LEARNING BY BIRDS [Ref]
Of course birds can learn. Indeed there's a trite saying that no animal has been discovered so low that it cannot learn. One of the simplest cases of learning is shown by parts of some experiments I carried out years ago on the curve-billed thrasher. I had raised a number of these thrashers by hand, and in connection with finding out about their tasting abilities I first fed them on the white of egg, hard-boiled and cut into little squares. They liked it. Then I soaked more squares of boiled egg white in evil tasting (to me) formalin. The birds came to the dish, and also ate them. But after that for a week they refused to eat such egg white. They had quickly learned to avoid the ill-tasting food.
BAD-TASTING FOOD Once I hand-raised a barred owl from a nestling to adulthood. Sometimes getting food for it was a problem, and upon occasion I fed it frogs, which it seemed to like well enough. But then came a day when I fed it a toad. The owl seized the toad at once. Now toad skin, presumably as a defense weapon of the toad, secretes a substance irritating to the mucous membranes of some animals. And this was evidently irritating to the owl, for it did not hold the toad long in its bill. It spat it out, and the owl's face gave evidence of disgust. After that the owl not only refused to take toads, but it also refused to take frogs such as it had found palatable before. Evidently frogs looked too much like toads. The learning was effective, and extended not only to the original object, but also to other, similar objects.
BUCKET-DRAWING When in Florida with the Archbold Expeditions I was studying blue jays. A very simple but amusing thing that chickadees learn is to sit on a perch and pull up a little container of food that dangles far below the perch on a string. Jays, along with crows, are among the most clever of birds, as I've said before, and I gave two jays in one cage a chance to learn the trick. In three days one of the jays was regularly and quickly pulling up the little bucketlike container and getting its food from it. The process was simple: the jay reached down, seized the string in its beak, secured the slack under its foot, and reached down again for another pull. Sometimes five separate pulls were needed to raise the food bucket the eight inches to the perch.
The jays were regularly fed in this manner. Soon I noticed that only one of the two birds pulled up the bucket, though the other also fed from it. In effect one was depending on the work of the other. After this had gone on for a month, I wondered if the second jay, which had never done any of the work, would be able to pull up the bucket if left alone. Certainly it had had lots of opportunity to learn by seeing its cage mate go through the motion. So I left it alone in the cage. This second jay, despite its chances to learn by observation, took one day longer to learn how to pull up the bucket of food than had the first jay. The jays certainly learned the trick quickly through a trial-and-error process, but simply watching the process seemed to be of little help in learning it.