Heady, Steady, (Optic) Flow!

Birds are overwhelmingly visual animals, and image quality really matters to them. Hunting kestrels are able to detect the UV reflecting scent marks and urine-trails of voles (their main prey). They can literally see where the voles have been. Hovering and holding the head still is critical to that ability – by holding the head still the bird can keep the visual background steady, so any relative movement against the background gives away the prey’s position. https://player.vimeo.com/video/564528082?app_id=122963

This video taken by our very own Kate Reynolds illustrates how Kestrels use head stability to hunt their pray.

How birds keep the head steady is made pretty clear in onboard footage from the back of a Gyr falcon. The bird keeps its head, and its visual field, fixed relative to the background as it turns, before rapidly shifting to a new visual position. These rapid movements are called saccades, and we do the same, but by moving our eyes rather than our heads (birds eyes are so big relative to their skulls that they can’t really move them, so they have to move the head instead).

The system that keeps birds heads steady involves both visual stabilisation, and the 6th sense we share with birds – proprioception, position and acceleration sensing, based in the semi-circular canals of the ear. Fluid in the semi-circular canals flows around the canals whenever the head rotates (due to the fluid’s inertia), and that flow is translated into a rotation and acceleration signal. The semi-circular canals evolved early in the origins of vertebrates, and recent research has shown that the semi circular canals went through dramatic evolutionary changes as birds evolved from terrestrial bipedal theropod dinosaurs (like velociraptor), and as those fast-running upright bipedal dinosaurs evolved from their more sedate sprawling crocodile-like ancestors.

Even in modern terrestrial bipedal dinosaurs like chickens, the head-stabilisation technique is spectacularly successful, as comparisons with camera stabilisers demonstrate.

It seems likely that any agile autonomous visually-guided vehicle will require bird-like sensor-stabilisation systems.

Reference for evolution of semicircular canals

Bronzati et al., Deep evolutionary diversification of semicircular canals in archosaurs, Current Biology (2021), https:// doi.org/10.1016/j.cub.2021.03.086

Reference for saccadic eye movements

Michael Land 2019. Eye movements in man and other animals. Vision Research, Volume 162, 2019, Pages 1-7, ISSN 0042-6989,

https://doi.org/10.1016/j.visres.2019.06.004.

Reference for kestrels detecting vole scent marks and urine using UV vision

Viitala, J., Korplmäki, E., Palokangas, P. et al. Attraction of kestrels to vole scent marks visible in ultraviolet light. Nature 373, 425–427 (1995). https://doi.org/10.1038/373425a0