Shorebirds Size in Field Guides

I thought this exchange in the Shorebird class might be of interest to B3. First Lance’s question, then my response.

From Lance Felber (2020 Shorebird student)

I noticed while looking at Least Sandpiper in a couple of different guides a fairly large discrepancy in lengths listed. So I looked at Least, Semi, and Western in several guides. Here’s what I noticed:

The Shorebird Guide

Least 4 1/2 – 4 3/4

Semi 5 1/4 – 6

Western 5 1/2 – 6 3/4

Sibley, Kaufman, Nat Geo

Least 6

Semi 6 1/4

Western 6 1/2

Museum of Natural History

Least 4 3/4

Semi 5 1/4 – 6

Western 5 1/2 – 6 1/2

I would guess that the Shorebird Guide would be the most accurate, but even in some of the pictures in that guide Westerns don’t look that much larger than Leasts. I don’t have much experience comparing the peeps, so I am curious:

Is the size range in Westerns (and Semis) because of different sizes of the sexes, including bills?

In your experience are Leasts really 1-2″ smaller than Westerns?

If so, any idea how the popular field guides have the size of Leasts, in particular, so far off?

Jeff’s Reply.

That’s a great question Lance!!! Thanks for bringing this up.

As Gandalf said of Saruman, “Great as his knowledge is, it must have it’s source”. The same is true of the authors of the field guides. They select sources and then summarize them in some fashion down to a one line statistic. The source of the data ultimately is field studies where birds are captured, weighed and measured. Those field studies are conducted at different times, locations, seasons, and include individuals of different ages, genders, and life cycle situations (e.g. molting, post or pre-migration, breeding, etc.). Below is a table I excerpted from Birds of the World online of mass measurements for Least Sandpiper. I think this illustrates my point – and that there’s a whole lot more room online than in a field guide .

I won’t go into additional details, but when you take a measurement, even one that seems as straight-forward as measuring a bird’s mass or length, the measurements vary. Thus, in these published studies the mean, standard deviation and range are all provided. The mean value along with these additional statistics shows how much the data varies based on measurement error (and by “measurement error” I don’t mean “a mistake”, I mean the inevitable differences that creep into any measurement. There’s a whole field of study on measurement error).

Here’s what Sibley and Nat Geo field guides say on their size measurements:

If you asked a seemingly simple question like “what’s the average size of a Least Sandpiper”, here are the variables I can imagine in the data (and I’m sure ya’ll can think of others):

Gender,

bill length (most measurements of a birds length consist of tail-to-tip-of-bill),

age,

current life-cycle status (breeding, post-breeding, pre-migration, in-migration, post-migration,

location (different populations of widespread species vary in size),

time-of-day,

measurement technique and

measurement error.

So, to summarize an answer to your question “why do field guides quoted sizes differ?”: because each author chooses data from a different study or studies, chooses a different statistic from that study or studies, and presents it in a way that seems best for their purposes.

My personal experience with shorebird size is this. I see a lot of variation even within the same species. (This is part of the “recalibration” effort I spoke of when a new shorebirding season starts). Sometimes I’m convinced a bird “can’t be a Least, because it’s almost the same size as that Western”. Other times I see two Least Sandpipers of obviously different sizes. One recent spring (2017) I was filming shorebirds at Hornsby and I was struck by a comparison between some very skinny Pectoral and what appeared to be normal sized White-rumped. Sibley lists them as Pectoral (8.75 inches, 73 grams) and White-rumped (7.5 inches, 42 grams), but I trouble telling the two apart based on size.

I don’t find size to be instantly diagnostic for the 5 peeps (Least, Western, Semipalmated, Baird’s & White-rump), especially for new shorebird students. As you’ll see in class on Sunday (I plan to show at least one filmclip) sometimes the size difference is obvious. But other times less so. I certainly wouldn’t guess that there is a 1-2 inch size difference between Least & Western from what I see in the field. Some of the quoted size difference might be based on the bill length as you suggest.

So here’s my advice in dealing with size of shorebirds (and other birds in general) given in field guides.

1) Remember that perceptions of size are generally less reliable than other field marks.

2) Think in terms of relative rather than absolutesize.

3) Use the measurements in field guides on a relative basis: All of the sources you quote, show the sizes in this order: Least, Semipalmated, Western. And this is the way I perceive them.

4) Use the reference species we’ve talked about in class: Killdear, Least Sand, Lesser & Greater Yellowlegs to judge the relative size.

5) Take your time and try to determine species based on other things than size – then after getting that down at a particular time/location, try to notice the sizes of the species you’ve already IDed. I think you will start seeing difference between the 3 smallest peeps (Least, Semipalmated, Western) and between them and the 2 larger peeps (White-rump & Baird’s)

That’s what I know,

Jep