My wife is a naturalist and she recently went down a rabbit hole on the internet when researching "dimorphism" (male birds having a different appearance than females) and discovered that many bird species that don't at first appear to have dimorphism actually do.
That is because bird's eyes see a different spectrum than we do. When my wife discovered this, she perked up and said, "Just like Will and Blue!"
For those of you who have not yet read "Not Alone," what my wife was referring to is the genetic trait that
allows the main protagonists, Will and Blue, to communicate with their eyes by being sensitive to the infrared spectrum. Now bird's eyes tend to be sensitive toward the ultraviolet end of the spectrum, but the concept is the same.
In Will and Blue's case, they don't "see" infrared, instead, their brain processes it into sounds, but again, it is their eyes that enable that ability. The challenge in the book was to have Will's science-professor dad explain this (chapter 12, "I Think Therefore IR") in a way that didn't bog down the reader (or Will). Maybe I succeeded, maybe I didn't, but either way, I think this article on bird vision (What Birds See) does a great job of illustrating how birds (and possibly some humans!) can sense things that we can't because their eyes are just a little different.
This just scratches the surface of this topic and hopefully will prompt you to go down your own search rabbit hole. Or if you really want to dig deep, check out this Wikipedia entry on the subject: Bird Vision (Wikipedia)
If you have read "Not Alone" or the recently released sequel "The Innocence of Westbury" I truly hope you enjoyed them and are looking forward to the final book of the trilogy "Forest" coming this fall. And if you haven't already, please please do leave a rating and/or review on Goodreads or your retailer. It really helps new authors get off the ground!
Thanks again and have a great day!