Hawk owls rely on vision to hunt, demonstrate adaptability

Posted Jan. 31, 2009, at 12:10 a.m.

In response to last week’s column about the northern hawk owl, Joe Fernandez of Orono — a friend and former co-worker of mine — said he was inspired to do more research on this bird. He came across the Cornell Lab of Ornithology’s “All About Birds” Web site (www.birds.cornell.edu/AllAboutBirds/) and shared the in-formation he found there.

“According to this Cornell U. site, this bird can find its prey under one foot of snow!” he said.

Incredible as this may seem, this owl’s ability to detect and capture prey hidden beneath the snow is one of the prerequisites for its survival in the North. But it started me thinking about the similarities and differences between this owl and other owls, as well as other birds of prey. It also made me wonder if this owl was as adept — and as adapted to — hunting by sound as are other owls.

Most people are familiar with owls as hunters of the night. Owls do have several adaptations that allow them to hunt in near darkness and, as is the case with barn owls, complete darkness, as described by R.S. Payne in the Journal of Experimental Biology.

The structure and placement of an owl’s ears and facial feathers facilitate their auditory power. The ear openings are asymmetrical, and this allows the bird to pinpoint the location of the sound with a high degree of accuracy. In addition, the feathers that compose the familiar facial “disk” further catch and funnel sounds into the ear openings.

Knowing this, I was intrigued when I read the northern hawk owl lacks this ear asymmetry. Other northern owls, such as the great gray owl — which relies primarily on voles for food — exhibit this to the extreme.

In addition to asymmetrical ear openings, great gray owls also show asymmetry in their skulls. This, in addition to their large, very pronounced and complex facial disks, allows for extremely precise location of sounds and superior hearing.

The decreased auditory acuity may force the hawk owl to “critically depend on birds in winter, when small mammals are less accessible under snow,” according to the “Birds of North America” species account.

Accordingly, hawk owls are more apt to hunt during the day and, as I mentioned in last week’s column, their short, rounded wings and long tail give them superior maneuverability — perfect for pursuing other avian prey among trees. In fact, birders have observed the owl in Bristol pursuing and capturing a white-winged cross-bill (a large northern finch).

Interestingly, there is a hawk that displays owlish characteristics. The marsh hawk, or northern harrier, sports a facial disk much like an owl’s. It isn’t as pronounced, but it is visible and unique among hawks. This hawk doesn’t hunt at night, but the prey it hunts — primarily small ground rodents — is often concealed by thick grassland or marsh vegetation. So it relies more often on sound than sight to capture this prey.

How intriguing — an owl that relies more on sight and a hawk that relies more on hearing.

There is much more study that needs to be done on the hawk owl, especially, according to the BNA, as it “may be in the process of adapting to different prey species, such as a greater availability of larger prey — e.g., snowshoe hare.”

The hawk owl is already known for being a great opportunist in its hunting habits and choice of prey, and it looks as if it is continuing to adapt — an encouraging sign.

bdnsports@bangordailynews.net

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