December 3, 2021

The science magazine of Imperial College

# Science behind the photo #48

A Persian falconer’s prize, the Black Shaeen (Falco peregrinus peregrinator) is one of the fastest and treasured falcons, reaching speeds of up to 320km/h ...

A Persian falconer’s prize, the Black Shaeen (Falco peregrinus peregrinator) is one of the fastest and treasured falcons, reaching speeds of up to 320km/h. Biologists and falconers have studied the Peregrine falcon’s eyesight and approach to his prey extensively.

Peregrine Falcon’s have binocular vision eight times better than our own. The fovea is a region in the falcon’s eye responsible for producing sharp, focused images. The sharpest image is produced when an object is viewed in a line of sight both central as well as 40 degrees to the left and right of the head axis. This means when a falcon observes prey, his head will alternate between looking directly at the prey as well as turning his head left and right. The further away the object the longer a falcon will spend looking at the object on a 40 degree angle.

When in flight this poses a problem. If a falcon’s head is turned sideways to create a sharp image of his prey the aerodynamics of flight are compromised. A turned head can slow a falcon’s aerodynamic drag coefficient by a factor of two. How can a falcon achieve such high speeds of flight while maintaining a focused image of his prey?

Tucker and Colleagues [1] have created a mathematical model that incorporates both maximal flight speed and image focusing.  They found a spiral path of flight would reduce aerodynamic drag most significantly. Observations of flight confirmed these results; the spiral path allows the falcon to maintain a 40 degree line of site of the prey, with one eye, without having to turn his head. Even though the curved path is longer than a straight one, the falcon will reach the prey more quickly as there is less aerodynamic drag. A falcon’s speed is much faster when his head is held straight; this more than compensates for the increased flight path distance.

Flight paths are also thought to curve towards the direction of the sun. This means a falcon’s presence is lost in the glare of the sun, allowing it to be hidden from the prey. This research has provided a greater insight into the Black Shaeen’s incredible ability to reach such extreme speeds while maintaining high visual acuity of his prey.

Reference

1.Tucker, V.A, Tucker, A.E, Akers, K., Enderson, J. H. Curved flight paths and sideways vision in peregrine falcons (Falco peregrinus) The Journal of Experimental BiologyI (2000). Vol 203; pp 3755-3763.

PHOTOGRAPH: Natasha Mehrabi