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Identifying a bird is like solving a mystery. Every gumshoe birder gathers clues from the subject’s appearance. But what happens when those clues don’t materialize, or when they add up to a species that defies logic? With no point of reference, judging size can be tough. Catch a bird in the wrong light and it looks washed out. Even worse are oddballs that turn up with missing feathers or field marks painted on by brushes with nature. To crack those cases, you’ll need to know both the environment and the nuances between species—but also be ready to second-guess yourself. Follow the tips our experts gleaned from their own hard-won experience. This is the universe testing your skills. You’ll become a better birder because of it.

Size Can Be Deceiving

In spring and fall, Greater and Lesser Yellowlegs are easy to spot along marshes and shorelines across the United States. What’s not as easy? ID-ing them. Both birds have a white eye ring, checkered backs, white bellies, and, of course, bright-yellow legs. The only obvious difference is their size: The Greater is greater and the Lesser is lesser.

There are quite a few examples of such puzzling pairs. Cooper’s Hawks and Hairy Woodpeckers both have avian “Mini-Me’s” (Sharp-shinned Hawks and Downy Woodpeckers, respectively). There are greater and lesser versions of scaup, black-backed gulls, and prairie-chickens. White herons come in a bunch of sizes, too.

Identification can be a snap when two species of differing sizes stand next to each other, but that hardly ever happens because birds are jerks. So then what do you do?

My advice is to pretend like you’re playing Pac-Man. In the video game, Pac-Man and Ms. Pac-Man look identical, except that Ms. Pac-Man wears a bow. You’ll need to find that bow—the feature that distinguishes each pair of lookalike birds.

For the Greater Yellowlegs, it’s the head- to bill-length ratio, which is larger than that of the Lesser Yellowlegs. (Hey, I didn’t say it would be a cinch.) The bill length also differs for Hairy and Downy Woodpeckers. Adult Lesser Black-backed Gulls have yellow legs, while adult Great Black-backed Gulls wear shades of pink. Also focus on leg and bill color to separate the white herons.

Once you learn to look for these features, you’ll be able to size up most birds. The yellowlegs, however, might haunt you forever. —Nick Lund

Tips to Build Your ID IQ

The Լ����Ƶ Birds of North America app lets you narrow species by size (look under “Explore Birds,” then “Advanced Search”). But when size is distorted, try these other options.

Wing Shape

Choices such as tapered, pointed, swept, and fingered could help lead you to the correct species. A short-winged bird, for example, could be a Northern Bobwhite, a Virginia Rail, or even a Least Bittern.

Locomotion

What is the bird’s flight pattern like? If it hovers, it may be a hummingbird, a kestrel, or a White-tailed Kite. If it gets in formation, it could be an ibis, a cormorant, or a European Starling. —Purbita Saha

Lighting Changes Everything

Silhouettes of birds can be diagnostic when you know the species well, but not when you’re just learning. For a bird’s every field mark to be exposed, you’ll need perfect lighting. The best times for birding are an hour after dawn and an hour before dusk, when the daylight is at its richest and warmest, and the shadows under leaves and shimmering heat waves are absent. To make the most of every minute, use an app like Sun Surveyor or Helios to track the sun’s position each day.

If you’re still having trouble seeing field marks, you may need better optics. Even the low-priced birding models on the market today are far superior to the bins you may have inherited from your grandfather’s Army days. When testing them, don’t look at objects in bright sunlight—aim at dark corners or the undersides of tables. The best pairs will have coated glass, which gives you a brighter image no matter how rough the lighting. —Sharon Stiteler

Go-To Bins

For a starter pair, the (above; $269) has great light-gathering ability. The dielectric, phase-correcting coatings on the ($400) offer superior transmission. ($3,249) binoculars are worth every penny when it comes to clarity and brightness. (Disclosure: The writer has a sponsorship with Swarovski.) ���.��.

Bottoms Up

The most common profile of dabbling ducks isn’t one you’ll find in a field guide. Luckily, you can learn to identify them while they’re feeding—simply by getting to know their butts.

Northern Pintail

The four-inch tail feathers on the male are key: They’re longer than those of any other dabbling species.

Mallard

For the telltale sign of a male mallard, find two black curls sticking up like the ends of a well-twirled mustache.

Green-winged Teal

Two gold patches flash beneath the male’s tail. (They’re visible when the bird is upright, too.) ‱�.��.

Birds of a Different Feather

Sometimes, even when you see a bird in perfect light, its feathers might not match its expected form. But the occasional curveball is part of what makes birding fun. The only way to prepare is to learn common species and not jump to any conclusions. Remember, even birds can get weird. 

Balding birds. A few missing feathers can utterly change your impression of a bird. On one birding boat trip off the New Jersey coast, a bizarre bird flying low over the waves grabbed my attention. With a small head and stubby tail, it had the entire crew guessing: Was it a dark petrel or an odd shearwater? Could it be a member of the auk family? After circling at a distance, the mystery bird landed on the boat. It was a Mourning Dove that had lost its tail feathers. (Above left: a molting Northern Cardinal.)

Dirty birds. Once in Arizona, a friend phoned to say he’d found a flock of Harris’s Sparrows—the only North American sparrows that have black faces. At the scene I instead found White-crowned Sparrows that had been feeding on fallen olives. Discoloration like this happens a lot. Hummingbirds may end up smeared with pollen. Swans’ heads and necks are often tainted orange by minerals. And in one case, a Greater Yellowlegs covered in oil was identified as a Spotted Redshank. (Above middle: a Black-chinned Hummingbird with pollen.) 

Leucistic or albino birds. As an obsessive 10-year-old birder, I learned about two oft-seen conditions that can cause birds to lack their usual pigments. At the time, I’d found a pure-white bird I was thrilled to ID as a Snow Bunting. Sure, it seemed odd that this Arctic species should be hanging out in Kansas in July, but I couldn’t imagine what else it could be. After watching it for a couple of days, I crossed Snow Bunting off my life list. Clearly I was looking at a pigment-free House Sparrow. (Above right: a leucistic Eastern Screech-Owl.) —Kenn Kaufman

Contributing writers: Nick/Nicholas Lund, Sharon Stiteler, Purbita Saha, and Kenn Kaufman.