The Eye of the Falcon

 

Peregrine Falcon

Kiting in a strong breeze above the Halibut Point overlook this Peregrine Falcon caught my eye. It hung there almost stationary on long tapering wings while it surveyed the shoreline, giving me time for a photograph. A few seconds later it was half a mile away on a mission.

The eye of the falcon

The photograph revealed the remarkable protrusion of the bird's eye, like the pilot's canopy at the head of a jet fighter. That image raised a lot of questions.

Juvenile Peregrine on hunting perch

The visual acuity of peregrine falcons is legendary. One website enthusiast reported, or conjectured, that they can spot their prey more than one mile away which to us is "like seeing a rabbit from 17.6 football fields." Another made the case that "a peregrine falcon’s vision is eight times better than that of humans. They can spot small prey from 2 miles away." No matter how you squint at it, their eyesight is phenomenal.

A Merlin, a small falcon

The Merlin is local falcon with similarly outsized eyes providing a long focal length for telescopic magnification. Where we humans have about 30,000 receptors in our retinas, birds of prey have over a million. These rods and cones are distributed between two foveae in each eye, allowing the bird to focus on close and distant objects at the same time. This has been described as "seeing the world through both a macro and zoom lens simultaneously."

Peregrine Falcon

The falcon's binocular vision is optimized by the location of its eyes towards the front of its head pinpointing prey which, on the other hand, have eyes on the sides of their heads to scan in all directions for danger. The difference has been quipped as “Eyes in the front, they hunt. Eyes on the side, they hide.”

Falcons have three lids around each eye. The top and bottom eyelids are opaque for closure when the bird is sleeping. The third, a translucent membrane, moistens the eye and protects it from damage during flight. Like see-through goggles it facilitates high-speed hunting.

The distinctive plumage below the falcon's eyes likely evolved to minimize reflected sunlight from obstructing its vision. Known as malar stripes they serve the same purpose as an athlete's "eye black" in reducing glare. Furthermore, by soaking up a blinding flare, they improve contrast sensitivity, or the ability to see details and discern objects from their backgrounds—like high-speed prey against a bright sky. 

The human eye and brain can process light images comfortably at up to 60 per second to form meaningful vision for normal activity when walking or running. At higher speeds the world appears blurrier to us. The Peregrine Falcon, diving on its prey in excess of 180 miles per hour, gathers information at over 100 images per second.  The qualities of its vision have taken it to the upper limit of aerial performance.