Marc D. Hauser in American Scientist:
The British philosopher Bertrand Russell once said that “it must have required many ages to discover that a brace of pheasants and a couple of days were both instances of the number two.” That discovery, however, was made not by the brace of pheasants, but the philosopher himself, presumably as an adult human being. And what of the pheasants? Are they capable of understanding that as a pair they represent the number two?
Birding wisdom holds that to watch most birds without disturbing them, it is best to hide behind a blind. If the bird sees you enter, however, you’re not much better off because it is now aware of the blind. One way around this problem is for two people to enter the blind together. Some time later, one person leaves and the bird, apparently assuming the coast is clear, goes back to business as usual. Why? Because most birds observed in this situation are incapable of computing a simple subtraction: 2 – 1 = 1!
It would seem that, if birds are any indication, animals are far from the most astute of mathematicians. But…
More here.