In early October, my sister brought me dead bird she found in our yard. Despite having the bird “in hand” (I didn’t actually hold it) and recording it in my nature journal, I came away uncertain of the species. While I was convinced it was some kind of thrush, probably a hermit thrush, there were a few things that left me uncertain. The main point of confusion was its size. It was much too small.
According to my edition of The Sibley Guide to Birds, the length of an adult hermit thrush should be about 6.75 inches. My little bird was only 5.5 inches long. However, I couldn’t find any other species in my guide that seemed like a good match. I left the bird ID as a maybe and forgot about it.
Then I read an interesting fact in David M. Bird’s Watching Bird Behavior column in the September/October 2020 issue of Bird Watcher’s Digest. One of the questions he addressed was about the shrinking in size of some species of North American migratory birds. Was this why my bird was so small?
I found a report about the study he cited and discovered that while the measurements showed a statistically significant decrease in body size, that decrease was 2.4%, not nearly large enough to explain my little bird.
I decided I needed more data. I went to the Cornell Lab of Ornithology’s All About Birds page for hermit thrushes and found something interesting. A hermit thrush can be anywhere from 5.5 to 7.1 inches long! That Sibley’s cites 6.75 inches as the average size* suggests that they normally are closer to 7 than 6 inches. But my little 5.5 inch bird is actually within the size range reported for this species.
The whole thing has been a good reminder to me that while our field guides are helpful in pointing us in the right direction, they have to present a simplified view of the world. The amount of variation that occurs in nature is much higher than even the best guides suggest. They are giving you what is average or most common, but there are plenty of birds in the world that won’t meet the criteria exactly.
So my mystery bird was a hermit thrush after all. Next time I have a bird in hand, I’ll start with a description that includes ranges instead of averages. It should make it easier to identify the bird.
*In the front of my Sibley’s, it says that bird lengths may vary from the reported length by at least 5%, so he tried to address the variation found in nature.