When birds gave up their teeth and traded their “hands” for wings, obtaining food may have become challenging. But through evolution, birds developed an ingenious assortment of bill shapes to enable them to keep eating.
In my column in the July/August 2018 issue, I described how bills evolved from primitive snouts and what researchers have learned from studying the skulls of modern and ancient birds. Here I give examples of various birds’ bills and how they use them.
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Finches have conical bills designed for cracking seeds. Their bill size correlates with preferred seed size, which reduces competition in feeding flocks. Seeds are held between the edges of the mandibles and kept in place with pressure from the tongue. The side of the upper mandible is grooved, while the side of the lower mandible is narrow and knife-like. With a seed in place, the bird bites hard while moving the lower mandible forward in a “slicing” motion that facilitates cracking the shell. The inner soft parts of the seed are separated and swallowed and the shell fragments expelled.
A unique bill modification allows crossbills to feed almost exclusively on conifer cones. A cone looks like a fist full of flattened scales; its seeds are located at the base of the scales. To reach the seeds, the scales must be separated (pushed apart). A biting bill can’t do that. Cleverly, bills of young crossbills are finch-like but, as they grow, the distal parts of the mandibles curve. The lower curves up and to the left or right, and the upper curves down. When the bill is closed, the tips cross. When feeding, crossbills open the bill slightly until the tips are even and insert the bill between cone scales. By biting down, the bill tips cross, pushing the scales apart and exposing the seeds, which are removed by their tongues. This technique allows the birds to use the more powerful muscles that close the jaws, to pry the cones open.
Flycatchers typically perch near the edges of trees or shrubs and fly off to grab insects in mid-air. Their bills are adapted for this feeding method by being wide at the base and somewhat flattened, which creates triangular shapes with large surface areas for capturing insects — sort of like slapping your hands together to smash a mosquito.
If you’ve ever watched feeding flights of nighthawks, you had to have been amazed by their erratic flights. They dive rapidly and constantly change directions, as if they were insects. They’re not, but they’re chasing insects. Their bills are very short and wide, resulting in a gape about an inch in diameter. To capture an insect in flight, the bird opens its mouth like a net and scoops the bug out of the air, which leads to its erratic flight. To get a better idea of this, hold the center tube of a paper-towel roll in your hand and try to catch a fly.
Woodpeckers, of course, have stocky, chisel-like bills that are well suited to burrow into insect galleries just below the bark and to excavate cavities. In many cases, the digging is made easier because of a brown fungus that softens the wood. And after the wood has been removed by foraging woodpeckers, their elongate tongue is inserted to pull out a hoped-for beetle larvae. Everyone should have a chance to observe an excavating Pileated Woodpecker. They are spectacular. Crow-sized with a stocky 2-inch bill, they can pitch large wood chips into the next county.
Stilts and avocets use their long bills like forceps, as most shorebirds do, but avocets offer a new twist. The edges of their mandibles have sensitive nerve endings that provide great touch. They often swing their partially open bill from side to side near the water’s surface, clamping down when they feel small insects or crustaceans.
Oystercatchers have an elongate bill that is laterally compressed to a “knife-like” shape that is well adapted for opening bivalve molluscs, such as oysters and mussels. The birds insert their thin bills between the valves and cut the muscles that hold the shells together (adductor) by biting, making the soft tissue on the inside an easy lunch.
Usually, bills are shaped to specialize on certain kinds of preferred prey. It’s curious, therefore, to see both the Snail Kite and Limpkin, two totally different birds with totally different bills specializing on a single prey — the apple snail — a large mollusc about 3 inches in diameter. The Snail Kite is a raptor with a sharply hooked beak, a bit longer than most raptors, that is designed to easily reach into the shell and remove the snail. The kite anchors the shell with its foot. The Limpkin is a heron-like bird with no close relatives. It has a slightly decurved, long, narrow bill. It holds the shell down by its foot and inserts the bill like a lever and pries the snail out.
Bills come in a variety of shapes and sizes that permit birds to exploit a wide range of food sources, demonstrating an amazing array of form and function.
This article from Eldon Greij’s column “Amazing Birds” appeared in the January/February 2019 issue of BirdWatching.