196 A HISTORY OF BIRDS 



constant, as for example in the Plover tribe, which lay three or 

 four, or in the Gulls and Terns, which lay two or three, or of the 

 Pigeons, which never exceed two. 



In form birds' eggs present certain peculiarities ; thus in the 

 Owls, Bee-eaters and Diving Petrel they are spheroidal, and 

 bi-conical in the Night-jar and Sand-grouse ; in the Cormorants 

 they present a long ellipse, while in many of the Plovers and 

 Guillemots they are pyriform. Only in the case of pyriform 

 eggs does shape appear to have any significance. Thus in the 

 case of the Guillemot, which deposits a single egg on a bare 

 ledge of rock in the face of some cliff, this shape is obviously 

 advantageous, since, if blown by the wind, or struck by the 

 bird as in diving off the ledge, instead of rolling over into the 

 sea or on to the rocks below, it will revolve in a circle on its 

 own axis ; though in spite of their shape thousands of eggs 

 are annually lost, either because laid on too small or sloping 

 a ledge, or because they are struck too violently by the sitting 

 bird, as when it leaves the work of incubation through fright. 

 In the case of the pyriform eggs of the Plovers this shape is of 

 advantage, since it allows the eggs to be more closely packed 

 — the sharp ends inwards — and so more easily covered by the 

 sitting bird. This being so, it is a little surprising to find that 

 even among nearly allied birds of this group pyriform eggs are 

 by no means the rule. 



The size of the egg does not by any means always cor- 

 respond in proportion with the size of the bird which laid it, 

 but depends rather on the relative development of the newly 

 hatched chick. Thus where the eggs are relatively large — com- 

 pared with the size of the parent — the chick at hatching is able 

 to run about and feed itself, under guidance, while on the other 

 hand, when the egg is small the chick is hatched in an ex- 

 tremely helpless state — before the eyes have opened, or the 

 body has developed a covering of any sort (p. 246). In propor- 

 tion to its size perhaps the Apteryx lays the largest egg. This 

 bird indeed was described on one occasion as " the bird which 

 lays eggs bigger than itself" ! By way of contrast a comparison 

 is often made between the egg of the Humming-bird and that 

 of the extinct .(Epyornis, the cubic contents of whose egg 

 equalled about three gallons. The Curlew, Raven and Guil- 

 lemot are birds of similar size, and so also are the Snipe and 



