Ducks lay eggs with a greasy exterior. 



Eggs have commonly a special ground-colour, the intensity 

 of which seems to increase with the strength and vigour of the 

 individual. Upon this a variety of markings may be super- 

 imposed as small speckles, or round spots, or irregular blotches 

 or spiral streaks. The colour is not invariably the same in all 

 the eggs laid by a bird in one season. Thus the Tree-sparrow 

 seems always to have one egg different from that of the rest 

 laid in the same nest. The Guillemot is quite exceptional for 

 the extraordinary amount of variation in the colour and mark- 

 ing of its eggs. There is a great variety of coloration in the 

 class. Professor Newton affirms that hardly a shade known to 

 the colorist is not exhibited by one or more, and some of these 

 tints have their beauty enhanced by their harmonious blending, 

 or by the pleasing contrast of the pigments which form markings, 

 often most irregular and often regular in shape. 



Por the most part coloured eggs are laid in open nests, and 

 white eggs in covered nests ; but white eggs are sometimes laid 

 in open ones, as by Ducks. On the other hand, some spotted 

 and coloured eggs are laid in covered nests — as by the Jackdaw, 

 the Magpie, and the G-rass-warbler. 



The changes of development in the egg can only go on at a 

 certain temperature, to maintain which birds sit on their eggs, 

 or, as it is called, incubate. The period of incubation varies, and 

 is much related to the size of the birds. The egg of the Ostrich 

 requires to be incubated for from fifty to sixty days, while that 

 of the Wren needs but ten days. Mostly it is the Hen which 

 sits, the male often bringing her food ; often, however, the two 

 sexes takes turns in incubating. In some birds the male is said 

 to incubate, as in the Cassowary and Emeu, the Australian Frog- 

 mouth (Eurostopodus albogularis), and the Ostrich. 



With the Cuckoo, however, both sexes avoid the labour 



* See 'The Ibis,' 1860, p. 74. 



