THE AUKS 281 



suggests that the chief functional value of their peculiar shape lies in 

 a larger surface being in contact with the bird's belly during incuba- 

 tion. The great auk, with one brood spot, had a pyriform egg, too. 

 Puffins and little auks, nesting in safer situations, have somewhat round- 

 er (yet still pyriform) eggs (and two brood-spots). Both sexes incubate. 

 There is evidence that in guillemot colonies of above a dozen pairs, 

 eggs may sometimes be pirated and adopted by neighbours (Lockley, 

 1934). Johnson states that the common guillemot (or murre, as it is 

 called in America, where his study was made) recognises its egg and 

 moves it back into the old position if it rolls away or is experimentally 

 transposed by the observer; in three instances the adults whose eggs 

 had been moved "hovered" first on the vacant egg-site, then seeing 

 the egg in a new postion, went to it and by degrees shuffled it between 

 the breast and legs until the original site was regained. But one 

 guillemot which had lost her egg was seen by Johnson to "steal" 

 another from the mire accumulated in a depression on the ledge and 

 "moved it about twelve feet to the location where last evening it 

 covered (the lost egg)'', further evidence of the determined pirating 

 of eggs by bereaved adults. 



Some eggs become so fouled with slime, which dries to a hard coat, 

 that the original beautiful colour is lost, and many must be only 

 recognisable by their position on the ledge. 



Gulls patrol cliffs where guillemots breed, and become bold in 

 the search for eggs and small chicks. We have watched a herring- 

 gull alight on a ledge and try to force a sitting guillemot from its egg. 

 After visiting a guillemot colony, many observers have watched gulls 

 seize the eggs before the slower-flying guillemots, frightened away by 

 the human intruders, returned. Small guillemot colonies may lose 

 all their eggs as a result of a single visit by a human being. More 

 rarely the sudden swoop of a gull will cause a panic in which the 

 frightened guillemots leap from the ledges before disengaging their 

 feet and breast from the egg, which is then accidentally carried out 

 and dropped in mid-air. The guillemot will lay at least once again 

 if the first egg is lost when fresh. Winn (1950) found that black guille- 

 mots replaced fresh eggs, which had been destroyed, in 15 days; 

 but there was no re-laying after June. 



Very little reliable data on the incubation and fledging-periods 

 of auks was published until the work of Lockley (1934c) on the puffin, 

 and Keighley and Lockley (1947, 1948) on the razorbill and common 

 guillemot, and Winn on the black guillemot appeared. The results. 



