The jay feeds both in trees and shrubs and on the ground. Acorns are usually a staple food. Insects are also taken in quantity and form the principle food given to nestlings. However, in fact jays are omnivorous, and berries, fruits, eggs, young birds, mice and other small mammals are eaten as well.
Hunting and killing of recently-fledged young small birds is usually, and perhaps only, practised by adult jays that are feeding well-grown young. Many distasteful or poisonous insects are habitually ignored by jays. This is almost certainly due to learning. Walter Rothschild proved that a jay was able to distinguish which of two insects, one poisonous, one innocuous, the former eaten shortly before the latter, had caused its discomfort even although both were vomited up together.
In towns and suburbs, jays readily learn to eat bread and other human food and become very fond of peanuts.
When seeking food the jay will dig in the likes of dead leaves or loose soil with side-to-side swings of head and bill and try to open crevices, rolled-up leaves or any other seam or small hole by inserting and then opening the bill.
Just like other crows, food that needs breaking up is held under their feet. Usually it’s held under both feet, less often under one foot. The jay dislikes holding anything at all sticky under foot and food of this description, if it needs further breaking up, is usually held in the bill and rubbed about on the perch or on the ground. To eat an acorn the jay holds it between its feet on the perch. They seldom, if ever, hammer at an acorn but bites and levers with its bill till the shell is pierced, when it is soon removed.
The jay habitually hides food, mostly acorns and usually in the ground, but sometimes in trees as well. When an acorn is being hidden the bird usually gives it a few hard blows with the bill after inserting it and before covering it up. Jays in the London parks hide bits of meat and cheese above the ground more often than they do nuts and acorns, perhaps through learning that such foods are not improved by being buried.
Intensive storing of acorns takes place as soon as they are ripe and while they are still on the trees. They are, at any rate, by resident adult pairs, carried back to the birds’ living areas. This is very apparent in times of widespread failure of acorn crop when jays may fly miles to obtain acorns. In places, such as inner London, where the oaks tend to be scarce and scattered, but much less so under other conditions as, naturally, the jay collects acorns as near home as it can.
Acorns are habitually recovered. Observations suggest that, at least in south-eastern England, most of the many acorns eaten by Jays in winter and spring have been previously hidden by them. Hidden acorns may be regularly recovered from under a foot or more of snow. The bird retrieves hidden food by flying or hopping straight to the spot.
Jays may learn to correlate baby oaklings with the presence of a hidden acorn, through finding the former when they return for an acorn they have buried or possibly by chance. Comparable learning is apparently shown in relation to birds-nesting. Many jays, presumably as a result of experience, will at once begin to search for a nest if a passerine starts to mob or attack them. It is at first astonishing to see the way in which the jay, which may be high in a bare tree or on a garden fence when for example a blackbird begins to mob it, immediately flies to the nearest bush, hedge or other likely nesting place and determinedly searches there.