Many studies show that the largest number we are able to capture at a glance, without counting, is about four or five. The following experiment was performed in 1930.
A squire was determined to shoot a crow which made its nest in the watch-tower of his estate. Repeatedly he had tried to surprise the bird, but in vain: at the approach of man the crow would leave its nest. From a distant tree it would watchfully watch until the man had left the tower and then return to its nest.
One day the squire hit on a ruse: two men entered the tower, one remained within, the other came out. The experiment was repeated on the succeeding days with two, three, then four men, yet without success. Finally, five men were sent: as before, all entered the tower, and one remained while the other four came out and went away. Here the crow lost count. Unable to distinguish between four and five it promptly returned to its nest.