When I first met Carson, our resident red-tailed hawk, I was fascinated by her eyes. The light grey iris was striking compared to the dark eyes of other hawks I have known. This isn’t just a difference between individuals; eye color hints at the bird’s age. Red-tailed hawks hatch with light-colored eyes that darken over time. Carson is just three years old now, so we expect her eyes to turn a dark mahogany over the next few years. Some other raptors change eye color, as well. Eagle eyes change from brown to yellow, while sharp-shinned hawks start bright yellow and transition to a blood red. We can’t know for sure why this happens, but it may be an important cue for birds looking for a mature mate.