272 SEA-BIRDS 



and the little auk with their eggs entirely shielded by the rock cover; 

 and the puffin nesting in the safety and darkness of its burrow. It is 

 even more interesting to find that of these living auks the common 

 and the Briinnich's guillemots (Hke the extinct great auk) have a single 

 large brood-spot between the thighs. The razorbill, the little auk 

 and the puffin normally lay only one egg, yet, like the black guillemot 

 (which lays two eggs), they carry two brood-spots. On the razorbill 

 and the puffin the brood-spots are remarkably small and narrow, 

 situated on each side of the body close to the wing and thigh. They 

 are so reduced that only a comparatively small portion of the egg can 

 be accommodated against the bare skin, the rest being covered by one 

 wing which, being insulated by feathers, cannot, of course, provide 

 the same amount of heat as the brood-spot. 



The double brood-spot may be a relic of an earlier period in the 

 evolution of these birds when two eggs were normally incubated. 

 It might be argued that incubation of two large eggs within the narrow 

 confines of an underground burrow, at least in the case of the puffin, 

 would be inconvenient and therefore disadvantageous to the species. 

 However, in the little auk this cannot apply; there is normally plenty 

 of room in the little cavern under the rocks. The Ggg of the puffin is 

 white, like that of most hole-nesters, but when held up to the light it 

 exhibits a washed-out lilac or brown pattern, suggesting a protective 

 colouring at an earlier stage of evolution, when presumably the species 

 was an open-site breeder. It may well be that the puffin has become a 

 hole-nester from an open-site breeder and that the razorbill is in a 

 preliminary stage of evolution from the period when it brooded two 

 protectively-coloured eggs in the open. Two eggs are sometimes, 

 though rarely, brooded by both razorbill and puffin, but there is no 

 record of their successful hatching. Further conjecture on this sub- 

 ject, however, does not seem called for in the present state of our 

 knowledge. It must suffice here to point out that the razorbill incu- 

 bates its egg under "one falling wing" not because the egg is so large 

 (as has been suggested by one writer — an invalid reason: the guillemot 

 is scarcely as large as the razorbill but it has a larger egg which it 

 incubates between the thighs) but because of the situation of its brood- 

 spots. 



We have seen that territory-finding and claiming in the auks 

 begins early in some species, later in others. The sexual bond brings 

 the auk pair to the familiar nesting site in the spring. It has been stated 

 that auks, on their arrival on the water below the cliffs, are already 



