chick when but a half-hour old gets only small bits of predigested fish, but at the age of three weeks it may be invited to bolt an entire squid. Again a bird like the black-billed cuckoo, which has repeatedly tried to serve a large insect and failed, has been seen to quickly withdraw it, mince it fine with her bill, and then offer it with success.
Birds quickly acquire a habit of going to their nest, by a definite path, through association, and if the branch which holds it is suddenly removed, they try to follow the established course, and will hover at the point in space which the nest formerly occupied, even when their young are in full sight, and these actions may be repeated many times, until the old habit is broken by an actual visit to the new position (compare
Fig. 33. Flicker feeding Young, with nest-hole opened at the back, illustrating the force of old habit. The bird for some time continued to enter the hole by the usual course.
Figs. 33 and 34). The habit of entering the nest from a certain side, of facing the same way while sitting over the eggs, of grasping the same branch when inspecting and cleaning the nest, and of leaving it in a definite manner, are all more or less fixed by habit in a brief course of time. In the same way drinking and bathing places, perches, spots for dusting, sun-bathing and sleeping are resorted to by habit for longer or shorter periods, according to the other conditions which modify behavior.
Do birds discriminate their own eggs and proper young? Very many do not, yet some do, sooner or later. The success of the European
