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• Top image © marctonysmith

Birds are built for what they do.
Every part of the bird you're looking at is a clue to what it is.

The combination of size and shape is one of the most powerful tools to identification. Though you may be drawn to watching birds because of their wonderful colors or fascinating behavior, when it comes to making identifications, size and shape are the first pieces of information you should examine.

With just a little practice and observation, you'll find that differences in size and shape will jump out at you. The first steps are to learn typical bird silhouettes, find reliable ways to gauge the size of a bird, and notice differences in telltale parts of a bird such as the bill, wings, and tail.

Soon, you'll know the difference between Red-winged Blackbirds and European Starlings while they're still in flight, and be able to identify a Red-tailed Hawk or Turkey Vulture without taking your eyes off the road.

Apply your size & shape skills to the parts of a bird

After you've taken note of a bird's overall size and shape, there's still plenty of room to hone your identification. Turn your attention to the size and shape of individual body parts. Here you'll find clues to how the bird lives its life: what it eats, how it flies, and where it lives.

Start with the bill – that all-purpose tool that functions as a bird's hands, pliers, knitting needles, knife-and-fork, and bullhorn. A flycatcher's broad, flat, bug-snatching bill looks very different from the thick, conical nut-smasher of a finch. Notice the slightly downcurved bills of the Northern Flickers in your backyard. That's an unusual shape for a woodpecker's bill, but perfect for a bird that digs into the ground after ants, as flickers often do.

Bills are an invaluable clue to identification – but tail shape and wing shape are important, too. Even subtle differences in head shape, neck length, and body shape can all yield useful insights if you study them carefully.

Noticing details like these can help you avoid classic identification mistakes. An Ovenbird is a common eastern warbler that has tricked many a bird watcher into thinking it's a thrush. The field marks are certainly thrush-like: warm brown above, strongly streaked below; even a crisp white eyering. But look at overall shape and size rather than field marks, and you'll see the body plan of a warbler: plump, compact body, short tail and wings, thin, pointed, insect-grabbing bill.

Measure the bird against itself

This is the most powerful way to use a bird's size for identification. It's hard to judge a lone bird's size, and an unusual posture can make shape hard to interpret. But you can always measure key body parts – wings, bill, tail, legs – against the bird itself.

Look for details like how long the bird’s bill is relative to the head – a great way to tell apart Downy and Hairy woodpeckers as well as Greater and Lesser yellowlegs, but useful with other confusing species, too. Judging how big the head is compared to the rest of the body helps with separating Cooper’s Hawks from Sharp-shinned Hawks in flight.

Get in the habit of using the bird itself as a ruler, and you’ll be amazed at how much information you can glean from each view. Good places to start include noting how long the legs are; how long the neck is; how far the tail extends past the body; and how far the primary feathers of the wing end along the tail (or past the tail).