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Owls & Owl Pellets
General Information
Of the 180 or so species of owls that occur worldwide,
eighteen species occur in North America. These are divided into two groups,
the Barn Owl (1 species) and the typical owls (17 species).
Owls range in size from the tiny North America Elf Owl,
which at 6 inches long is the smallest owl in the world, to the large and
powerful Great Horned, Snowy and Great Gray Owls.
Most owls are nocturnal, but some species, such as the
Short-eared Owl, Barred Owl, Snowy Owl, Hawk Owl and Pygmy Owl, will hunt
during the day. Notice in the following images that owls are cryptically
colored and patterned to blend in with their preferred habitat.
Owls perform a valuable service by helping to keep rodent
populations in check. In some areas, such as the sugar cane plantations of
Florida, land owners have recognized the value of owls in controlling rodent
populations, and have placed nest boxes for Barn Owls to encourage these
birds to stay, nest and help control rodent populations.
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The Long-eared Owl is Holarctic,
occurring over most of North America, Europe and Asia. The long ears are not
really ears, but tufts of feathers that help the bird to blend in with its
surroundings. In winter months, these owls form communal winter roosts,
usually in a cedar tree. These wintering birds are sensitive to human
disturbance, so if there is a winter roost near you, it is important that
they not be disturbed.
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Figure 1 - Long-eared Owl (Asio otus)
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Figure 2 - Burrowing Owls (Athene cunicularia)
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The Burrowing Owl occurs from
North America to the southern tip of South America. It will nest and roost
in the abandoned burrow of a prairie dog, fox, badger, or even the burrow of
a gopher tortoise. The distress call of the young owl mimics the rattling
sound of a rattlesnake, and helps to scare off predators. |
The Snowy Owl is circumpolar in
Arctic regions. Young Snowy Owls have heavy black mottling in the plumage.
Owls have large, rounded heads with large eyes that face forward, giving
them good binocular vision. The eyes of large owls, about the size of a
human eye, are fixed in their sockets, so owls must turn their head to look
sideways or behind. Birds have a third eyelid, called a
nictitating membrane, that is transparent so the bird can blink without
shutting out the light. In owls, this membrane is opaque, and helps protect
their sensitive eyes from bright daylight. |
Figure 3 - Snowy Owl (Nyctea scandiaca)
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Figure 4 - Snowy Owl (Nyctea scandiaca)
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The adult Snowy Owl is truly
the ghost of the Arctic. Snowy owls are known for periodic irruptions when
they leave their northern grounds and move farther south than usual. This
southward migration usually coincides with shortages of their favorite foods
of mice and lemmings. These large, powerful owls also prey on ducks, geese,
crows, ptarmigans, small gulls, various seabirds, fish, and other
vertebrates. Scientists examined the stomach of one owl and found that it
contained 17 mice! |
The Short-eared Owl is a
cosmopolitan species that occurs across all of North America, most of
Central and South America, Europe, Asia, and parts of Africa. One race
occurs on Pacific Islands. In Hawaii, they are called Pueo, and were
worshipped and revered by ancient Hawaiians as a god and guardian spirit.
They are often seen hunting during daylight hours. |
Figure 5 - Short-eared Owl (Asio flammeus)
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Figure 6 - Short-eared Owl Face
(Asio flammeus)
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The owl’s facial disk, large
eyes and the hooked beak can be seen in this long telephoto image. The
facial disk of an owl helps to concentrate sound, greatly assisting them to
locate their prey by at night detecting the faint sound of a mouse rustling
in the grass or a lemming running under the snow. Flight and body feathers
are also adapted to allow these birds to fly silently. |
The Short-eared Owl in flight
shows a pale wrist patch on the upper wing and a dark wrist patch on the
under wing.
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Figure 7 - Short-eared Owl in Flight
(Asio flammeus) |
Figure 8 - Short-eared Owl feet (Asio flammeus)
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The business end of an owl is
its strong feet and sharp talons. These are used to grasp, hold and kill its
prey. Note that the feathers extend down the legs to the toes, helping to
keep the legs and feet warm.
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The cryptic color and pattern
of an owl allows it to blend in well with its environment. Owls can sit all
day in just the right place to blend in and not be seen. (Hint: It is
sitting in the tree canopy near the center of this photo). The next photo
zooms in on it.
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Figure 9 - Can you find the owl in this photo? |
Figure 10 - Spotted Owl (Strix occidentalis)
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This is a good example of how
the cryptic color and pattern of the owl breaks up its profile and helps it
to blend in with the light and shadows of its surroundings.
The gentle Spotted Owl is a western species closely related to the Barred
Owl. It has received lots of publicity because its need for old growth
forest brings it into conflict with the logging industry. So
how can you find an owl in your neighborhood?
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A clue to the presence of a
roosting owl are the white splashes and pellets on the base of a tree and
the ground. A well known characteristic of owls is their
habit of regurgitating a pellet after a meal. Opening a pellet is an
interesting exercise, and is a good way to determine the identity of the
prey by the bones contained in the pellet.
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Figure 11 - Owl Droppings |
Figure 12 - Owl Pellets (castings)
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These are the pellets (or
castings) of a Long-eared Owl. They consist of undigested fur, feathers,
bills, claws, teeth, skulls and bones of the prey animals. In
owls, the pellet forms from 6 to 10 hours after the meal is eaten, and is
regurgitated from 10 to 16 hours after the meal. This sounds disgusting, but
studies show that this process is necessary to keep the bird healthy.
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Can you identify the various
bones shown here? These are the remains of a rodent from a Great Horned Owl
pellet. Scientists and amateur naturalists learn a great deal by studying
the remains in a pellet. This is a good method of study as it does not harm
the bird.
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Figure 13 - Remains from an Owl Pellet |
Figure 14 - Rodent Jaw
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This is the mandible, or lower
jaw bone of a rodent. Notice the long incisor teeth on the right, and the
molars at the center of the jaw. By studying the dentition, scientists can
identify the particular species of rodent or mammal.
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These are the femur bones of a
rodent. They look like miniature leg bones of a human. Notice the rounded
end at the right of the lower femur. This is the part that fits into the hip
socket.
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Figure 15 - Rodent leg bones |
Figure 16 - Rodent bones
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Can you identify what part of
the rodent skeleton that these bones came from?
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These are the bones of a bird
that were found in the pellet of a Great Horned Owl. The top bone is the
skull and beak. The middle bone is the jaw bone. The other three bones are
the long bones of the legs and wings. Sometimes scientists find a bird band
in a pellet like this, thus providing important information on the movement
and fate of the banded bird.
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Figure 17 - Can you identify these remains? |
Additional Information
Hawks, eagles, shorebirds, terns, herons, grebes, gulls, rails,
shrikes, warbles, swallows and many other species of birds also regurgitate
pellets. Study of these pellets or castings is a good way to determine the
food preferences of birds. Scientists sometimes discover the presence of a
species that was previously not known to be in an area.
Pellet studies provide information useful to ecologists and
conservationists who try to protect and manage these species. It also
provides scientific data that tells us how important these birds are to man
because they do so much to control rodent populations.
As with
any animal study, if you handle pellets from these birds, be sure to
thoroughly wash your hands and take the necessary precautions to prevent you
from getting any infections from disease organisms that might be contained
on the pellets.
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All images are courtesy of CWBO. All image copyrights are owned by CWBO.
Any use of these images must have permission of CWBO. |
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