CAN BIRDS COUNT? [Ref]
If birds can count, it's a rather rudimentary thing—perhaps no more than impressions of the size of groups. The widely known example showing that birds don't seem to distinguish between one and two persons is the ruse used by bird photographers and students of birds who are using blinds from which to watch the birds at close range.
The hide, or blind, is a little hut built perhaps a few feet from the nest to be photographed. If the photographer enters the blind in the sight of the parent birds, and conceals himself there, the birds who saw him go in will be a long time in coming to the nest and in resuming their normal activities. But if the photographer takes a companion with him, both go into the blind and conceal themselves, and then one of them goes away leaving the other concealed, the bird quickly disregards the intrusion and goes about its activities as though no one were left in the blind. This subterfuge has long been used and is very successful. Apparently the bird is unable to distinguish between the two people that arrive at the nest and the one only that leaves, and behaves as if both had gone away.
In my duck-hunting days a duck hunter who used wooden decoys told me he was sure that there was a certain number of decoys necessary before they were effective.
The decoys were wooden blocks, carved and painted to resemble life-sized ducks, weighted to float like them, and anchored in shallow water in a flock within gunshot range of the blind in which the duck shooter sat. The idea was that ducks flying by would see the flock on the water, assume that here was a safe resting place, and fly in and light, or attempt to light among them, giving the wild-fowl gunner an opportunity to shoot the wild birds.
The duck shooter claimed that if less than twenty-five or thirty decoys were put out in the flock, the setup was much less effective than if more than twenty-five to thirty decoys were used. He thought that the ducks could distinguish between less than twenty-five or thirty and more than twenty-five to thirty, and favored the latter. Though this is distinguishing between greater and lesser amounts, it hardly comes in the category of counting.
DISTINGUISH "MORE" FROM "LESS" However, a series of experiments summarized on Page 121 in the periodical Bird Banding for 1940 seem to indicate that birds can distinguish between different numbers of things, such as peas and numbers of dots. The birds, including pigeons, parakeets, and jackdaws, were trained either to choose a certain number of objects under certain circumstances, or to choose between two quantities of objects with reward and punishment motivation. It was found that these birds were able to distinguish up to a maximum of six. That this is really counting in the human sense of the term, which is linked with speech or written symbols, is improbable, but it does indicate, as one would expect, that birds do at times distinguish between different quantities, and sometimes with considerable precision.