NESTS AND EGGS, AND NESTLING BIRDS 101 



becomes red, inflamed by the accumulation of blood contained 

 in certain of the abdominal veins, and a consequent increase of 

 temperature. Such inflamed areas are known as " brood spots." 

 When this area is normally covered with feathers these are shed 

 to facilitate the brooding. 



While many reptiles may lay as many as sixty eggs, birds never 

 lay more than twenty, as in some of the game-birds ; but a far 

 smaller number is the rule. Thus the guillemots, auks and 

 petrels lay but a single egg during the breeding season, though 

 this will be replaced if, by accident or other causes, destroyed. 

 Pigeons never lay more than two ; plovers four. 



For the most part the external layers of the shell of birds' 

 eggs are more or less highly coloured. Among birds which build 

 no nests, but lay their eggs in slight depressions in the ground, as 

 in gulls and plovers, the coloration of the shell harmonises so 

 perfectly with the surroundings as to be almost invisible. Where, 

 however, the eggs are deposited in a more or less bulky nest the 

 colours may often be relatively conspicuous, as in the blue eggs 

 of the thrush and hedge-sparrow. But in these cases the pro- 

 tective coloration is transferred to the exterior of the nest ; for 

 unless this is rendered as inconspicuous as may be, no amount 

 of protective coloration which the eggs might possess would 

 be of any service. 



When the eggs are laid in holes and other dimly lighted places 

 the shells are white, and this because eggs having coloured shells 

 would inevitably be smashed sooner or later by the sitting bird, 

 which, returning to the nest, would be unable to see the eggs in 

 so uncertain a light, and would thus break them in the endeavour 

 to settle down. White shells, on the contrary, in such a situation 

 would be visible. That this is so is shown by the case of the 

 puffin. This bird lays its eggs in a burrow, and the shell is white. 

 But this whiteness is formed by a thick chalky layer which, on 

 being scraped away, reveals a coloured shell. 



There are some birds, it should be remarked, which are ex- 

 ceptions to the rule just laid down, as, for example, many pigeons, 

 such as the wood-pigeon, which constructs a flimsy nest of sticks 

 in the topmost branches of some tall tree. But here they are 

 too high up to be in danger of enemies from below, while the 



