By Bill Pulliam
In the last two articles, I talked about birds’ feet and bills, and how they are tuned to each species’ lifestyle. But, really, it is the wings that truly make the bird.
Probably the first thing that most people think of when they think about birds is flight, and with good reason. Birds are the biggest flying animals around, and other than the bats, they are the only flying vertebrates. Also, all of our North American birds fly. Some don’t fly very often, or very far, but they all do it. There are flightless birds in the world, such as Ostriches and Penguins, but these are all native to other continents and only show up here in captivity.
Given that wings are mostly for flight, and that all birds have them, it’s worth talking about two properties of flying things that will already be familiar to anyone who knows about airplanes. These are wing loading and wing aspect ratio. Wing loading is the ratio of the weight of the bird (bumblebee, airplane) to the area of its wings. Wing aspect ratio is a measure of whether the wings are proportionately long and narrow (high aspect ratio) versus short and wide (low aspect ratio). So let’s look at some of the different types of wings that you’ll find in the bird world.
One group of birds can be described as the “big soaring birds.” These are creatures like hawks, eagles, and vultures, of course, but also pelicans and others you might not think of at first. These birds have large bodies connected to large, broad wings. Their wing loading is relatively low considering how big they are, and their aspect ratios are also low (broad wings). They are built for catching thermals and updrafts and coasting with little or no flapping over great distances. When they do flap, the wing beats are usually slow and deep.
Another group could be called the “big flapping birds.” These also have broad wings (low aspect ratio) but are not as adept at soaring. They flap regularly, with slow beats often mixed with soaring glides. This is also how the “big soaring birds” behave when they are first launching, and when weather conditions are not good for soaring. The “big flapping birds” include many groups, from crows to herons to geese. Even turkeys could be considered here, if you have ever seen them when they get well up in the sky, flying in the open air from ridge to ridge.
In contrast to these big broad-winged birds, are the “fast pointy-winged birds.” These are birds, large and small, that are built for speed and maneuverability. They have narrow pointed wings (high aspect ratio) and a heavier wing loading than the soaring birds. Falcons, swallows and martins are good examples of this group. They can twist, turn, dart and dive with amazing speed and agility. The falcons are among the fastest living things on earth. An extreme example of this group is the Chimney Swift, with long sickle-like wings on a slate-gray cigar-shaped body. All of these birds are aerial predators that pursue their prey on the wing. The Peregrine Falcon might be after ducks and doves, while a swallow is after mosquitoes and flies, but the need to be swift and agile is the same for both.
There are some birds that are especially drawn to open water that combine features of all three of these groups. The gulls and terns are the most numerous examples. They have long wings (high aspect ratio) that are proportionately bigger than that of the falcons and swallows (lower wing loading). They are not fast flyers, but they are agile, swooping and capable of either soaring high on fixed wings, or powering along with deep, slow wing beats.
Most of the land birds could be lumped together as “fluttering birds.” These are birds of forest and field that have relatively short and small wings (high wing loading, low aspect ratio). These are most of the songbirds and other small- to medium-sized birds from chickens to chickadees. They fly with fast wing beats, sometimes flapping constantly, but often with brief glides (wings out) or bounds (wings folded, cannon-ball style) between bursts of flapping. I’ve lumped together a huge range of birds here, and with experience, you will learn to recognize a warbler from a titmouse from a sparrow from a wren by the precise way that it flies. But all use short wings and rapid flapping to move themselves around in tight quarters.
An entirely different kind of “flight” happens among a few groups of birds that are more at home in the water than the air. The auks and murres have short pointy wings that seem to barely be able to keep them airborne when you see them flying over the ocean. But that is not their only use for those wings. They also use them to “fly” underwater when they dive deep to chase fish. The wren-like Dipper of western North America also uses this underwater flight. Dippers live in whitewater streams of the western mountains, feeding on small aquatic critters that they pursue through the rapids. Alas, there has only ever been one murrelet seen in Tennessee and no Dippers. So you will have to travel if you want to see this behavior in action.
Finally, we come to the most astounding flight of all, that of the hummingbirds. These tiny cousins of the swifts have short, narrow, pointed wings (high aspect ratio) that are quite small, even in proportion to the bird’s overall small size (high wing loading). The hummingbirds flap so quickly that to our eyes, the wings are just a buzzing blur. Slow-motion photography shows that they use a distinctive figure-8 motion of the wingtips to generate lift on both the “up” and “down” strokes of the wing beat. This lets them simply hover in place, and move in three dimensions in any direction they desire.
Whether you are sitting on your back porch, strolling on a beach or rolling down a highway, you are rarely out of sight of a bird on the wing for very long. The amazing variety of ways in which these creatures flutter, flap, swoop, swirl and sail through the air can provide a lifetime of fascination.
Bill Pulliam got started in birdwatching by his junior high science teacher in 1974, and has been an avid birder ever since in 48 U.S. states and 7 foreign countries. He is currently the Tennessee editor for eBird, an online project that compiles millions of observations from tens of thousands of birders around the world.