PARENTAL CARE AND NEST-MAKING 291 



susceptible. Some details are interesting. Thus some birds 

 (e.g. albatross, penguin) show bare patches where the eggs 

 come into direct contact with the warm skin ; some brooding 

 birds, e.g. snipe, increase their inconspicuousness by putting 

 soil on their backs ; some birds, e.g. eider-duck, draw a 

 coverlet over the eggs when they have to leave the nest. 

 These details suggest the danger of easy-going mechanical 

 interpretations. 



Mere bare patches require no special interpretation, but 

 there is sometimes a very definite transformation of the skin 

 and blood-vessels, which may be seen, as in the Crested 

 Grebe, long before the time for brooding — in the young 

 bird indeed. The transformation consists in an absence of 

 the fatty layer (panniculus adiposus) and in the marked 

 development of a network of blood-vessels. These include 

 the external thoracic artery, the abdominal cutaneous artery, 

 and the abdomino-pectoral cutaneous veins. The signifi- 

 cance of this " brooding organ " is that it brings the 

 warmth of the blood nearer the eggs. 



The modes of incubation may be classified : — 

 {a) Female only, e.g. the hornbill, albatross, eagle-owl. 

 In many cases, as in the hornbill, the male feeds his 

 brooding mate. 



(b) Male only, e.g. the emu and the Rhea. In the Grey 



Phalarope, where the female does the courting, the 

 male does the brooding. 



(c) Both parents, e.g. ostrich and oil-bird. In both these 



cases the female sits by day and the male by night, 

 while pigeons illustrate the ordinary method of 

 irregular alternation. 

 Brooding of Pigeons. — It may counteract the vagueness 

 of these general statements in regard to incubation to give 

 some details of the behaviour of pigeons, based on the care- 

 ful observations made by the late Professor Whitman (1919). 

 The time of incubation differs in different species ; in the 

 common pigeon it is 17-19 days, in turtle-doves 14-16 days. 

 In the now extinct Passenger Pigeon it was only i2| days. 

 Some species begin to sit on the nest before the first egg is 



