How to Use a Field Guide Part 2a: Size matters, but use with caution

How to Use a Field Guide Series

By Steve N. G. Howell

Part 2a: Size matters – but use with caution (for part 1 go here)

What is Size?

How much bigger is a Starling than a Song Sparrow? Well, the field guides say a Starling is around 9 inches, whereas a Song Sparrow is around 6 inches. So, a Starling is 1.5 times the “field guide size” of a Song Sparrow?

01 field guide (1 of 1)

Is this Startling 1.5 times the “size” of a Song Sparrow? Most people, I think, would say it looks at least twice as big…

Is this Startling 1.5 times the “size” of a Song Sparrow? Most people, I think, would say it looks at least twice as big…

How about a Dunlin vs. a Least Sandpiper? A Dunlin is apparently 8.5 inches, and a Least Sandpiper some 6 inches, according to The Sibley Guide. So a Dunlin is about 1.4 times the “size” of a Least?

(see following caption)

(see following caption)

This Least Sandpiper looks pretty bloody small compared to the Dunlins it is with, so how would you compare the “sizes” of these two species?

This Least Sandpiper looks pretty bloody small compared to the Dunlins it is with, so how would you compare the “sizes” of these two species?

But do you know how those field guide figures are derived? The conventional field guide “size” is a measure of “total length,” taken from museum skins laid on their back and measured bill tip to tail tip (with no undue stretching), but it seems most people copy somebody else and few people actually make their own measurements. This length is a carry-over from old manuals designed for ornithologists collecting birds and having them in hand, freshly dead. It has continued into modern field guides for the simple reason that nobody has come up with a better way to covey the “size” of a bird. But then again, how often do you see birds lying on their back in the field to compare?

Related to “size” is a pet peeve of mine: virtually all field guides give just a single length for any species, yet every species varies, from a little to a lot. For example, my own measurements of series of museum specimens give a length for Song Sparrow (not including the huge Alaskan birds) as 5.7-6.3 inches, and for a Starling as 8-9 inches; and for a (North American) Dunlin as 7.7-9.5 inches, and a Least Sandpiper as 5.2-5.7 inches.

And remember, the area of a bird is more what you see in the field, and is a square of length (assuming the bird is square!). Hence, a bird 6 inches long x 2 inches high = 12 inches in area, whereas a bird 9 inches x 3 inches is 27 inches in area. So in terms of surface area, a Starling looks more than twice the “size” of a Song Sparrow, and a Dunlin can look relatively “huge” compared to a Least Sandpiper.

Photos, and the associated size illusions they promote, can make things even more puzzling. Compared to the foreground Least Sandpipers and single Western Sandpiper (the whiter bird), the Dunlin at back in the water looks “huge” – yet you field guide says it is 8.5 inches vs. 6 inches for a Least Sandpiper.

Photos, and the associated size illusions they promote, can make things even more puzzling. Compared to the foreground Least Sandpipers and single Western Sandpiper (the whiter bird), the Dunlin at back in the water looks “huge” – yet you field guide says it is 8.5 inches vs. 6 inches for a Least Sandpiper.

Moreover, birds are 3D, so what you see is perhaps even the cube of field guide length, creating an even greater discrepancy between the “size” you see in the field and the “length” you see in the book. And all this supposes the birds you are comparing have the same proportions, otherwise the length comparisons are even harder to make! And then, to compound all this, you have the classic problem of a lone bird with no sense of reference… So yes, while field guide “lengths” can give you an idea of “size” it is good to use that idea with appropriate caution.

This note isn’t intended to make you throw up your hands, but rather to encourage you to spend more time in the field, in direct experience with birds in life, getting to know them in different contexts, and, especially if you’re a beginner, comparing the “size” in field guides to the “size” you see on a bird whose identity you are confident about. Even if you live in city, how much bigger is a Feral Pigeon than a Starling? Next time take a second look, see what your field guide says, and ponder how this information is presented, and how that might help you with other identifications in the field…