Index of suspicion. I had never heard the birding term until Julie Keene used it during last year’s birdathon. Julie and I are teammates on The Raven Loonatics — a squad of Maine Audubon birders who annually compete against other teams to see who can find the most birds in one day. The Loonatics are a formidable team, at least in our own minds.
I Googled the term and discovered that it is a routine phrase in medicine that is used to indicate how seriously a particular malady is being considered when making a diagnosis. It makes sense that Julie would use the term. She’s a veterinarian at Veazie Vet Clinic.
But the term fits birders perfectly. Good birders are highly suspicious. In this case, an index of suspicion is a mental checklist of possibilities which helps to make an identification of a particular bird in a particular location, looking a particular way, doing particular things. The more a bird fits the description and pattern witnessed, the higher it is in the likelihood — the index — of being the correct identification.
For instance, if you see a bird in a tree, you may think of many birds that it could be, but I guarantee that a loon is not on the list. It is not within your index of suspicion because there is no chance that a loon would be in a tree.
Everyone has an index of suspicion. The difference between newcomers and experts is just a matter of degree, informed by experience. When a brand new birder sees a bird the size of a chickadee, without even thinking he will eliminate crow from the list of possibilities. Size matters.
Location matters. Most sparrows stay near the ground. Most warblers are up in trees. Even though they are roughly the same size, a bird in a tree is more likely to be a warbler than a sparrow and, as your brain starts to work on the identification, it will likely sort through the warbler possibilities first. Sparrows do sit and sing from treetops, but they never feed there. If the mystery bird is feeding on something in the tree, sparrows drop completely off the index of suspicion. Behavior matters.
Behavior and location are handy clues. Somewhere back in grammar school we learned that if two species are competing in the exact same niche, one will eventually outcompete the other. Maine has a couple of dozen breeding warblers, all feeding on insects. How can we have so many warblers without one of the species eventually eliminating the competition?
The reality is that warblers have divided up the pie. Some feed at the top of the canopy, some at the bottom, some on the outside of the tree, some on the inside. Some fly out to catch insects, some stay on the branch and glean bugs from leaves. Some probe bark. Each warbler has a unique strategy that is different enough that it doesn’t compete directly with neighbors. As a result, most warblers act just a little different from each other. Experts pick up on these clues.
So if I see a small bird feeding high in a mature tree, my index of suspicion places high canopy birds nearer the top of the list of candidates. These include black-throated green, pine and blackburnian warblers. Northern parula is surely a possibility. But if I see it down low, near the ground, then common yellowthroat, mourning warbler and Canada warbler rise to the top of the index of suspicion. In fact, if the bird is midway between the two habitats — let’s say in a high bush or low tree — American redstart, magnolia warbler, Nashville warbler and chestnut-side warbler nudge to the top of my index.
You can’t have an index without knowing what birds are possible in the locale. I know Maine very well, but if you drop me in Costa Rica, I’m just another novice. I’m a Dewey Decimal System birder in a Library of Congress jungle. In order to avoid feeling completely helpless, I often review a field guide on the plane to my destination just to get a sense of what is possible upon arrival. I’m building my index of suspicion, as crude as it may be in an unfamiliar place.
You’ve got a more refined index of suspicion than you think you do. Even if all you do is feed birds, you know what comes to the feeder and you know when something is different. You’re suspicious.
Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter. Bob developed the Maine Birding Trail, with information at www.mainebirdingtrail.com. Bob can be reached at firstname.lastname@example.org.