THE DEVELOPMENT OF BIRDS. 229 
sometimes so fine that the surface of the ege is quite glossy. 
Those of the Tinamous look like glazed porcelain, Some other 
Birds, however, as the Grebes 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 era- 
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. 
Eges 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 alw ays 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. 
For 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 son some ype 
and coloured eggs are laid in covered nest 
the Magpie, and the Grass-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 (Hurostopodus albogularis), and the Ostrich. 
With the Cuckoo, however, both sexes avoid the labour 

* See ‘ The Ibis,’ 1860, p. 74. 
