There was much excitement this winter, among birders and photographers, over the sightings of a northern hawk owl on the wetlands. What captured my fancy was its face. In addition to some classy plumage markings, hawk owls have a facial ruff or facial disc. So do many other owls, but not all. And I began to wonder about the factors that might lie behind and perhaps explain the variation.
First, a little background. The facial disc is framed by a ruff of stiff, curved feathers on both sides of the face. These feathers are thick, coarse, and acoustically reflective, so they serve rather like a parabolic reflector, focusing sound waves toward the ears. The ruffs, and the fluffy, acoustically transparent, feathers that cover the face, are adjustable as needed, to focus the sounds.
Some owls have complete facial discs that form a nearly full circle around the eyes and bill (barn owl, great grey owl). Others have sizeable semicircular patches on each side of the face; the patches do not fully meet over the forehead, leaving a triangle of ordinary feathers (e.g., screech owl, hawk owl, boreal owl, great-horned owl). Still others have small, less well-defined patches on either side of the head (e.g., snowy owl, pygmy owl, burrowing owl). Take a look at a good field guide to see this variation for yourself.
In addition to this variation in facial discs, the ears of owls also differ among species. The ears of many owls are not symmetrical. Right and left ear openings may differ in shape and location on the side of the head. Furthermore, folds and flaps of skin around the ear opening may differ on right and left sides. These asymmetries allow owls to pinpoint more accurately the rustling movements of prey animals in both the vertical and horizontal planes. Some owls, including the barn owl, are capable of locating prey very accurately and precisely in total darkness, using acoustic cues alone. Great gray owls, boreal owls, and snowy owls can find and capture prey that is invisible under deep snow cover, plunging through the snow, using acoustical cues. But the skull and ear symmetries vary greatly, from so asymmetrical that the head may look rather lopsided to quite symmetrical.
I wanted to know if the variation in facial disc and ear symmetry might be associated with some simple ecological differences, such as foraging habitat or primary foraging time (day vs. night). Or maybe species that are close relatives, in the same genus, tend to have similar acoustical arrangements.
I could find no very clear association with foraging habitat. Although forest and woodland foragers all tend to have strong facial discs and skull asymmetry, open-country foragers are more varied. Two open country foragers (snowy owl, burrowing owl) have small facial discs and symmetrical ears, but another (barn owl) has a complete facial disc and strongly asymmetrical ears.
These hearing adaptations seem to be associated with foraging time, in general. Owls that typically forage during the daylight or twilight hours (pygmy owl, burrowing owl) tend to have small facial discs and symmetrical ears. Strongly nocturnal owls tend to have large facial discs and asymmetrical ears. However, the snowy owl forages in constant daylight during the summer and constant dark during the Arctic winter, but its facial disc is small and the ears are symmetrical.
Finally, I asked if closely related species always share similar hearing adaptations. Certainly, all the screech owls (genus Megascops) do. However, there are two species in the genus Bubo (great-horned owl and snowy owl, now classified in this genus), and one of these has a good facial disc and asymmetrical ears while the other one (snowy) does not.
Thus, none of the three simple potential associations (habitat, timing, relationships) proved to be completely useful in describing the pattern of hearing adaptations in owls; exceptions abounded. As Stephen J. Gould famously noted, it is the exceptions that probe the rule and tell us how good that rule is. In the present case, the obvious conclusion is that other factors must be involved! Given that ear asymmetry in owls is thought to have evolved at least five separate times, and that my three potential associations are admittedly simplistic, perhaps this is not surprising. But it was fun to see if any simple patterns could be detected.