One interesting aspect of raptors is how eye color changes as birds age from juvenile to adult. In some raptors, eye color takes a shorter time to change than in others, and sometimes this is even related to the sex of the bird, as males will typically change quicker than females. Certain species take as much as 5 years to change eye color completely (Figure 1, Bald Eagle -- brown in juvenile on left, to yellow in adult on right), others, such as the North American falcons, are born with eyes that never change from dark brown (Figure 2, Merlin on left, Peregrine on right). This is helpful when trying to tell the similar-looking Merlin and juvenile Sharp-shinned Hawk (both are streaked below) apart when trying to decide which one is perched in your yard. The yellow eye of the juvenile Sharp-shinned will rule out Merlin (has dark eyes) immediately.
One raptor that barely changes eye color is the Osprey, whose pale orange eye as a youngster changes to yellow as an adult. The Northern Harrier is unrelated to the Osprey, but interestingly it shows a very similar eye color change to an Osprey relative, the Bald Eagle. Harriers change from grayish or yellowish in males to lemon yellow (Figure 3), but juvenile females (Figure 4, top left) have dark brown eyes that gradually change from pale brown to bright yellow through the first few years of adulthood, identical to the brown-to-yellow change of the Bald Eagle. The three accipiters (Sharp-shinned, Cooper’s, and Goshawk) are quite the opposite, starting off with bright yellow eyes as juveniles that turn to dark red as they age (Figure 5, Sharp-shinned Hawks).
The eye color of most buteos (Red-tailed, Swainson’s, Ferruginous, etc.) changes from pale yellowish to dark brown over time (Figure 6, Red- tailed Hawks), but that is not completely true in some respects. Swainson’s, Red-shouldered, Rough-legged, and Broad-winged Hawks in particular can have brownish eyes even as juveniles (Figure 7, Broad- winged Hawk juvenile on left, adult on right). Another interesting tidbit is that it takes Ferruginous Hawks more time than the other buteos to attain dark brown eyes. Remember, there are exceptions to every rule regarding raptor plumages and colorations. I have seen several juvenile accipiters with dark brown eyes, an adult Cooper’s Hawk with white eyes, and various other “funky-eyed” raptors. So, if you see a bird with an oddly colored eye, consider yourself lucky...and feel free to share it with us!