How to Use a Field Guide Part 2b: Size Matters – But Use With Caution

How to Use a Field Guide Series

By Steve N. G. Howell

Part 2b. Size matters – but use with caution (For part 1 go here and part 2a go here)

But wait, what about weight?

Some field guides are also now giving (usually a single value for) mass (or weight, to us non-physicists), which can be a more useful measure of “size” than length. But then again, mass can vary hugely depending on whether a bird has eaten recently, is fattened for breeding, is a lean immature after migration, and so on.

Moreover, the structure of the bird can also be a huge factor in trying to relate mass to “size.” Heavier birds do not necessarily appear “bigger” to the human eye.

For example, Sooty Shearwater and Buller’s Shearwater are about the same “size” in a field guide, with a length of about 45 cm and a wingspan of about 100 cm. Hey, wait, I hear some readers cry: I don’t do metric. Well, you know, it doesn’t really matter as long as the same system is used: I assume you can tell quickly if 60 is larger than 40, or if 100 is the same as 100, whether meters, inches, leagues, grams, ounces, football fields, or even bottles of tequila?

So, same wingspan and same length, but one has an average weight of 800 grams, while the other has an average weight of 425 grams. Which might lead you to think one is twice as “massive” as the other, but which one (if either) looks bigger in the field?

Figure 1 shows crude approximations of these birds’ flight shapes – Buller’s has broad wings, Sooty has narrow wings. Buller’s is a lightly built bird with broad wings, designed to live in calmer subtropical latitudes where it flies with buoyant ease. Conversely, Sooty is a heavy, narrow-winged bird that does well in windy high latitudes where it can exploit strong winds and arc high like a boomerang.

Fig 1. Compare the dimensions and mass of these two figures, the upper representing a Sooty Shearwater, the lower a Buller’s Shearwater. While your first thought might be that the lower figure is “bigger” it has the same length and wingspan as the upper, and the mass is almost half of the upper bird!

Fig 1. Compare the dimensions and mass of these two figures, the upper representing a Sooty Shearwater, the lower a Buller’s Shearwater. While your first thought might be that the lower figure is “bigger” it has the same length and wingspan as the upper, and the mass is almost half of the upper bird!

Fig 2. Sooty Shearwater has the heavy body and narrow wings (high wing-loading) to arc high in strong winds.

Fig 2. Sooty Shearwater has the heavy body and narrow wings (high wing-loading) to arc high in strong winds.

Fig 3. Buller’s Shearwater has a light body and broad wings (low wing-loading) such that it tends to stay low over the water, even in strong winds.

Fig 3. Buller’s Shearwater has a light body and broad wings (low wing-loading) such that it tends to stay low over the water, even in strong winds.

These are extremes, and somewhat atypical in the world of mass versus length (at least for non-seabirds), but they should help make you aware that any measurement of “size” in a field guide, whether it be length or mass or wingspan, should be used with due caution. In short, experience and the human eye can often make helpful judgments, ones that cannot be divined from stats in a book.