FF-01: Owl Pellets
Materials List:
Mounted horn or barn owl or owl study skins
Owl wing and talons(optional to use)
Stuffed pocket gopher
Saran wrapped display of assorted rodent bones
3 covered dioramas
- skulls found in owl pellets
- pocket gopher bones
- owl pellet display
1 bag vocabulary cards
1 bag bone cards (illustrations of parts)
1 picture of owls (in script folder)
1 leaflet on barn owl (in script folder)
1 baggie of large bones for comparison
1 spray bottle
9 magnifying glasses
1 pair scissors
2 plastic tweezers, 7 metal tweezers
1 book "Owls - Whooo Are They?"
****Owl pellets (freeze dried and found in box so marked)
Handouts:
Bone sorting chart/guide
food web
full skeletal diagrams
Vole
Snowy Owl
Barn Owl
Eastern Screech Owl
bones word search
Note: Look for these in binder or
manila envelope!
Renewable supplies
Paper plates
Ziplock bags
Paper towels
Large bag for dumping pellet parts
Xerox copies showing different rodent bones (one for every two students)
Toothpicks
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OWL PELLETS
SUMMER 1999
FF–01
OBJECTIVES:
To understand the owl's relationships within its food web, and to identify and compare bones
found in dissected owl pellets.
SCIENCE THEMES:
Systems and interactions, scale and structure.
PROCESS SKILLS:
Observing, relating, inferring, categorizing
GRADE LEVELS:
Grades 3 – 6
FOCUS WORDS:
Owl, owl pellet, regurgitate, carnivore, predator, prey, digestion, nocturnal, dissection, talon,
crop, gizzard, food web, binocular vision
BACKGROUND INFORMATION:
Most owls are nocturnal birds of prey. They fly silently through the sky, hunting small rodents,
insects and, occasionally, birds. Equipped with excellent hearing and highly developed binocular vision,
they are extremely efficient hunters in low or no light conditions. An owl’s eyes’ retina lack cones hence
they do not perceive color. However, the pupils can be adjusted independently which gives an owl more
flexibility in spotting her prey. An owl's ears are set at different levels on each side of her head, so it can
hear from different distances and heights. This difference in ear alignment enables the owl to pinpoint
her prey accurately in the dark.
It's not true that owls turn their heads 360°, but they are capable of more than 180° turns, which
is helpful in hunting as their eyes are fixed within their bony sockets.
Owl flight is silent. The feathers of most birds are stiff on the edges. Owls have a soft fringe to
cushion their wing beats.
Like all birds of prey, owls have beaks with a sharp hook for tearing flesh and sharp claws to
help the owl grasp his prey and pull apart the meat. When an owl eats, he swallows his prey whole. His
internal organs then sift through the material and digest the meat of the prey. Since bones, scales, teeth,
fur and feathers cannot be digested , the gizzard compacts them into pellets which the owl regurgitates
18-20 hours after having eaten.
Because they burn a lot of calories, owls must eat enormous amounts of food and must process
the equivalent of their own body weight in food. Because of a poor sense of smell, owls are known to eat
skunks.
Owl pellets are masses of bones, teeth, hair, feathers, scales and insect skeletons. They are
produced and regurgitated, not only by owls, but also by hawks, eagles, and other raptors (birds of prey)
which swallow their prey whole or in large pieces. Proteolytic enzymes (protein digesters) and strong
acids that occur in high concentrations in the stomachs of raptors dissolve the soft parts of the prey. The
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relatively weak stomach muscles of the bird form the undigested fur, bones, feathers, etc. into wet, slimy
pellets. In this process, even the most fragile bones are usually preserved unbroken. Because the pyloric
opening of the stomach into the intestine is narrow, only finely divided materials pass into the intestine.
As a result, tiny bones only occasionally pass through the digestive system. Each whole owl pellet
usually contains virtually complete skeletons of the animals which the owl ate the night before the pellet
was formed.
Barn owl pellets are egg-shaped masses averaging 47 mm in length by 33 mm in width. Their dry
weight is about 52 percent fur and 48 percent bone, each pellet contains an average of 3.3 animals.
From these data, it is estimated that an adult barn owl requires 80 to 100 grams of prey daily. The
kinds of prey species in each pellet varies, but a sample of 135 barn owl pellets in the Pacific Northwest
contained the following averages: 2.5 voles (Microtus), 0.7 shrews (Sorex), and 0.1 of the other species.
Major factors determining the kinds and ratios of species found in owl pellets are where the owl lives
and what prey are seasonally most abundant.
Owl pellets themselves are ecosystems, providing food and shelter for communities, which may
include clothes moths, carpet beetles and fungi. Clothes moth larvae are frequently abundant in pellets,
feeding on fur and feathers. The black spheres about the size of periods (.) that are found in the pellets
are the droppings of the caterpillars (larvae). The larvae metamorphose near the surface of a pellet in
cocoons made of the fur.
(The following is a transcript of the presentation by Bill Kirsher, on Sept. 18, 1986, at EV Fall 86
Training.)
"Let's talk about owls a little bit. All birds are wonderful animals, but owls have certain attributes
that make them particularly interesting. They fly at night. In the dark, they catch their prey. They are
carnivores. You know they're carnivores; you can tell by the talons that they have and their beaks. That's
how you identify birds, by their feet, their claws, and their bill. And it's pretty obvious when you look at
these birds. (show stuffed owl – either barn, and great horned).
"We have about seven kinds of owls in the area. Only two of them are fairly common. One is the
barn owl and the other is the horned owl. The horned owl is a big majestic looking owl. They are fierce
looking and really beautiful when you see them out in the open.
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BACKGROUND INFORMATION:(continued)
"Owls have certain physiological characteristics which set them apart from other birds. These
birds have their eyes on the front of the head. They have stereoscopic vision, whereas other birds have
the eyes on the side, and they must cock their heads so that they can get a good look at it out of one eye.
Owls look straight ahead and when they want to see something on the side, they turn their whole head
around.
"Owls eat a variety of things. You might be interested to know that the primary prey of the
horned owl is the skunk. That might suggest to you that they don't have a sense of smell. They probably
eat more rodents than anything else does. They eat a variety of rodentsand birds. The owls swallow their
prey whole, except in the case when they eat other birds, they always tear off the wings since there is not
much nourishment in the wings.
"The major item of prey for the owls in this area are pocket gophers. (Show a pocket gopher) Why is this
called a pocket gopher? - because it fits in a pocket! Not really, but if you get down and examine it, you
will see on the sides of the cheeks, there are little pockets with which they can carry food. (Pass around
a stuffed gopher or other rodent)
"The pocket gopher has an average of six young, three times a year. They reach maturity in about
six weeks. If you include the young that breed that year and the original adults, a gopher population has
the potential to expand to several hundred in less than one year. (This could be a great math lesson; how
many gophers total in one year, from one breeding pair, if all of the offspring live and they all reach
maturity at six weeks.?)
So, if something didn't happen in a very short time, we would up to our ankles in pocket
gophers. In a couple more generations, you would be smothered in jumping, squealing pocket gophers.
But that's not going to happen. Why? Well, to being with, when they get to a certain point they would
begin to eat all the food in their food supply. That is one way to control the population. Other controls
are owls, coyotes, foxes, snakes and hawks. In an examination of over 300 owl pellets from this area, 1/3
to 1/2 had pocket gophers in them
"When the owl catches his pocket gopher, he swallows it whole. The animals go down into the
gizzard of the owl where it is digested. It doesn't go to the stomach. The gastric juices come up from
below, and the soft tissues of this animal are dissolved away. These are powerful juices. It takes about
eight hours for a pellet to form. The indigestible remains, the skull, the fur, and the bones form the
pellet. That's all done in the gizzard. After about 8 hours, the owl regurgitates the pellet – the
indigestible remains that are all wrapped up neatly. Regurgitation is something like throwing up, but it’s
a neat, clean little pellet.
"After the owl finishes feeding, it goes off to sit passively while the pellet is forming. They are
reluctant to fly while the pellet is in them. If the do fly, or are scared, sometimes they will regurgitate
prematurely. It takes about eight hours for the digestion to be complete. They will regurgitate just prior
to leaving for their night feeding."
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ACTIVITY:
• Prior to doing the activity, talk about some characteristics about owls as described in the Background
Information. Also, pass around the wing, talon and mounted (or study skin) of an owl. Ask the
students if they can explain how an owl's beak and feet are designed for hunting. Have them inspect
the wings and think how these are adapted for hunting.
•
Ask the students to cover one eye, to simulate how most birds see, ask them how owl's vision is like
ours. Ask them if they know how it is different.
•
Hold up a box with intact owl pellets – Can one of you guess what kind of animal made this?
•
Describe how an owl makes a pellet: A photo of an owl is a good visual; a diagram of the owl's
gizzard and stomach to show where the pellet comes from is also helpful.
•
When an owl eats, he swallows his prey whole or in large chunks.
•
The owl's inner organs sift, sort and absorb the nourishment from the food.
•
The owl has an inner "garbage compactor" which mashes the indigestible parts into a pellet which
he spits out.
•
•
Show a previously dissected pellet. Sort the contents into any type of grouping you choose. Describe
the owl's diet by observing the pellet contents. Estimate the number of animals that made up the
owl's meal.
Have a helper pass out the paper plates, tweezers, toothpicks, and magnifying glasses.
•
Pass out the pellets. Each learning station should have 8 - 10 students. Two students receive one
pellet each.
•
Pass out mounted bones that have been found in a pellet to show students what to look for.
•
Pass out small plastic "bone" bags , so students can keep the bones they find.
It is important to impart the necessary information prior to passing out the pellets. Once the dissection
starts, general conversation and discussion is impossible. The children become very interested in the
pellets and what they find.
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STUDENT DATA SHEET (optional to leave with teacher)
1.
Number of skulls (or pairs of jawbones) found in your owl pellet:
2.
Species and number of prey mammals found in your pellet:
Number of Different Species
Number of Individuals
3. The Class Record:
Total number of prey animals found:
Total number of pellets examined:
Average number of prey animals per pellet:
Prey Animals
Numbers
Bird
Rat (Rattus sp)
Vole (Microtus sp.)
Deer mouse (Peromyscus maniculatus)
House mouse (Mus musculus)
Shrew (Sorex sp.)
Mole (Scapanus sp.)
4. If a barn owl produces one pellet each day, estimate how much food the owl would eat in a year.
One class study suggested the following annual consumption for one owl: 906 voles, 259 shrews, 8
moles, 5 deer mice, 5 rats, 5 jumping mice and 2 birds.
5. The skeletons in these owl pellets provide information about the small animal populations found in
owl habitats in Northwestern Washington. But how accurately do you think information gained from
pellet analysis reflects the kind and abundance of ALL the small animal populations in the owls'
habitat? (Hint: Think about where the owls feed.)
Since barn owls feed during the late evening and nights in open fields, animals that are active during
the daytime and animals that live in woodlands will not appear in significant numbers in owl pellets.
6. Environmentalists are concerned about the accumulation of certain poisons in the body systems of
predators. Why do you think owls, eagles, swordfish, lions and humans are often the most threatened
organisms of a community that is exposed to DDT or mercury? (Both DDT and mercury are stored
and not broken down in animals.)
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Being at the highest trophic levels in food webs, these animals will concentrate poisons that
accumulate in tissues of their prey.
7. Make a diagram of a food web in which the Northwestern Washington barn owl is positioned at the
highest trophic level. Make your diagrams as comprehensive as possible. Use dotted lines to indicate
uncertain relationships. If you have problems, refer to the owl food web poster.
8. Reexamine the bones that have been separated from fur, feathers, and other debris and grouped
according to type. These bones may be laid out corresponding to their position in the natural
skeleton. The poster picture of the skeleton of the vole will help in classifying, positioning and
naming the bones. Bones of a skeleton may be laid out flat on a piece of cardboard (in this position
the bones can be glued to the cardboard and hung as a picture),
Or the bones of an animal may be glued together to form a freestanding skeleton.
9. Make a collection of drawings or specimens of organisms found living in unpreserved owl pellets.
Most visible are the various stages in the life cycle of clothes moths, carpet beetles and fungi.
10.
Find out how many kinds of owls live in your area. Compare the way they live with what you know
about the barn owl that produced the pellet you studied.
REFERENCES
Duke, G.E., Jegers, A.A., Loff, G., and Evanson, O.A. (1975) Gastric digestion in some raptors. Comp.
Bioche. Physiol. 50A, 649-656.
Doerksen, George Peter. (1969) An analysis of barn owl pellets from Pitt Meadows, British Columbia.
The Murrelet. 50(1), 4-8.
Farner, D.S. (1960) Digestion and the digestive system. In Biology and Comparative Physiology of
Birds, Vol. 1. (Edited by Marshall, A.J.) Academic Press, New York.
Glading, B.D., Tillotson, D.F., and Sellect, D.M. (1943) Raptor pellets as indicators of food habits.
Calif. Fish & Game. 29,92-121.
Kostuch, T.E., and Duke, G.E. (1974) Gastric motility in great horned owls. Comp. Biochem. Physiol.
51A. 201-205.
Reed, C.I., and Reed, B.P. (1928) The mechanism of pellet formation in the great horned owl (Bubo
virginianus). Science Wash. 68. 359-360.
KEY TO THE MAMMAL SKULLS FOUND IN OWL PELLETS COLLECTED IN WESTERN
WASHINGTON
Clyde M. Senger, Western Washington University, October 1980.
1. a. Teeth or tooth sockets present; skull without prominent ridge of bone forming an eye socket;
skull of relatively thick bone (fig.1)--Mammals, Class Mammalia--go to 2
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b. Lacking teeth and tooth sockets; skull with prominent ridge of bone forming a complete eye
socket; skull of relatively thin bone--Birds, Class Aves
2. a. A wide gap (diastema) between one or two front teeth (incisors) on each side and the chewing
teeth (cheek teeth - premolars and/or molars); 1st incisor in each jaw large, evenly curved and
evergrowing (Fig.1,2)--go to 3
b. Tooth row more or less continuous (Fig.7,8)--Shrews and Moles, Order Insectivora--go to 8
3. a. With only one incisor on each side, its front surface usually distinctly colored (tan, orange, or
reddish) tip sometimes stained dark grayish or blackish without vertical groove in front (outer)
surface (in some areas rodents do have grooved upper incisors); lower jaw with conspicuous
coronoid process--Mice and Rats, Order Rodentia--go to 4
b. With two upper incisors on each side, the second much smaller than the first, no special
coloration on front surface of incisors, first upper incisor with a vertical groove; no conspicuous
coronoid process on lower jaw; intact skull over 40 mm in length and rarely encountered-Rabbits, Hares and Pikas, Order Lagomorpha, probably Eastern Cottontail, Sylvilagus floridanus
(Fig 2)
4. a. Skull total length less than 32 mm; rows of cheek teeth separated in front less than 3 mm; lower
jaw with either row of cheek teeth less than 4 mm in length or teeth flat crowned--Mice--go to 5
b. Skull over 32 mm in length; cheek teeth rows separated by over 3.5 mm; lower jaw teeth row
length over 6 mm and teeth with cusps or worn to a dished out shape--Rats, Genus Rattus (R.
rattus or R. norvegicus)
5. a. Cheek teeth rows over 5 mm in length, chewing surface worn smooth and flat to give a pattern of
loops and triangles (most common type of skull) Fig. 1--Field Mice or Voles, Genus Microtus-go to 6
b. Cheek teeth rows less than 4 mm in length; teeth with cones or cusps on chewing surface or in
older animals the surface worn to a smooth but not flat surface and without a pattern of loops and
triangles--go to 7
6. a. Skull over 24 mm in length; rows of cheek teeth 5-7 mm long; last upper teeth with 4 vertical
ridges on inside (Fig. 3). (These are the inner points of the first loop, two triangles and the inner
projection of the terminal loop and may not be well developed in young mice where the initial
enamel surface of the teeth have not yet been worn away); most commonly encountered kind of
Townsend Meadow Vole, Microtus townsendi
b. Skull under 24 mm; rows of cheek teeth 4-5 mm long; last upper teeth with only 3 vertical ridges
on inside (Fig. 4) (terminal loop does not have a pointed inner projection--Creeping Meadow
Vole, Microtus oregoni
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7. a. Each first upper cheek tooth with a main cusp followed by a groove with two cusps on each side
of it; hard palate terminating about the level of the last cheek teeth; incisive foramina
terminating at about the level of the first cheek teeth (Fig. 5); for lower jaw, combined length of
last two cheek teeth greater than length of first cheek tooth, length of cheek teeth row 3.25-3.75
mm--White-footed or Deer Mouse, Peromyscus maniculatus
b. Each first upper cheek tooth with main cusp followed by two more central cusps each with a
smaller cusp on each side; hard palate terminating well behind the last teeth; incisive foramina
terminating well behind front of first cheek teeth (Fig. 6); lower jaw with combined length of last
two cheek teeth less than length of first, length of cheek teeth row 3-3.25 mm--House Mouse,
Mus musculus
8. a. Tips of teeth reddish; upper and lower first incisors elongated and projecting well ahead of bone
and with additional points behind main point; zygomatic arch not present (Fig. 7)--Shrews,
Family Soricidae--go to 9
b. Teeth all white or with grayish stain; first incisor with a single, broad rounded cusp; zygomatic
arch present or its former presence indicated by broken stubs (Fig. 8)--Moles, Family Talpidae-go to 10
9. a. Length of skull about 15 mm; length of each tooth row about 7 mm; length at row of tooth
sockets in lower jaw 4-5 mm--probably Wandering Shrew, Sorex vagrans
b. Length of skull about 22 mm; length of teeth rows about 10 mm: length at row of tooth sockets
in lower jaw 5-6 mm--Water Shrew, probably Bendire Shrew, Sorex bendiri
10. a. Length of skull over 22 mm; length of rows of teeth about 10 mm--Shrew Mole, Neurotrichus
gibbsi
b. Length of skull over 25 mm; length of rows of teeth over 12 mm--go to 11
11. a. Length of skull over 40 mm; length of rows of teeth about 18 mm--Townsend Mile, Scapanus
townsendi
b. Length of skull less than 40 mm, about 35 mm; length of rows of teeth about 15 mm--Coast
Moles, Scapanus orarius
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