Nest-building, Incubation, and Migration. 237 



horsehair. A second nest which the aviary greenfinch 

 built was also perfectly typical. 



In the same year Mr. Budgett reared from a few weeks 

 old a young hen bullfinch, and kept it in a cage till the 

 next spring, when he bought a cock, probably an old bird, 

 and turned them together into the aviary. The hen soon 

 began to build, and finished her nest in about four days ; 

 but she used neither roots nor twigs, of which there was a 

 plentiful supply. The nest was composed of nothing but 

 dried grass, with a little wool and hair. She laid therein 

 five eggs, two of which hatched, but the little birds soon 

 died. She then began another nest, this time a typical 

 bullfinch's nest of fine twigs and roots lined with horse- 

 hair. Here five eggs were again laid, all of which were 

 hatched, and three reared. She also built a third nest, 

 which was perfectly typical of her species. 



Further evidence of this kind is much to be desired. 

 But this, as far as it goes, appears to be decisive. Some 

 birds build their nests true to type, without opportunities 

 or with but the slenderest opportunities of imitation or 

 instruction. It appears to me that the evidence before us 

 justifies the conclusion that nest-building in definite ways 

 is an instinctive activity ; * but that it is modifiable by 

 individual experience. Whether the modifications are 

 inherited we do not know. It may be well to note how 

 largely the performance of this activity is due to internal 

 impulse, the external stimulus, being perhaps afforded by 

 the sight of the requisite materials. 



It is also advisable to draw attention to the complexity 

 and delicacy of the activities involved in nest-building. 

 There is necessarily a good deal of careful selection of the 

 appropriate materials ; and these materials are used in 



* The nest-building of the sticklebacks, both three-spined and ten- 

 spined, is without doubt truly instinctive. 



