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Why lighting conditions can throw birders a curveball

lighting conditions
A Common Tern angling toward the viewer catches the bright sunlight (left), but if it turns slightly away from us, the near side of the body is thrown into shadow (right). Art by David Sibley

One of the primary clues we use to identify birds is color, and the color we perceive is entirely dependent on lighting. The challenge of color perception is not limited to bird identification, of course, and we are constantly making adjustments (mostly subconscious) to determine the color of the things we see around us. Bird identification simply brings this challenge to the forefront. 

Most of the time, we do a good job of adjusting to lighting conditions, and we can recognize a color, for example, the particular shade of yellow of a male American Goldfinch, whether the bird is in bright sun, or deep shade, or the warm glow of sunset. We can do this because we judge colors relative to the colors around them, and the overall lighting of a scene tells us what to expect. It can take some time to “recalibrate” our color perception, but once we’ve adjusted to the current conditions, we can see a goldfinch and say, “ah, yes, the typical yellow color.”

Occasionally, though, lighting throws a curveball. In some cases, we misinterpret the scene — our color calibration is off. In other cases, there is just one small variation we didn’t expect — a greenish glow reflecting off a leaf or an unexpected shadow, for example. Something like this happened to me recently while watching terns in Long Island Sound, New York.

It was early morning, and I was looking south over the water. Dozens of Common Terns were flying by, mostly moving west (left to right). They were brightly lit by the early morning sun. Suddenly, an all-dark tern appeared among the passing groups; “Black Tern!” I thought. A few moments of study revealed that it was just another Common Tern, in shadow. But how could it be in shadow, in bright sun over the open water? 

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With the sun directly to my left, all the birds that were traveling on a line angling slightly toward me were lit by the early morning sunlight. But when a bird veered off that flight line and turned slightly away from me, the sun struck the far side of its body instead, leaving me with a view of dark shadows on the near side of the bird.

This was a very specific scenario and quickly resolved (although that didn’t keep me from making the same mistake again), but it is one example of the “tricks of the light” that we always must be alert for when birding. 

This article was first published in the September/October 2021 issue of BirdWatching magazine.

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David Sibley

David Sibley

David Sibley writes the column “ID Toolkit” in every issue of BirdWatching. He published the Sibley Guide to Birds in 2000, the Sibley Guide to Bird Life and Behavior in 2001, and Sibley’s Birding Basics in 2002. He is also the author of the Sibley Guide to Trees (2009), the Sibley Guide to Birds-Second Edition (2014), guides to birds of eastern and western North America (2016), and What It’s Like to Be a Bird (2020). He is the recipient of the American Birding Association’s Roger Tory Peterson Award for lifetime achievement in promoting the cause of birding and a recognition award from the National Wildlife Refuge System for his support of bird conservation.

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