Birding 101: Judging size in the field will make you cry

by Jan 12, 2020

This is the first in a series of posts I’m calling Birding 101, which are short comments or ideas about birding about things I think might be of interest to other birders or potential birders. They will show up randomly when I run across something that I think is worth talking about.

Today’s Birding 101 is about the challenge of judging bird size out in the field. This one trips me up more often than I should probably admit. In the case of this bird, Laurie and I were in San Luis NWR on the tule elk route when we noticed a bird in a tree at scope distance in the elk compound. My birder brain looked at it and said “White-tailed kite!”. Pulling out the scope to get a better look, it was immediately obvious that was an incorrect call, but I had no idea what it was.

It wasn’t until I grabbed the camera, shot a number of images of it with the long lens, and then sat down and looked on the camera LCD and magnified the image that my mistake was obvious. It was a Loggerhead Shrike.

In case it’s not obvious — a shrike is just a bit smaller than a white-tailed kite. It’s closer to the size of a large sparrow.

I’ve thought about how this happened, and here’s what I think: the bird was close to a 1/4 mile away (the photo below was taken at 560mm plus my Fuji’s crop factor, and then was cropped further in Lightroom). My brain decided it was a large bird in a large tree at that distance, so it had to be a hawk. In fact, the tree was smallish, but once I put a scale value to the tree, I scaled the bird to that size as well. And once I decided it was a hawk-sized bird, when it was clear it wasn’t the kite, I had no idea what it could be, because for a hawk-sized bird, there is no real second option with that coloration in this region. It wasn’t until I looked at the bird on the LCD of the camera in a fresh context that I saw it for what it was.

On the positive side: I actually located that bird at that distance at all, given how small it was. And for some reason, it did seem to me to be a predator, hence my plopping it into raptor land — the Shrike is a predator and if you see it close up you’ll notice it has the hook on the end of the bill to help it grab prey. But once I did decide it was a raptor (and therefore big), I struggled to reset my expectations once it was clear that wasn’t going to work — until I switched to looking at it in magnification on the camera. Which, by the way, I did because I’ve done this before and I know that helps me reset those size assumptions.

So if you’re out in the field birding, it’s good to remember that the size you think a bird might be can be quite deceiving and land you into the wrong ID. Unless you have some way to judge size compared to other known objects (like another bird you have positively IDed), assumed size should be quite low on the factors you use to decide the ID. Even, as above, judging size based on the tree its on can turn you sideways.

So it’s always good to be skeptical of size assumptions you can’t verify in some objective way….