Experiment seems, therefore, to show in a way, and with a clearness impossible of attainment by observation under natural uncontrolled conditions, that nest-building in birds is instinctive. That the manner and method of procedure is often modified in accordance with special conditions--that the instinctive outline of nidification receives its final touches through individual experience--is sometimes seen under nature, and more often under the semi-experimental conditions of domestication. Thus three pairs of pigeons in the Wilson Tower of Clifton College made their nests in 1898, as I am informed by Mr. H. C. Playne, of galvanized iron wire, pieces of which were left in a corner at the top of the tower, thus affording a parallel to the behaviour of the unconventional crow of Calcutta, mentioned by Mr. F. W. Headley,[59] which made its nest of soda-water bottle wires, which it picked up in a back yard. But even in this matter experiment serves to bring out clearly the selective influence which is exercised by intelligence. Bolton,[60] in 1792, observed a pair of goldfinches beginning to build their nest in his garden. They formed the ground-work of moss, grass, etc., as usual; but on his scattering small parcels of wool in different parts of the garden they, in great measure, left off the use of their own stuff and used the wool. Afterwards he gave them cotton, and they then used this instead of the wool; then he supplied fine down, and they finished their work with this, leaving the wool and cotton.