Owls

Owls have remarkable biology. 

Let’s start with their eyes, which are among the largest relative to the size of the body  of  any bird, and even large relative to many mammals . They are not global or gently ellipsoid like ours. Rather, they’re   tubular or cylindrical. Owls have a specialized skull that holds them in place  just like a pair of binoculars.   The eyes always stare straight ahead, and cannot swivel any more than  a binoculars lenses, confined to the barrels of the binoculars,  can swivel.

This peculiar arrangement gives owls two kinds of vision. Straight ahead, it’s indeed binocular: a rare characteristic among birds, but one with exceptional depth perception which greatly increases their hunting efficiency, especially when coupled with their gigantic light-gathering corneas.

But this arrangement still   allows them fairly good peripheral vision, because the very large size of their eyes , allows for more  scansion,  even if this is only monocular. Fortunately, owls have necks that let them rotate almost three-quarters of the way around, so they can quickly see what is going on, to good advantage in both hunting and in defense.

This maneuver is not quite as good as the dramatic scene featuring the young Linda Blair from the movie, The Exorcist. No, not the vomiting scene, the head-spinning one, although the owls habit of vomiting tells us a great deal about their diets.

Owls vomit what amounts to balls or cylinders called pellets. These contain the relative indigestible parts of their prey. While the range of items in owl   diets varies  with the size of the owl. Owls swallow prey whole whenever  they can, but will rip apart larger prey. This means that larger owls (for example the Great horned owl) will eat prey as large as skunks (owls have no appreciable sense of smell) , raccoons, small foxes, squirrels, as well as any mice, chipmunks, moles and voles. Owls can, and do take, bats on the fly.

While most owls nest in trees, a few of the very smallest, who usually live in dry, desert like country, literally burrow in the ground, where they eat insects and the occasional underground lizard. In Latin America, when small farm villages expand, or when a slum expands out into the scrubland, the mouse and rat population expands, and these small owls actually shift their diet and hunting, and start to tackling the larger rodents. While this strategy has its risks in the owl being injured, it greatly increases the number of matings and frequency with which these owls can lay higher quality eggs.

Owls are both fighters and lovers in their relations with other birds. They tend not to get along well with hawks, with whom they would compete for food.

However, the biggest danger to their health appears to come mostly from being mobbed during the daytime by flocks of smaller birds which attack them on sight in multiple numbers with the idea of each small bird contributing in rapid succession of screaming out, attempting to peck or to claw at the owl in flight, and then peeling off while another member of the flock strikes.

Indeed it has now been determined that the reason that owls tend to roost deeper in the woods rather than next to the fields in which mice, skunks or gophers might be more readily found is that  they are less likely to be seen and then subsequently mobbed. Owls shift to the fields at nightfall, when the small birds roost.

The pygmy owls would seem to be especially afraid of small swarming birds, for it is itself, as its name suggests, a small owl. But recent studies show that the special plumage that this owl has, which in effects, imitates a pair of eyes on the back of its neck, may have an offensive rather than a defensive intent. 

The old theory of this adaptation was that it frightened the mobbing birds by having them perceive that they were attacking the owl face on, obviously a high risk strategy given the beak and talons of the owl. But it now appears that these false eyes are actually intended to lure mobbing birds to attack the owls at daytime, so that the owl can subsequently determine how many such birds are in the area, where they might roost, and how the owl might then best plan an attack on them for food.

Owls are actually the friends of some seabirds who nest near the great snowy owl, for that owl is a ferocious defender of its nest, and egg-raiding rodents and arctic foxes are attacked with such ferocity by the owls that they avoid the area altogether optimizing the hatching of those seabirds, like the brent geese, whose ground nests are closest to the owls.  (The owls tend to know which birds predate on owl eggs and which do not, and geese are apparently smart enough not to try it.)

Of course there are some very well known endangered owls, particularly the spotted owl of old growth forests along the Pacific coastline.  Troubles caused by the timber industry, collisions with power lines and a   disinclination to breed wherever there is too much human activity, have given that species major protective status. It now turns out that the biggest threat to the species, or at least to its “racial purity” as it were, is not from human activity, but from the Eastern barred owl, which has stopped fighting with the spotted owl as the former spreads from its historic eastern and Midwestern strongholds, and now mates with the spotted owl, creating viable hybrids.

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Tony Stankus, tstankus@uark.edu Life Sciences Librarian & Professor

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