152 CHILDHOOD OF ANIMALS 



construct a platform nearly a foot in diameter and then gradually, 

 layer by layer, rear up a circular wall curving inwards until it 

 becomes a closed dome with an aperture at one side. The aperture 

 leads into an outer chamber, and there is a second inner chamber 

 placed high up and lined with soft material for the reception of the 

 eggs. Some of the swifts in Indo-Malay and Australia make 

 saucer-shaped cradles of thick saliva mixed with feathers and 

 fibres, while the swift from nests of which " birds '-nest soup " is 

 made uses only saliva. 



When the "nest " is merely a hole scraped in the ground, in most 

 cases it is the work of the female only. When it consists of a 

 quantity of material scraped up, or collected from a distance, or 

 woven or moulded into a specially shaped receptacle, both males 

 and females join in the task. There are a good many cases where 

 birds associate in colonies for nesting. We are all familiar with 

 rookeries, and with the massed nests of swallows and swifts. A 

 great many sea-birds lay their eggs or construct their simple nests 

 so close to each other that they almost touch, and there are one 

 or two instances where birds combine to form enormous structures 

 containing the individual nests of many pairs. It is probably 

 more the common choice of a suitable site than any social instinct 

 that has led to these associations. Although quarrels and robberies 

 are frequent, there is a certain amount of combination against 

 common enemies, but the families remain really distinct, and there 

 is nothing approaching the ordered communities that occur 

 amongst insects. 



When the receptacles are ready, the females place the eggs in 

 them. Eggs differ remarkably in shape. Some are almost spherical, 

 most are elongated ovoids with one end larger than the other, in 

 the extreme cases the eggs being pear-shaped. Rounded and 

 regularly ovoid eggs, if given a push on a smooth surface, will roll 

 a great distance ; pear-shaped eggs simply twist round in a circle 

 of which the narrow end is the centre. Such eggs would certainly 

 be little liable to breakage by being rolled off rocky ledges, and they 

 are found in the case of many birds which lay in dangerous 

 situations, but they also occur in shore-birds where there is no 

 similar danger. There appears to be no advantage or special 

 adaptation in the various gradations from nearly spherical to 

 oblong eggs. 



The shell of eggs is a transparent, organic membrane thickened 

 and hardened with deposits of lime, and the natural colour, 



