Judging size of birds

Size judgment is one of the constant quandaries of bird identification - critically important but fraught with error. In a recent online discussion about these photos of sandpipers in flight, I was intrigued by the question of how I, and others, "just knew" that these birds were too small to be Knot or Pectoral and must be Least or Semipalmated. There are no reference points in the picture, so the cues for judging size must come from the birds themselves, and proportions of different body parts must hold the key.

Mystery sandpiper photo by Corey Finger, who relates the whole story here

Now this photo (above) has been posted and discussed on ID-Frontiers, with no consensus on the identification, but with Least and Sharp-tailed - two species of very different sizes - both getting votes. Again I was intrigued by the question of size judgment. I traced bird outlines from two photographs to compare body size and proportions.

These outlines (below) show two species, different sizes in life but adjusted here so that the wing length and body size are about the same.
In this demonstration, the most striking difference is the relative size of the head, making the upper bird look big-headed while the lower bird looks small-headed. There also appears to be a difference in the back end, at least in these two images, but I can cover up that part of the image and still get a strong size impression from just the front half of these two birds.

The upper sketch is traced from a Least Sandpiper photograph, the lower is traced from a Sharp-tailed Sandpiper photograph (both photos in Crossley, Karlson, and O'Brien, The Shorebird Guide).

Does this actually help identify the mystery sandpiper in the photo above? I think so. To me the mystery bird's head size matches the Least Sandpiper better than the Sharp-tailed. And I think head size is also the key to judging the size of the flying sandpipers in the earlier mystery photos. It may not be definitive, but it's another point to add to the mix.

This is not to say that larger birds of all species look relatively small-headed (Cooper's vs. Sharp-shinned Hawk is an obvious exception) but whenever we are judging size in the field or from photos it's worth pausing to think about what goes into that judgment. Most often it does not involve actual direct comparison of sizes, and I suspect that experienced observers subconsciously rely on body proportions in a lot of cases.