He hops about and pecks, not for the love of any thing he expects to find, but just for the love of hopping and pecking.
The real explanation is that the food which he has taken is delivering up, within his system, the force stored in it that was received originally from the beams of the sun, while the plant which produced it was growing.

This force must have an outlet, and it finds this outlet in the incessant activity of the bird's muscles and brain.

The various objects which attract his attention without, _invite_ the force to expend itself in _certain special directions_; but the impelling cause is within, and not without; and were there nothing without to serve as objects for its action, the necessity of its action would be none the less imperious.

The lion, when imprisoned in his cage, walks to and fro continuously, if there is room for him to take two steps and turn; and if there is not room for this, he moves his head incessantly from side to side.

The force within him, which his vital organs are setting at liberty from its imprisonment in his food, must in some way find issue.
Mothers do not often stop to speculate upon, and may even, perhaps, seldom observe the restless and incessant activity of birds, but that of their children forces itself upon their attention by its effects in disturbing their own quiet avocations and pleasures; and they often wonder what can be the inducement which leads to such a perpetual succession of movements made apparently without motive or end.