BRITAIN'S BIRDS AND THEIR NESTS. 25 



excavated on the softer slopes by the birds themselves. 

 In these burrows the birds may be easily caught once the 

 position has been reached ; but all Petrels have the un- 

 pleasant habit in such emergencies of vomiting over 

 their aggressors a thick, oily fluid, the half - digested 

 contents of their stomachs. Oil is indeed typical of 

 Petrels ; their bodies are very oily, and the odour of oil 

 hangs about them and pervades their nesting-holes. A 

 slight nest of grass, &c., is sometimes made at the end 

 of the burrow ; but this is very often dispensed with. 

 Some of the birds begin to lay by the end of May, but 

 incubation is not general till well on in June. Only one 

 egg is laid, and this is pure white before it becomes 

 soiled, which it soon does owing to the rough texture of 

 its shell. Not uncommonly some very faint reddish specks 

 are present, often arranged in a zone near the larger end. 

 The Storm Petrel will lay a second and a third time if 

 robbed, and the fact that eggs have been found in the 

 middle of September has led to the supposition that two 

 chicks are sometimes reared in a season. But as incuba- 

 tion lasts about seven weeks, and the nestling is said to 

 take more than ten weeks to become fledged, this seems 

 hardly possible. The nestling is at first nearly naked and 

 helpless, but is later covered with very long black down. 



Small fish and various pelagic organisms form the 

 food of this Petrel, and fatty refuse of all descriptions 

 is particularly sought after, the birds flying close to 

 the surface of the sea and skimming off minute float- 

 ing particles. The name Petrel, although also widely 

 applied to the whole order, strictly belongs to this and 

 a few allied species. It means 'little (bird of St) Peter,' 

 and is probably an allusion to their habit of ' walking 

 on the water' — that is, of paddling their feet in the 

 water as they fly close to the surface. 



D 



