Raptors

The term “Bird of Prey” could be used to indicate any bird that preys upon other animals, particularly vertebrates.  Technically, this would include storks and penguins!  Since inclusion of species such as penguins can confuse the intention behind “bird of prey,” the term “raptor” is often used instead.  “Raptor” is a more specific term which identifies birds that are adapted to kill prey with specialized feet, or talons.

Raptors are also known for powerful keen eyesight (estimations of 6-8 times better than human eyesight, depending upon species) and sharp beaks for shredding meat.  The nictitating membranes, a third set of eyelids which are opaque and close sideways, serve as protective goggles for raptors’ eyes during dangerous situations such as high-speed stoops or while killing aggressive prey.  Raptors consume non-digestible material such as teeth and feather or fur along with the digestible meat.  These non-digestible materials are compacted and regurgitated as a pellet.

Most raptors demonstrate size-based sexual dimorphism: the males are generally one third smaller than the females.  Due to this trend, traditional falconry termed males, especially of the peregrine falcon species, as tiercels.  Most young are born with darker feathering to camouflage with the nest.  Young raptors are fully grown upon fledging, with feathers slightly larger than those of adults.  Therefore, raptors actually appear to diminish in size upon their first molt.  These larger feathers provide young, inexperienced birds with greater power to catch larger prey.  The smaller feathers of adults permit higher maneuverability to catch smaller, faster, but also less-dangerous prey which requires more skill.

Types of Raptors

Buteos

Buteos are moderately large birds with broad, rounded wings and short tails.  They are easy to see because of their size and tendency to fly in open areas.  In our region, buteos include the red-tailed hawk, broad-winged hawk, rough-legged hawk, red-shouldered hawk, and Swainson’s hawk.

These hawks tend to ride thermals, or pillars of rising warm air.  They can soar on these thermals for hours with only the occasional wing flap.  A hawk’s keen eyes can detect slight movements in a field several hundred feet below.  Field mice and voles are a staple food of buteos.

Accipiters

Accipiters have shorter, rounder wings and longer tails than buteos.  They are largely birds of the woods and can move through a network of branches with amazing dexterity and speed.  Accipiters in our region include goshawk, Cooper’s hawk, and sharp-shinned hawk.

Except during migration, they seldom soar, but flap and glide when in the open.  The sharp-shinned hawk, the smallest of these three, takes small birds while the other two hunt larger birds.

Falcons

Falcons are built for speed.  Sharp, pointed wings and a long, narrow tail allow falcons to slice through the air.  The crow-sized peregrine falcon can fly over 200 miles per hour in its stoop (dive).  Falcons also have dark markings under their eyes and a “tooth.”  This “falcon’s tooth” is a notch that allows the bird to kill prey and break bones with a quick twist of the head.  Finally, falcons have a boney tubercle or projection inside the nare (nostril).  This tubercle creates a vacuum, pulling air into the airway during high-speed flight, similar to the forward projection on jet engines.

The prairie falcon of the western U.S. is similar to the peregrine in size and habits and may be found in Nevada.  The gyrfalcon is larger and inhabits the arctic.  Our most common falcon is the American kestrel, no larger than a robin.

Owls

Owls belong to Family Strigidae which divides into 22 genera and 122 species.  Owls are found around the world and some are migratory.  These birds have facial disks that funnel sound to the ears.  Soft flight feathers with a comb-like structure on the leading edge muffle the air, allowing owls to fly silently.

More familiar to many might be the owls’ large eyes.  These large eyes capture the smallest amount of light and are capable of rapid and sharp focusing.  Further, owls can independently dilate their pupils (dilate one pupil more than the other).  Binocular vision provides owls with excellent depth perception.  However, these eyes are elongated and tapered, like a football, which prevents owls from being able to move their eyes in their sockets.  Owls can rotate their heads 270 degrees each direction due to the presence of 14 cervical vertebrae, which compensates for the lack of peripheral vision.

Owls have excellent hearing as well.  Their ears are directed forward and are much larger than those of other birds.  The ears are asymmetrical.  For some, like great horned owls, the ears are symmetrically placed, but directed asymmetrically: the central fold of one external ear points downward while the other points upwards.  For others, like barn owls, the ears are asymmetrically placed, with one ear higher on the head than the other.  These arrangements provide owls with incredibly accurate sound localization.  Some studies indicate owls can locate and catch prey by sound alone, especially the barn owl.

Of the raptors, owls are the only family that is unable to digest bone.  Therefore, one can dissect an owl’s pellet and find the bones of its meals.  In some cases, one could put together the entire, complete skeleton of the prey.

Mathematics of Predation

From studies of captive birds, ornithologists estimate that large raptors must eat approximately 10 percent of their body weight daily in colder weather and 8 percent in warmer weather.  Smaller raptors like screech owls must eat 25 and 17 percent of their body weight in these respective periods.  This increased consumption is due to the fact that the smaller raptors have more surface area relative to body mass.  These small raptors lose heat faster and require more food to maintain body temperatures.

Value of Raptors

Raptors maintain balanced ecosystems by keeping prey populations in check.  The overlap of raptors in an area further emphasizes this balancing power.  Large raptors such as eagles tend to take larger prey like hares while smaller raptors (e.g. kestrels) take smaller prey (e.g. mice and lizards).  Also, nocturnal raptors, such as great horned owls, consume the same types of prey as diurnal counterparts, such as red-tailed hawks, a pattern which creates a round-the-clock monitoring system.  This constant predatory pressure keeps prey populations from growing or expanding.  For example, such a collective population of raptors in Superior Township, Michigan (36 square mile study area) consumed the following average numbers of prey annually:

  • 56,068 meadow mice
  • 7,616 white-footed mice
  • 2,207 other mice, rats, shrews, and moles
  • 39 weasels
  • 348 muskrat
  • 946 rabbit
  • 537 fox squirrels
  • 2,307 pheasant
  • 131 bob-whites
  • 11,913 small- and medium-sized birds
  • 1,215 snakes
  • 2,023 frogs
  • 1,490 crawfish
  • 1,787 other items

Moreover, by maintaining healthy populations of prey species, raptors aid in preventing spread of disease.  Many of the prey species consumed by raptors carry disease, especially when approaching carrying capacity of the available resources.  Therefore, the presence of raptors reduces the number of disease-carrying pests.  In fact, many agricultural industries now create artificial environments or nest sites to attract raptors.  For example, farmers may build nest sites to attract owls, which then hunt the pests that would typically consume the crops and carry disease.

Flight

Only birds, insects, and bats are capable of true flight.  A bird’s wing, in cross-section, is shaped like an airplane’s wing.  When a bird’s or plane’s wing moves rapidly through the air, a partial vacuum forms on the upper surface which pulls the wing upwards (and the bird or plane with it).  One can see the same principle at work when a kite is pulled into the wind.  This action creates a vacuum or lift which keeps the kite aloft.

When a bird flaps its wings, it first thrusts them forward and down, then backward and up.  This circular motion keeps the wing moving through the air, creating the necessary lift.  A bird can move and twist individual feathers during flight.

A bird’s wing is mostly hollow flight feathers which are light and strong for their weight.  These comprise most of the wing’s wide surface area.  Underneath all the feathers, a bird’s wing is a modified arm: a skinny arm with light, thin bones.  The “wrist” and “finger” bones are fused together and it has only a few small, slender muscles.

Birds have fewer bones than other vertebrates.  Most bird bones are hollow, making them light.  In parts of the body which must be rigid in flight, such as the wings and backbone, many of the bones are fused together.

Though the muscles in its legs and wings are very thin and light, those in a bird’s chest are massive.  These are the muscles that power the movements of the bird’s wings in flight.  They are attached to a large keel or ridge on the breastbone.  Because these huge muscles concentrate a large percentage of a bird’s total weight in the chest, they keep the animal’s center of gravity below the wings, an ideal arrangement for flight.

Species housed at Animal Ark

  • Peregrine falcon, Falco peregrinus (Animal Ark does not currently have a peregrine falcon)
  • Gyrfalcon, Falco rusticolus (Animal Ark does not currently have a gyrfalcon)
  • North American kestrel, Falco sparverius
  • Red-tailed hawk, Buteo jamaicensis
  • Great-horned owl, Bubo virginianus
  • Barn owl, Tyto alba