Probability in bird identification

One of my regular birding spots is a small farm field near my house in Concord, Massachusetts. I can walk around the entire place in about 20 minutes, but I usually take about two hours and get in some good sparrow-study. The clump of taller vegetation shown here is always a good spot to study birds that come up out of the grass and weeds and perch a little more conspicuously, so I usually spend a lot of time standing here looking at sparrows through my telescope.

On one visit in September 2008, I noticed that the three common species of sparrows – Savannah, Song, and Swamp – were using different parts of the vegetation, and I made a note to watch for this on future visits. It seems to be fairly consistent, so I can now produce a kind of “map” of sparrow-regions within the shrub thicket.

I’ve divided the thicket into three zones that I’ll call the margins, the middle, and the depths. I was seeing mainly Savannah, Song, and Swamp Sparrows, and those are the species I’ll discuss here.

• The Margins are occupied by Savannah Sparrows and are rarely used by the other two species (almost never by Swamp)
• The Middle zone was used by all three species, although I would say it was preferred by Song Sparrow, and was used rarely by Swamp Sparrow.
• The Depths were the home of Swamp Sparrows, which rarely venture into the other zones. Song and Savannah sparrows would also go deep into the shrubs when alarmed (e.g. if a hawk flew by), but their typical behavior when flushed by me was to land on the outer edges of the thicket.

Of course, in real life this thicket is three-dimensional, but the zones I’ve drawn are more like a cross-section. So a Savannah Sparrow could head for the lower parts of the thicket and still land in the margins of that shrub.

This may seem trivial, but it’s really no different from habitat and other “soft” identification clues. You certainly can’t identify a bird solely by its chosen perch, but we should never doubt the capacity of the human mind to discern subtle differences in probability (just look at baseball batting averages). The more I watched the more I realized that these are helpful clues to identification, just as date and habitat are helpful clues. A sparrow that flies right up into the highest twigs of the cherry sapling is almost certainly a Savannah, and it doesn’t take much more to confirm that. A sparrow that goes straight into the deep cover and then barely pokes its head out is likely to be Swamp (or White-throated, or sometimes Song, but at least we can start with some idea of the likely species).

Check it out in your local area. I’d be interested to hear how it works in other places and with other species. And of course the same principle should be applicable to lots of other situations.