Link: http://www.audubon.org/bird/boa/F38_G1g.html

Text from

John James Audubon's 1840 Birds of America

Extremely suspicious and shy, this bird is ever on the look-out. Its sight is as acute as that of any Falcon, and it can hear at a considerable distance, so that it is enabled to mark with precision the different objects it sees, and to judge with accuracy of the sounds which it hears. Unless under very favourable circumstances, it is almost hopeless to attempt to approach it. You may now and then surprise one feeding under the bank of a deep creek or bayou, or obtain a shot as he passes unawares over you on wing; but to walk up towards one would be a fruitless adventure.

I have seen many so wary, that, on seeing a man at any distance within half a mile, they would take to wing; and the report of a gun forces one off his grounds from a distance at which you would think be could not be alarmed. When in close woods, however, and perched on a tree, they can be approached with a good chance of success.