Bird food

Rich, easily digested food makes up the bulk of most birds’ food. Because they need lots of energy, but have to keep their weight down for flight, very few birds eat bulky plant food such as grass or leaves. The majority are omnivores, taking a mix of seeds, fruit and small animals including insects. Without teeth to grind and chew, birds must make do with their bills and their muscular stomach. And they must digest their meals as quickly as possible to get rid of any excess weight.

Insatiable appetite. Small birds burn energy at an amazing rate and must eat vast amounts just to stay warm, let alone fly. In winter, a blue tit can spend 90 per cent of its waking hours feeding to stay alive. Hummingbirds use up fuel at 10 times the rate humans do.

Dropped from a height. Foods encased in a shell can be problematic when a bird’s bill lacks the power to crack them. One solution is to drop them from a height. This tactic is used by herring gulls on mussels, by lammergeiers to break bones, and by crows to smash eggs.

Feat of memory. Food is hard to find in winter, so some birds build up a secret stash during the autumn glut. Jays bury thousands of acorns, hiding each one in a different part of the forest and memorizing its location. Nutcrackers bury up to 100,000 nuts and seeds each year and can remember their locations nine months later.

Pigeon milk. Pigeons and doves are unique among birds in that they produce a kind of milk from the crop to feed their young. Crop milk, a thick soup of protein and fat, is made for the first three weeks of the chicks’ lives. After that the mother weans them onto solid food by swallowing mouthfuls of seeds and storing them in the crop, where they soak in the milk to form porridge.

Quick digestion. An average meal takes half an hour to pass through a bird’s body (compared to 24 hours in humans). A turkey vulture, for instance, can digest a whole snake in 90 minutes; the reverse process (a snake digesting a turkey vulture) takes weeks.

Eat dirt. Small birds hull their seeds to avoid carrying extra weight, but bigger seedeaters such as farmyard chickens swallow seeds, husks and all. They grind up their meals in the gizzard, a muscular stomach that contains swallowed grit and stones to mash the food. A wood duck’s gizzard can puree walnuts, and an eiderduck’s can crush mussel shells. Turkeys are said to be able to grind steel needles in theirs.

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