The best way to find raptors is to observe their prey. As soon as a shorebird tilts its head to point one eye up, a raptor must be looming overhead. Their skills in detecting predators exceed those of the best birder — after all, their life depends on their attentiveness. That is why scanning behaviour is commonly used as a reliable indicator of the danger level that animals perceive.
But, how reliable is it really? How do we know what a bird is really looking at?
My first doubts come up in Chile, in 2005. At midday we drive through the sweltering desert of Atacama. A carpet of salt crystals stands out against a bluegreen lake where flamingos are slobbering crustaceans. The sky is clear blue. The white clouds in the distance turn out to be the snowy peaks of the Andes.
Sure enough, there is a shorebird on the salt. It is a Baird’s Sandpiper (Calidris bairdii), and it is alone. The bird is not shy; we can even see that it is eating little white critters, probably amphipods. As I come even closer to take pictures the bird starts tilting its head several times as if scanning the sky overhead. I look up to seek for the raptor. There is none.
A little further down we see another loner on the salt plains: a Puna Plover (Charadrius alticola). Likewise, the plover is not shy, and as I approach to make pictures, this bird also tilts its head, just as the Baird’s did. Again, not a bird in the sky. I’m confused.
These observations linger in my head for some time, until Graham Martin, the King of Bird Vision, visits our institute. I bombard him with questions and pictures: Would the birds really look at the sky overhead? Could they be looking at me instead? Does the bright sunlight have something to do with it?
It is likely that the bird sees me as a predator. By scanning the sky, he may be anticipating his chances: if I decide to flee for this ground predator, could I be intercepted by an attacker from the sky?
However, birds can also keep their head tilted for another reason, explains Graham. Just like us, birds have a pecten oculi, the organ that causes the famous blind spot in the field of vision. This organ ensures that the retina receives sufficient nutrition. In birds, the organ may have yet another function: its dark dyes and velvety structure ensure that incoming light is strongly absorbed. By turning their heads, birds could make sure that the bright sunlight falls right on the pecten. As a result, they can reduce glare in the eye and prevent being temporarily blinded by the sun. Calculations by Graham confirmed that the head positions of the photographed birds were appropriate for his effect to occur. We published our ideas, thereby explicitly acknowledging that our sample size is only two (van den Hout & Martin 2011, Wader Study Group Bulletin 118: 18-21).
Since then I have been keen on trying to find out whether more birds exhibit such scanning behaviour in a predator-free context. And indeed, I observed and photographed several more scanning events, both from the northern and southern hemisphere, that could clearly not be connected to a raptor flying overhead. The pictures are shown below.
Like with the Atacama birds, it appeared that all pictures were taken around midday (and this was not planned!), when the sun was high in the sky. These observations seem consistent with the idea that by tilting their heads, the birds prevent disability glare from the sun, as in our 2011 paper. Although these observations do not conclusively solve our scanning issue, it makes the matter all the more intriguing, and compelling to solve.
Van den Hout, P.J. & Martin, G.R. 2011. Extreme head-tilting in shorebirds: predator detection or sun avoidance. Wader Study Group Bulletin 118: 18-21.