OWLS: FORM FOLLOWS FUNCTION

OVERVIEW
   While eagles, hawks, falcons, owls, ospreys and vultures are collectively termed birds of prey or raptors, this grouping together reflects traditional classification rather than modern scientific findings. Taxonomy, which is the process of systematically classifying plants and animals based upon their relationships to each other, really began in the 18th century with a hard-working genius named Karl von Linne. He is usually referred to as Linnaeus,which is the Latin form of his Swedish name. Linnaeus developed the concept of binomial nomenclature, the idea of giving plants and animals two names, usually based on Latin or Greek descriptive words. The first name is an organism's genus name and the second is its species name. Linnaeus also visualized larger, related groups he called "orders" and "classes." Birds of prey were originally lumped together in one group, the Raptores. Because eagles, hawks, falcons, owls, ospreys and vultures all have hooked beaks, strong grasping feet, excellent eyesight and masterful powers of flight, it was once thought that they must be related. Modern taxonomists recognize a phenomenon called convergence or convergent evolution, whereby unrelated organisms come ultimately to resemble one another because they have evolved similar adaptations in response to similar life demands. Birds of prey therefore are not necessarily closely related, or even related at all. For example, New World vultures like the Black Vulture and the Turkey Vulture are related to storks but not to eagles, hawks and falcons. Owls on the other hand share a common ancestry with a group of birds known collectively as nightjars. Nightjars are nocturnal and feed largely on flying insects. Examples found in New England are the Common Nighthawk and the Whip-poor-will. While owls and nightjars diverged from a shared line of development millions of years ago, they are still considered each other's closest relatives, and owls are not thought to be related to other birds of prey. In fact, because owls are nighttime hunters they have evolved a host of specialized features and behaviors that make them unique in the bird world. To focus on these owlish specialties, we will consider owls separately. For information on the other birds of prey, see "The Diurnal Birds of Prey: A Very Mixed Bag" on this Website.

Owls, then, are adapted to hunting at night. While they are usually termed nocturnal (active at night), many species are really crepuscular (active at dawn and at dusk). Worldwide there are between 160 and 175 different owl species. There is some disagreement about classification, and new owl species have been discovered in recent years, so the exact number of owl species is hard to pin down precisely. All owls are classified in one group known as Strigiformes (Strig uh FORM eez). This large group is subdivided into two families. One family, which only contains 13 species, includes the barn owls and the bay owls and is called Tytonidae (ty TON uh dee). The other family is much larger, because all the other owl species are included in it. This large family is called Strigidae (STRIG uh dee); the owls included in it are sometimes referred to as typical owls.


North America is home to 19 owl species. Only one, the Common Barn Owl (Tyto alba), comes from the Tytonidae family. The other 18 species belong in the Strigidae family. These 18 North American typical owl species are:
Flammulated Owl (Otus flammeolus)
Eastern Screech Owl (Otus asio)
Western Screech Owl (Otus kennicottii)
Whiskered Owl or Whiskered Screech Owl (Otus trichopsis)
Northern Pygmy Owl (Glaucidium gnoma)
Ferruginous Pygmy Owl (Glaucidium brasilianum)
Elf Owl (Micrathene whitneyi)
Burrowing Owl (Athene cunicularia)
Boreal Owl (Aegolius funereus)
Northern Saw-Whet Owl (Aegolius acadicus)
Northern Hawk Owl (Surnia ulula)
Long-Eared Owl (Asio otus)
Short-Eared Owl (Asio flammeus)
Spotted Owl (Strix occidentalis)
Barred Owl (Strix varia)
Great Gray Owl (Strix nebulosa)
Great Horned Owl (Bubo virginianus)
Snowy Owl (Nyctea scandiaca)

The eight owl species that are resident to New England (where the two people who make up the Wingmasters partnership happen to live) are individually profiled on this Website. Now let's look at owls' overall structure and see what physical characteristics (morphology) set them apart from other birds of prey.

FORM
Owls look the way they do because of one overriding fact: they evolved to hunt rodents during the hours of darkness, when the majority of rodents are active. That's owls in a nutshell - they are nighttime rodent hunters. Of course there are many exceptions to this general statement. Some owls like the Ferruginous Pygmy Owl, the Northern Hawk Owl and the Snowy Owl routinely hunt by day and are therefore considered diurnal (active by day). Some owls like the Flammulated Owl are considered insectivorous (preying upon insects) because insects and arachnids like spiders constitute their major food source. Several Asian and African species specialize in hunting fish. Still, these are exceptions that prove the general rule: most owls, to a greater or lesser degree, hunt rodents in darkness. Many of the physical characteristics that give owls their unique appearance stem from this shared lifestyle.


Let's start at the top. Owls have big, wide heads that appear to be set directly on their bodies without the benefit of a neck. Many species wear ear tufts, bunches of feathers that sprout from the top of the head. The huge eyes are set in the front of the flattened face, and each eye is surrounded by specialized feathers that together form a facial disk around the face. Other specialized feathers form "eyebrows" above the eyes. The feathers of the facial disk and eyebrows are paler than the body feathers, and the facial disk is usually set off by a rim of differently colored feathers. Hidden behind these specialized, square-tipped feathers are the ear openings, which may be asymmetrical (not matching) in size, shape or position.

The upper mandible (beak) is sharply decurved (curving downward) and partially obscured by stiff bristle feathers whose overall shape suggests a moustache. The stocky body is held upright and a rounded look is imparted by dense, loose plumage. Plumage color is always subdued, usually tones of brown, cream, gray and black.

A perched owl with wings folded gives no hint of some owlish specializations. One is wings that are very large for its body weight, wide from front to back as well as from side to side. The wings' soft upper surface gives owls a uniquely quiet flight. The characteristically short tail may not show when the owl is perched.

What is obvious on a perched owl (although many makers of owl toys and collectibles fail to notice this detail and consistently get it wrong) is that its large feet hold a branch differently from other birds. Most birds, including the diurnal birds of prey, grip a branch (or their prey) using three toes in front and one behind. Owls can move the outside toe of each foot forward or backward, and when they perch, almost invariably they grip with two toes in front and two toes in back. Prey seems always to be gripped this way. The legs are fully feathered and the toes are feathered right down to the talons. Sometimes the toe feathering is sparse (in the case of the Common Barn Owl), but more often it is thick. The toes of the Snowy Owl are so luxuriantly feathered the bird seems to be wearing slippers. The toes are thick and relatively short, with a gripping power that exceeds that of hawks and falcons. The talons are more strongly curved than those of other raptors (with the exception of the osprey).

FUNCTION
Owls don't look the way they do in order to sell toys or nail movie roles. As we have seen, they have evolved numerous specialized features to help them hunt primarily rodent prey in low levels of light. So now let's take a general look at how those specialized features work, and then we'll move on to detailed descriptions of those features.

The most obvious, most owlish feature of any owl is the outsized head. The head must be huge because it houses very large eyes (as large as human eyes in the case of some species, such as the Great Horned Owl and the Snowy Owl) and very large ear openings. The eyes must be as large as possible to let in as much light as possible. The head is wide to allow as much distance as possible between the ear openings. The face is flattened to give owls, which usually hunt with the built-in handicap of reduced light levels, a clear field of vision. The specialized feathers around the eyes are probably aids not to vision but to hearing, amplifying sound and perhaps channeling sound waves into the ear openings behind the eyes. The neck, which is hidden under the dense plumage, actually has more vertebrae than our own necks. All mammals, from people to giraffes, have seven cervical (neck) vertebrae. Owls have 14. They're also endowed with flexible neck muscles we lack. As a result they can rotate their heads 180 degrees from front to back without moving their bodies. This head movement is necessary because owls cannot move their tubular eyes more than 1.5 %.


The upper mandible (beak) is angled downward more sharply than is the case with diurnal birds of prey so it doesn't block its owner's field of vision. The bristle feathers that surround and obscure the beak are sensory receptors, just like the vibrissae (whiskers) that sprout from the muzzles of mammals like cats and dogs. Perhaps the bristle feathers help owls with close-up situations such as eating.

The upright stance of owls, so different from the more nearly horizontal posture of most birds, helps these hunters to watch for prey. Diurnal birds of prey like red-tailed hawks use the same posture while still-hunting (hunting by perching and waiting motionless for the movement of prey) and for the same reason - the more upright the bird, the taller it will be and the more of its environment it can survey. Perched this way, owls have a distinctly rounded look because of their densely layered, fluffy plumage. Owls actually have more feathers than many other types of birds. As many as 10,000 feathers of various kinds cover their heads and bodies, even unlikely places like eyelids. With most owls species, the only skin surface not covered by feathers is the underside of the feet. One reason for the extra feathers is that owls typically hunt when the sun is down, whether they are truly nocturnal or (more often) crepuscular. When the sun goes down, so do temperatures, particularly in wintertime. Owls therefore need protection from the cold, and they achieve it with extra feathers to hold in body warmth. Because they are in large part rodent hunters, preying on animals that can bite hard (the word rodent comes from a Latin word that means "to gnaw"), they must catch and dispatch their prey before it has the chance to bite them. So owls have evolved feathers with a velvety upper surface that do not rustle as the feathers rub against each other. The soft-surfaced feathers on owl wings muffle the sound of air flowing over the wings. On most owls species the outermost primary feather of the wing has a saw-toothed leading (front) edge, a stiff row of filaments called fimbriae that looks like they've been spiked with hair gel. The second primary feather has a much shorter saw-toothed edge, and the rest of the wing feathers lack this specialized feature. However, the tips and trailing (back) edges of owl primary feathers and secondary feathers (the major flight feathers of the wing) are soft, with a fringe that gives them a somewhat ragged look compared to the crisp edges of a falcon or eagle wing feather. These two wing-feather refinements - a spikey leading edge and soft, ragged edges on the feather tips and backs - help to silence the passage of air over the wing surface and so enable owls, uniquely in the avian (bird) world, to fly almost soundlessly.

Owl wings are large for the birds' weight (which is never as great as it seems; remember that owls are wrapped in extra layers of soft feathers that give them a deceptively heavy look). Birds like owls that have a big wing surface and a relatively low weight are said to exhibit low wing-loading. Birds like falcons that have a smaller wing surface and relatively high weight are said to exhibit high wing-loading. The owls' low wing-loading means two things: they don't have to flap their wings as often as birds like falcons, yet another contributor to quiet flight; and they can fly off with heavier prey than a bird with high wing-loading.


So owls have evolved several methods for cutting down on noise as they fly, and this virtually silent flight allows them to sneak up undetected on their prey. There is one more reason that owls fly quietly. Owl hearing is the best in the world (at least it is in some species, notably the Common Barn Owl), and many owls locate their prey largely by listening. How would it benefit owls to have this superior auditory ability if they made a noise as they flew? So owls have evolved uncannily quiet flight to help them hear their prey and to prevent their prey from hearing them.

Owl plumage is never brightly colored. The brilliant hues of male songbirds like cardinals, goldfinches and orioles is not present in any owl species. The question we should ask about their cryptically-colored (colored to conceal) plumage is this: Why is it necessary? Owls' ability to fly virtually soundlessly means that they can aerially ambush their prey without the benefit of camouflage. So why are owls colored and patterned to blend in with their surroundings? The answer lies in a phenomenon known as mobbing. Birds of prey are understandably feared by other birds; they represent a real or implied threat to those other birds. In response to this threat, passerines (songbirds) will gang up on a raptor, vocalizing to one another using an alarm call that is understood by many different kinds of birds. Some particularly aggressive species such as crows, blue jays and mockingbirds will even attack the predator, although the real intent of mobbing is probably not to hurt but to harass and drive away. While all raptors are the targets of mobbing behavior (which means that raptors have recognizable raptorial characteristics that identify them as such to other birds), owls seem to be marked out for particularly intense mobbing from passerines. The reason for this is probably that owls represent a greater threat to passerines than other raptors, because owls can see and navigate in the dim, dark hours when passerines are helpless. Owls can and do help themselves to songbird snacks at night, and songbirds by instinct know this. So by day the songbirds will frantically mob any owl they find, using the advantage of greater numbers. If the mob includes some murderously inclined crows the consequences to the owl may be fatal. So owls hide by day, using their camouflage (and any tree cavity that's handy) to conceal themselves from avian vigilantes.

The thick, strong toes of owls exert a tremendously powerful grip. For their own safety, owls need to subdue their prey as quickly as possible, particularly if their prey is a rodent that can turn and bite. The unusually powerful grasping ability of owl feet accomplishes two things simultaneously - the rodent is held immobilized in the viselike grip; and the toes drive the dagger-sharp talons into the prey, quickly ending its life. The strong curve of their talons (Great Horned Owl talons, for example, are curved in a full half-circle) helps owls to maintain their grip. So does the zygodactyl (two toes in front, two toes in back) arrangement of their toes. The two-in-front, two-in-back toe arrangement also enables owls to lift greater weights proportionately than other birds of prey, since the weight is evenly distributed.

The powerful owl foot grip is necessary partly because of something owls in general don't have, and that's speed. Falcons and eagles in particular often use a speedy, wings-closed dive known as a stoop to either knock prey out of the air or bowl it over on the ground. Their speed allows these raptors to take down prey larger than themselves. Owls, by contrast, usually capture their prey on the ground after a short glide from a nearby tree. They are not built for speed and (with the exception of a few species like the Northern Hawk Owl and the Snowy Owl) rarely achieve it. So they depend upon a powerful grasp, applied without the benefit of a speeding blow.


The feathering on owl legs and toes helps quiet their movement as they're swung down like the landing gear of a plane, ready to grab. The feathering also helps protect owls from the bites of their prey. It's often written that the feathering on owl toes is a protection against cold. That may be true, at least in part - northern species like the Northern Saw-Whet Owl and the Snowy Owl sport very heavily feathered toes, while the Common Barn Owl, with a more southern distribution, has sparsely feathered legs and toes. However, I'm inclined to think that owls need feathered legs and toes as much for protection from bites as for protection from cold temperatures.


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