We’re going to wander slightly into Michael O’Brien’s Field Identification territory with this column, but I’ll jokingly say it’s Michael’s fault - we recently co-taught a Cape May School of Birding Shorebird Workshop and spent lots of time using size comparisons as a key tool to identification. And does that ever simplify matters!
There are specific tricks to using size in the field, and that’s where Fieldcraft comes in. Size is notoriously difficult to judge in the field. I’ll bet I’ve seen that last statement in print a couple dozen times, and it is true, but if you understand the pitfalls, size is the first thing you should evaluate in an unfamiliar bird. About those pitfalls:
Know that which you are comparing: The best situation of all is when an unknown bird is next to a known bird. Try learning the actual measurements and weights of a few common species (these can be found in any of the Sibley field guides). This can be a real education. For example, did you know a Sanderling weighs about twice as much as your average “peep” like Semipalmated Sandpiper? Sanderlings really are a lot bigger and bulkier than the peep.
A robin is 10 inches long, measured tip of bill to tip of tail, and everyone knows robins. So what if a robin flies over in the company of a slightly smaller blackbird? You know right away the blackbird is not a grackle, because it’s too small.
Inject a dose of caution in comparing size. Check out the yellowlegs-dowitcher photo for one example of what I mean (and of the great value of using size in the field).
Compare the size of these two birds. The one in back looks larger - until you remember to compare just body size. Now they look about the same. Remember, bird measurements don’t include the legs, and long legs make a bird look bigger. If you know the bird in front is a dowitcher (in this case, a Short-billed Dowitcher of the hendersoni race) and you know the back one is a yellowlegs but you are not sure which one, you can now be sure it is a Lesser Yellowlegs because it is about the same size as a dowitcher. Greater Yellowlegs are much larger.
If the bird is closer than you think it is, it will look bigger - When does this happen? Perhaps the leading situation is when fog blankets the birdscape. You can’t see well, so when you do see a bird you think it is farther away than it really is. Your mind tells you it must be pretty big, since it looks so far away and you still can see it well. One story I often tell is of a group of three expert birders eyeing a perched “large raptor” across a foggy field that, on closer inspection, turned out to be a Common Redpoll.
Another time to beware is when you upgrade binoculars, especially if you go up in power. I remember borrowing a pair of the old Zeiss Classic 10X42s a few years ago at Sandy Hook, and suddenly there was something wrong with the Bonaparte’s Gulls. They were all looking too big, kind of like the larger and rarer Black-headed Gull.
Yet another time to beware is when you are expecting birds to be far, and have been looking at them far, for example, hawk-watching. Then something flies by close and it looks huge. . . until it turns out to be a monarch butterfly.
If you know the bird is very far away, and you can still see it, it’s probably pretty big - This is a good eagle trick. The dark dot over the City of Cape May or over that distant knob at Hawk Mountain that is growing ever so slowly as it approaches is a potentially interesting bird.
If you don’t have a bird to compare with, use something else you are familiar with - For example, a soda or beer can is five inches end to end, and so is a Carolina Wren (without the bill), though the wren is considerably slenderer and will look smaller. A Least Sandpiper is also about the length of a beer can, while a Pectoral at an average 8.75 inches is noticeably longer. Just beware of magnum cans!
Remember bird sizes are averages - There can be substantial size variation within a species, particularly between males and females. I’m pretty sure I’ve never called a female Peregrine Falcon a Merlin, but I have called snap-called male Peregrines as Merlins more times than I’d like to admit.
Consider the background - Birds in front of a clear background they contrast with, such as clouds on an otherwise clear day or a dark bird on sand, will tend to look bigger. Birds in front of a messy background, such as woods or a variegated mud flat, will look smaller.
If the bird is alone in a uniform background with nothing to compare it with, and you are not sure how far away it is, you don’t know how big it is - Single gulls, ducks, or alcids way out in the ocean, or single hawks way up in the sky come to mind. In these situations, switch your initial focus to shape.