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Birders mostly use songs, calls, and field marks to identify birds. Field marks are things like colors, eye rings, wing bars, crown stripes, bill length, and leg color. These are physical marks you literally can see “in the field.” And check out “primary projection,” useful for Empidonax flycatchers and a few other species.

But several other kinds of useful information don’t involve the bird itself. The phone app, Merlin, makes use of three of these.

Terry Rich can be contacted at

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