Birds learn solutions to their problems
Marsh harriers, which frequent wetlands in many parts of the Old World, have been observed drowning waterfowl that they have caught in pools or shallow streams. They hold the victim's head under water until it stops struggling. But as these birds never try to drown fish they have caught, it must be assumed that this behaviour is learned and not instinctive.
Crows, too, are excellent problem solvers, both in the wild and in captivity. Fatty treats hung on string in gardens to attract tits, for example, are often taken by crows. They pull up the string with their beaks while holding the slack against the branch with one foot.
Cracking the problem
Foraging successfully in an uncertain world takes skill, and crows are very adaptable - in western Canada some of them take whelks from the seashore at low tide.
Experiments have shown that the crows pick up only those shells that weigh enough to contain a live whelk. They seem to know that an underweight shell is likely to contain only the shrivelled remains of a dead whelk. To get at the meal inside the shell, the crows have learned to drop them onto a rocky surface time and again. On average, it take four drops from a greater height, may be more effective in breaking the shell, but it often results in the contents being scattered or contaminated with sand, so judging the correct height is crucial and must be learned.
Many other birds have acquired the skill of breaking a shell, too. European thrushes hammer snail shells on stones until they splinter, and the bearded vulture of Africa and Asia drops bones onto rocks from a height in order to break them open and allow the bird to reach the marrow inside.
to be continued...