Birds have evolved an extraordinary wide range of beaks in all manner of shapes and sizes depending on the environment in which they live and the food available to them.
Filter feeders, such as flamingos and some species of duck and swan, eat by straining food particles from water by passing it over a specialised filtering structure. Their bills have evolved to scoop up mouthfuls of water and separate mud and silt from food using tiny, complex hair-like structures called lamellae.
Unwanted items and water are pushed out of the side of their mouths using their large, rough-surface tongue, while small crustaceans, invertebrates, and algae are left to be ingested.
Flamingos’ beaks have also evolved to be used upside down. The upper mandible functions like the bottom mandible of most birds and vice versa, and they are one of the few species of animal that can move their top jaw while eating.
Most species of pelican feed by dip-netting. They have a long-straight upper mandible often with a hook on the end, and a large fold of skin, called a gular pouch attached to the lower mandible.
The pouch is highly flexible and can expand and contract and the lower jaw can bow outwards so they can use the pouch like a fishing net. They scoop up their prey, which is mainly fish but can also include crustaceans, turtles and amphibians, and then tilt their head and contract the muscles in the pouch to drain off any excess water. They usually swallow the catch whole.
Larger fish are often caught with the tip of the bill which they then toss into the air before catching and sliding down the gullet headfirst. Some species of pelican can hold up to 13 litres of water in their bills and 3 times more fish than in their stomach, but they do not store food in their pouch contrary to the well-known limerick.
Water shovelers, such as spoonbills, have long flat bills with a spatula-shaped tip. They feed by walking or running through shallow water over sand, mud, or clay sweeping their bill from side to side to find small fish, aquatic insects, amphibians and crustaceans.
The bill is serrated and covered with papillae which detect vibrations so they can feed by touch alone. Water drains through the serrations in the bill before they lift their head to swallow the food.
Scything is a feeding technique unique to avocets. They have long, thin beaks that curve upwards and which are used to forage in mud and shallow water for worms, crustaceans, aquatic insects, and plant matter.
They skim the water and hunt prey either by picking it from the surface of clear water, or by sweeping their bills from side to side to locate hidden prey if visibility is poor. Some stilts, which are closely related to avocets but have straight bills, will also use a similar method to scything to find food.
Mud-probers may have bills that curve downwards such as curlews, godwits, and ibises, or they may have straight bills such as snipes and dowitchers. Their bills have very sensitive tips which means that they can forage for food by touch alone.
They use them like forceps or chopsticks to pull prey from the mud up to depths of 20 cm. In very deep mud-probers the tip of the upper bill is able to open independently in a process called rhynchokinesis so they can grab and eat prey, such as crustaceans, molluscs, and small amphibians, even when the beak is submerged.
Surface skimmers feed by flowing low over water and skimming it with their lower beak to find small fish, insects, molluscs, and crustaceans. The lower mandible is thick and elongated and the upper mandible curves downwards into it part way along its length. As skimmers fly along the surface of the water, they open their bill dragging the lower mandible along until they find a fish. They then relax their neck, quickly snapping their jaws shut and snatching the prey out of the water.
There are just three species of skimmers in the world known to feed in this manner and because they feed by touch alone they often forage at night. Black skimmers have another unique adaptation which helps them fish. They are the only species of bird to have vertical slit-shaped pupils which means their bill falls within their field of binocular vision. This allows them to accurately position their bill to have the best chance of catching prey.
Plunge divers, such as terns and gannets, look for prey from the air and then dive into water to catch it in their beaks before returning to the surface to eat it.
Kingfishers are probably the best-known species of bird that practise plunge diving, although many of them don’t fish at all and feed on small mammals, reptiles, and other birds on land. They have long spear like bills shaped like a narrow cone which presents little resistance when the kingfisher enters the water. This means they can dive in barely making a splash and water flows smoothly around their beak giving the bird precious milliseconds to reach a fish before it has a chance to escape.
Once in the water the kingfisher catches the prey between its beak and returns to its perch where it will use its beak to rotate it so it can be held by the tail. The kingfisher will hit the fish on a branch several times to kill it before swallowing it whole headfirst.
Engineers working on Japan’s famous bullet trains were inspired by the shape of the kingfisher’s beak when they needed to fix the problem of loud booms when trains exited tunnels. They designed the front of the train to mimic the kingfisher’s beak which minimised the build-up of atmospheric pressure and not only reduced the noise but also allowed the train to travel faster using less electricity.
Pursuit divers are birds that chase their prey underwater. This group includes penguins, sawbills, grebes, and cormorants. They have large, strong beaks with curved tips or serrated edges, which help prevent their slippery catch from escaping. While their diet primarily consists of fish, which they swallow whole, they also feed on aquatic insects, worms, crustaceans, and molluscs.
Birds that catch insects in flight tend to have short, flattened beaks often with a hook at the end, and wide gaping mouths. Swallows, swifts, flycatchers, and nighthawks all have beaks adapted for insect-catching. They have small bumps on the roof of their mouths which point backwards and help the bird guide the food down their throat.
Some species of insect-catchers use their rictal bristles, tiny stiff feathers around a bird’s beak, to help guide the prey into their mouths using them a bit like a net. However, studies of flycatchers showed that their rictal bristles don’t play any part in capturing prey and they simply catch insects between the tips of their mandibles.
Insect-catching hummingbirds’ beaks are shaped to allow for an elastic snap so they can snatch flying insects in a fraction of a second, faster than could be achieved by jaw muscles alone.
