BIRDING BY HINDSIGHT: A Second Look at Measurements

I frequently notice statements like these when reading through birders’ accounts of their observations: The wren on the fence post was five feet away. A pipit stood on the road a mere ten feet away. The grackle on the telephone wire was ten feet from the observer’s car. There was only a 25-foot distance between the observer and the kite. The ibis stood in the marsh not more than 40 feet away.

Needless to say, I am jealous. How are these stealthy birders able to get so close to these birds? Not only that, I tend to note in many of these accounts that observers spend 20 or 30 minutes or more watching an individual bird which may be marginally significant but not hard to identify or exceptionally rare. Where do they get the patience?

I also seem to be arithmetically challenged. When in a group of birders watching a concentration of migrating geese or shorebirds, I may hear someone calling out an estimate of thousands of birds being present. Meanwhile, after scanning over the flock, the best I can come up with is typically a half or a third of what others are seeing.

Of course, I may be no better or worse than other birders at judging distance, time, or numbers. It’s just natural, I guess, for birders to underestimate distances and to overestimate both the passage of time and the magnitude of an impressive flock of birds.

For example, the next time your birding group spots a kestrel or some buteo perched in a distant tree, first have everyone estimate how far away it is, and then drive up to it measuring the actual distance with your car’s odometer. I’d be willing to bet if that raptor is, say, a half mile (880 yards) away, several of the guesses would be around 200 or 300 yards.

Or, try approaching to within five feet of a wren, within ten feet of a pipit, or to find a telephone wire only ten feet from the driving lane of a highway. I’m not sure any of this is possible. Get out a tape measure and see how far it is from your window to your bird feeder or to that robin foraging in the far corner of your yard. Don’t be surprised if what you thought was a distance of 10 or 20 feet is actually 50 or more.

Next, as time goes by, get out a watch and actually time yourself watching a bird until you’re finished studying its plumage or simply appreciating its presence. Does 20 or 30 minutes pass by, as is often stated in the reports I read? Or is it more like two or three minutes? Try watching a bird for a few minutes, or even just 30 seconds — it’s not that easy to do. In reality, these are significant chunks of time if devoted to studying a bird.

As far as the actual numbers of birds we see, I’m not so sure our guesstimates have a pattern of being too high or too low. I suspect that we might overestimate numbers when it’s a single impressive flock viewed at one sitting. (I remember a few years ago when a certain lake was drawn down and attracted lots of shorebirds: many observers reported a few thousand birds there — actual and careful counts were never more than a thousand.) But at the same time I think we may tend to be too conservative when guessing, for example, how many raptors we saw flying over Hawk Ridge during an extended period of a day or several hours.

Now, perhaps all this is irrelevant as we work on improving our identification and documentation skills. I’m certainly unaware of any record ever being disregarded just because it was thought that a birder failed to accurately measure the distance or time involved in an observation. At the same time, though, it is taken into account if a difficult-to-identify bird is only viewed for a few seconds or at a distance of several hundred yards.

The size of a bird, though, is often a crucial consideration in the bird identification process. Whether it’s the bird’s bill length or primary extension or its entire body length, a correct ID may depend on getting these measurements right. And the reported size of anything carries little credibility unless there is direct comparison with something in view at the same time. If someone reports that a bird was 20 inches tall or its bill two inches long, you can’t help but wonder how these were measured. If it’s stated that a gull was smaller than a Herring Gull but that no Herrings were around for comparison, we still don’t really know for sure how big the gull in question was.

So, is it hopeless to accurately determine a bird’s size if nothing is available for direct comparison? I used to think so until just a couple weeks ago. A group of us were birding on the Dry Tortugas in Florida when we were confronted with a nighthawk resting on a horizontal tree branch. (Please don’t ask me how high off the ground the branch was — how should I know?!) As most birders know, an Antillean Nighthawk is something to look for in South Florida, but, unless it is calling in flight or perched next to a known Common Nighthawk, trying to tell one for sure is a difficult challenge at best.

This nighthawk, though, clearly had a relatively short primary extension — that is, we could see its wing tips did not extend beyond the tail tip. A Common Nighthawk’s wing tips, like on the one we saw the previous day, extend beyond the tail tip. According to Sibley’s field guide and other sources, this difference is a big diagnostic deal! It was tempting to call it an Antillean on that alone, but I was looking for more.

Indeed, the field guides report that the overall length of an Antillean Nighthawk is 8 to 8.5 inches, at least a full inch less than a Common Nighthawk. (And this certainly seemed smaller than yesterday’s Common Nighthawk.) So, as I was trying to figure out how to find a ruler, climb the tree, and measure the bird without waking it up, someone with more common sense came up with a novel solution. I, at least, had never heard of this or tried it before, and it occurred to me this could be a helpful technique and solution to some ID problems.

We set up our scopes so that the tip of the nighthawk’s bill was at one edge of the scope’s field of view and its wing/tail tips were at the opposite edge. To accomplish this with a scope’s zoom eyepiece, we just zoomed in to get the desired field of view; with my fixed power scope, I just moved the entire scope and tripod up to the proper distance. Someone then opened his trusty Sibley guide to the inside front cover, which has a ruler printed along its edge, and stood under the nighthawk branch. Then, without changing any settings on the scopes, we reaimed the scopes at the ruler in Sibley, which was the same distance from us as the nighthawk. We then simply read the number of inches visible across the field of view, and accordingly we had our nighthawk’s length.

Each time we did this, no matter which scope we used and which person peered through the scope, we consistently came up with eight inches. In other words, we had successfully measured an Antillean Nighthawk. (Now, please don’t ask me why it wasn’t a Lesser Nighthawk....)

Of course, this technique will only work if the bird in question sits still long enough, if it is close enough to fill your scope’s field of view, and if it is oriented fully perpendicular to your position. You also might have to make allowances for a bird’s posture or position — a species’ stated length in the field guides is when the bird lies flat, from bill tip to tail tip. Because of the margin for error involved in trying to place birds and rulers at the exact edges of a scope’s field of view, this might not be reliable with smaller species — your bird probably needs to be large enough so there is at least an inch or more difference between it and its ID contender.

What could this work on in Minnesota? Keep it in mind the next time you run into those birds whose measurements might assist you with making a correct ID: a potential second-state-record Neotropic Cormorant, Ross’s Goose, Trumpeter vs. Tundra swan, accipiters, Gyrfalcon vs. smaller falcons, King vs. Virginia rail, yellowlegs, Whimbrel vs. Long-billed Curlew (remember the bird in 2002 in Anoka County?), various gulls, Boreal vs. Northern Saw-whet owl, shrikes, and American Crow vs. Common Raven.

And, while an Antillean Nighthawk may never turn up here, given the records for it in Ontario, a Lesser Nighthawk might. (Funny you asked about it earlier!) I can imagine a suspicious nighthawk showing up here some year, perhaps well out of season, whose identification might be confirmed in part by using this measuring technique.