Bird identification clues, bird size

Bird size is one way we often use to identify birds, since bird size is often the most obvious difference between closely related species. Size is never the only difference, true, but, out in the field, size may sometimes be a good clue when more information is needed for bird identification. However, using size to identify birds is fraught with danger and is a method that all good birders use with a healthy dose of caution. Size is a feature best saved for last or when all else fails.

The problem is that we humans are not that good at estimating absolute size of things in our environment when they are at a distance. We do fine with objects within arm’s reach, but that’s about it. Objects out at a distance in our environment are subject to far too many variables of light, shadow, perspective and so on to make size estimates reliable. This has been supported by plenty of research. So when are we safe using size as a way to identify birds when we are out to do some birdwatching?

Size is reliable only when it is relative to objects of already known size. If that shorebird in question is standing next to a Killdeer, you can check the size of the Killdeer in your bird guide and get a good idea of the size of that unknown shorebird. However, even this is subject to caution, since the two shorebirds must both be at the same distance. If not, your estimate is not reliable; move on to another feature you can use for identification. Be especially careful of this foreshortening issue when using birding binoculars or birding spotting scopes. Magnification in our binoculars and spotting scopes robs the observer of depth perception; magnification can make objects appear to be at the same distance when, in fact, they are not and it only takes small differences in distance to make your estimate unreliable.

The Hairy Woodpecker and the Downy Woodpecker are very similar in terms of appearance, but actually quite different in terms of size. In the pic, both are at the same distance and the difference is obvious. However, out in the field, even this much difference in size is not always obvious, especially when you only catch a fleeting glance at the bird as it moves through the branches. Hey, that's the fun of birdwatching, though. It's not always so easy.