We have talked a little bit about how the pupils of some birds can serve as field marks in the wild and you might be surprised to learn that oystercatchers are in this group as well! Take a look at this American Oystercatcher that I spotted in the Galapagos and see if you can tell what looks odd about the eye

It’s a weird shape, right? What you’re seeing in this bird’s eye is actually an iridial depigmentation that ornithologists commonly call an “eye fleck”. Here’s a closer look:

We don’t really know why, but recent studies suggest that the presence of this eye fleck is a really good field mark for identifying sex in American oystercatchers, as only females tend to have the feature. Based on this finding, the bird I spotted is female! Though the study I’m referencing was conducted on American oystercatchers in Texas, the researchers found 97% agreement with the sex identification using eye flecks and genetic testing for sex. The eye fleck method is far less invasive and may be as accurate or even more accurate than physically assessing morphometric measurements of each bird. I really love these shorebirds and I just think this is the coolest identification trick ever.