Have you ever had the privilege of approaching an owl only to see it doing this with its head?

What is this owl doing? What does this strange and seemingly random behavior accomplish? That’s what we’re going to investigate today. But first, we’ll need to discuss some basic owl anatomy. If you were to look at an owl, you would notice that it differs from many bird species in one major area: its eyes. Unlike many birds that we are likely to see frequently in our daily lives, owls have both eyes situated on the same anterior plane of the cranium. This feature allows for stereoscopic vision, a highly useful ability in predators such as owls and other raptors. However, owls also differ from many species in that their eyes are not structured in the form of a rough, ovoid sphere. Instead, they are more tube-like than spherical with a tight-fitting orbital socket that for many years was thought to preclude the possibility of any eye movement whatsoever (Steinbach & Money, 1973). This belief was supported by the commonly known fact that owls can rotate their heads as much as 270 degrees in certain species, something many scientists believed was an adaptation circumventing the lack of eye movement. In actuality, owls can move their eyes a nominal amount (from 1 to 5 degrees), but most certainly use their enhanced cervical spinal mobility to compensate for that limited ocular mobility. 

That’s interesting for sure, but how does it relate to the strange head bobbing? It’s all related to a phenomenon known as motion parallax. Take a second to look out a window and focus on an object a medium distance away. Now move your head side to side and up and down while maintaining your focus on that object.  Motion parallax refers to the perception that objects closer to you will seem to “move” more than objects in the distance during the type of movement you just did. This phenomenon is enormously helpful in the mental estimation of distance, and you and I use it every day without realizing it. It’s also a strategy used by many animal species ranging from birds to mammals to certain insects.

Many owl species, notably the Barn Owl (Tyto alba), have stereoscopic depth perception (stereopsis) comparable to that of humans. However, their extremely limited eye mobility means that their ability to quickly estimate distances is somewhat lower. The back and forth, up and down bobbing of their heads allows them to quickly estimate approximate distance/depth in novel situations (van der Willigen, Frost, & Wagner, 2002). This finding is supported by the observation that in order to attain comparable accuracy to stereoscopic tests, owls that had one eye covered exhibited far more pronounced head movements.

 

References

Kral, K. (2003). Behavioural–analytical studies of the role of head movements in depth perception in insects, birds and mammals. Behavioural Processes, 64(1), 1-12. http://dx.doi.org/10.1016/s0376-6357(03)00054-8

Steinbach, M. & Money, K. (1973). Eye movements of the owl. Vision Research, 13(4), 889-891. http://dx.doi.org/10.1016/0042-6989(73)90055-2

van der Willigen, R., Frost, B., & Wagner, H. (2002). Depth generalization from stereo to motion parallax in the owl. Journal Of Comparative Physiology A: Sensory, Neural, And Behavioral Physiology, 187(12), 997-1007. http://dx.doi.org/10.1007/s00359-001-0271-9