ANIMAL BEHAVIOR. 327 



(3) Periodicity. The periodicity of the disposition to sit 

 coincides in the main with that of the recuperative stage. Its 

 length, however, at least in birds, is nicety correlated with, 

 though not exactly coinciding with, the time required for hatch- 

 ing. It may exceed or fall short of the time between laying 

 and hatching. The wild passenger pigeon (Ectopistes) begins to 

 incubate a day or two in advance of laying, and the male takes 

 his turn on the nest just as if the eggs were already there. In 

 the common pigeon the sitting usually begins with the first egg, 

 but the birds do not sit steadily or closely until the second egg 

 is laid. The birds do not, in fact, really sit on the first egg, but 

 merely stand over it, stooping just enough to touch the egg with 

 the feathers. This peculiarity has an advantage in that the 

 development of the first egg is delayed so that both eggs may 

 hatch more nearly together. The bird acts just as blindly to 

 this advantage as Ectopistes does to the mistake of sitting 

 before an egg is laid. Ectopistes is very accurate in closing 

 the period, for if the egg fails to hatch within twelve to twenty 

 hours of its normal time, it is deserted, and that too if, as may 

 sometimes happen, the egg contains a perfect young, about 

 ready to hatch. Pigeons, like fowls, will often sit on empty 

 nests, filling up the period prescribed in instinct, leaving the 

 nest only as the impulse to sit runs down. It happens not 

 infrequently that pigeons will go right on with the regular 

 sequence of activities, even though nature fails in the most 

 important stage. Mating is followed by nest-making, and at 

 the appointed time the bird goes to the nest to lay, and after 

 going through the usual preliminaries, brings forth no egg. 

 But the impulse to sit comes on as if everything in the normal 

 course had been fulfilled, and the bird incubates the empty 

 nest, and exchanges with her mate as punctiliously as if she 

 actually expected to hatch something out of nothing. This 

 may happen in any species under the most favorable condi- 

 tions. It is possible by giving an abundance of rich food to 

 wind up the instinctive machinery more rapidly than would nor- 

 mally happen, so that recuperation may end in about a week's 

 time, when incubation will stop and a new cycle begin, leading 

 to the production of a second set of eggs in the same nest. 



