Energy Flow Through a Food Chain: Barn Owl Pellets
LABORATORY PURPOSE
How energy moves through living organisms is best understood by analyzing food chains. Studying
the energy laws will help a student understand why there are so few predators in comparison to the
number of prey.
1. A barn owl food chain will be examined by dissecting a regurgitated barn owl pellet and identifying the prey species and number.
2. The student will successfully use mathematical calculations to illustrate the Second Law of Thermodynamics, energy changes and entropy increases in a barn owl’s food chain.
3. The student will attain skill in using a dichotomous key to identify the bones of barn owl prey.
Materials needed
1. One owl pellet per student from Pellets Inc., Bellingham, Washington, 2. Tweezers, 3. Latex
gloves, 4. Dust masks, 5. Dissection trays, 6. 15 cm rulers, 7. Hand magnifying lenses, 8. Bone indentification chart.
INTRODUCTION
The flow of energy through an ecosystem can be studied by assessing the energy requirements of a
top predator in a food chain. The barn owl, Tyto alba, like all birds of prey regurgitates a pellet containing the indigestible bones, fur, and feathers of its prey. Because birds of prey can regurgitate the
indigestible parts of their prey they can get rid of unnecessary weight quickly. Since birds fly, anything they can do to reduce their weight is advantageous. The pellet is a compact mass of the waste
bones and fur. Barn owls form this pellet about six hours after feeding, but do not usually regurgitate
it until 11 or 12 hours have elapsed. Seeing fresh prey will often stimulate an owl to regurgitate the
pellet in preparation for a new meal. Many other birds, from curlews and bitterns to snipe and terns
also form pellets. Tern pellets can be analyzed for specific “otoliths”, the ear bones found in fish, to
determine the species of fish eaten. Regurgitating compacted waste pellets is efficient. There is less
need to carry waste mass in the bird’s digestive systems for long periods. Fur and the larger indigestible bones are quickly and easily expelled. Since barn owls commonly live in barns, these regurgitated pellets are not destroyed by rain, and are available for analysis long after they are deposited.
Figure 2a. (Center) A long-billed Figure 2b. (Right) A raven pelFigure 1. (Left) A
burrowing owl regurgitating curlew regurgitating a pellet filled let contains rodent and bird bones
along with blue robin’s egg shells.
with crab and small clam shells.
a pellet.
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Figure 3. Bird skull found in a barn
owl pellet.
Figure 4. Skulls from owl pellets. From left to right, pocket
gopher, rat, Townsend vole, deer mouse, and least shrew.
Owl pellets contain rodent skulls which can be identified using a taxonomic key. The key chosen
will depend on the area of the country the owl pellet was obtained. For this lab, pellets from Western
Washington state will be examined. The mammal identification key is for small animals commonly
found in Washington state barn owl pellets. Appropriate keys must be obtained for pellets from other
parts of the country.
A good estimate of the caloric needs of an owl may be obtained by identifying the number and kinds
of organisms eaten and knowing the caloric content of each prey animal. In a typical food chain,
no more than 10 percent of high quality energy is transferred from one trophic level to another. The
remainder is waste heat. This rough “10 percent rule” is in accordance with the Second Law of Thermodynamics, which states there is an increase in entropy, or disorder, when energy is transformed.
Actual food chain energy measurements show the percentage of energy transferred ranges from one
to ten percent. The energy is conserved, not destroyed, according to the First Law of Thermodynamics, but a tremendous amount of low temperature waste heat is produced, which radiates out into
space. A food calorie is a kilocalorie. The total kilocalorie (kcal) content in a small mammal varies
with time of year and is primarily due to differences in the amount of fat tissue. It is usually highest
in the summer at 1.7 kcal/gram, and drops to a low of 1.3 kcal/gram in winter. For this lab, an average value of 1.5 kcal/gram will be used as the calorie content for small mammals.
100 tree, 900 waste heat 10 rat, 90 waste heat
1 cat, 9 waste heat
Total, 999
units waste
heat to space.
1000 units
sun energy
Figure 5. Example of the Second Law of Thermodynamics. For every 1,000 sun energy units sent
to plants, the producers, 90 percent is converted to waste heat during photosynthesis and other
metabolic activities. This waste heat radiates out into space. The plant keeps and stores 100 energy
units. Consumers, including the Kahalari rat (center), which eat the vegetation will convert 10 heat
units to rat stored energy and 90 heat units to metabolic waste heat. The African wild cat (right) eats
the rat and converts 9 rat energy units to waste metabolic heat, which radiates out into space. That
leaves only 1 sun energy unit stored in the form of cat tissue. In summation, of the 1,000 units of sun
energy, 999 units degraded to heat in only three steps of this food chain.
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PROCEDURE
1. Obtain an owl pellet and place it in a dissecting tray. Using your fingers, gently and slowly ‘crack’
the pellet apart, taking care not to crush any skull parts. Using tweezers and a dissecting needle,
separate the hair from the bones. In a separate pile, place other material such as chitinous insect parts
or feathers. Try to identify the feathers as being either owl feathers, which might be on the outside of
the pellet, or prey birds which may be in the middle of the pellet. (Figure 6.)
2. After carefully cleaning the bones, separate the skulls from the other bones. Using the key provided on the following pages for Prey Species in Owl Pellets of Western Washington, identify all the
animals by their skulls.
3. Record the data on the data page. If parts of animals such as the larger rats or rabbits are found,
record the approximate percentage of those animals, based on the percentage of bones present. If
feathers other than owl feathers are found, record this information.
4. Record the class’ cumulative data and calculate the average number of calories obtained by an
owl in one night foraging, assuming the owl produces two pellet every night.
5. Rearticulate one of the prey by choosing bones that seem to be from one individual animal. To
identify individual bones, use the skeletal illustration for the Townsend Vole (Microtus townsendii), the most common rodent found in the pellets from Western Washington. Glue these to a piece
of heavy cardboard or thin plywood (See Figure 28). If a shrew or mouse is found, then carefully
rearticulate its skeleton.
Figure 6. (Above) Barn owl pellet. The
feather on the outside of the pellet is an owl
feather. (Below) Crack open the pellet carefully to avoid crushing bones.
Figure 7. (Above) Barn owl, Tyto alba.
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DATA
ANIMALS
COMMONLY
FOUND
IN WASHINGTON
OWL PELLETS
Townsend vole
Microtus townsendii
Creeping vole
Microtus oregoni
Deer mouse
NUMBER IN
YOUR
OWL
PELLET
TOTAL Mass in
NUMgrams
BER
IN
CLASS
47-83 ( 65
grams ave.)
17-20
House mouse
Wandering shrew
Sorex vagrans
Pocket gopher
Thomomys talpoides
Shrew mole
Townsend mole
Coast mole
Rat, Norway or black
Birds
10-30 (20
ave.)
17-25 (21
ave.)
7
60-160 (110
ave.)
10
138
62
300
Varies- A
meadowlark is 113
grams
Kilocalories/
gram
1.5
KILOCALORIES PER
ANIMAL
(1kcal-1 food
cal)
98
1.5
28
1.5
30
1.5
32
1.3
10
1.5
165
1.5
1.5
1.5
1.5
1.8
15
207
93
450
203
TOTAL
KILOCALORIES
Other
Sum total number of calories in classes owl pellets =
Total number of owl pellets analyzed by the class =
CALCULATIONS AND QUESTIONS
1. Calculate the number of calories one barn owl needs per 24 hours.
Average number of kilocalories per pellet
= Sum of all kilocalories / total number of pellets analyzed =
Assume two pellets are produced daily by each owl (The average is actually 1.8 pellets/
day). The total number of kilocalories consumed by the owl = (average number of calories
per pellet) × 2 =
Total number of calories = kilocalories × 1000
2. Assume that only 10% of the energy is transferred from the grass and seeds the prey animals eat,
and only 10% of the energy stored in the prey animals goes to the owl. How many kilocalories of
grass and seeds does it take to feed the prey species per day, and how many kilocalories of prey species is needed to feed the owl?
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Example calculation: Suppose your class averaged 165 kcalories/pellet, so 330 kcalories is needed
by a barn owl every 24 hours.
10% × (#calories of seeds consumed by the prey animals) = # calories in barn owl prey/day
0.1 × (Energy Consumed by Rodents) = EnergyConsumed by owl
0.1 × (ERodents) = EOwl
Example: (ERodents) = EOwl/0.1 = 330kcal/0.1= 3300 kcalories/day
Your calculations of (ERodents) = EOwl/0.1
3. Calculate the calories of plant material needed to produce the calories of prey animals in an average pellet, assuming a 10% efficiency and using the equation:
0.1 × (Eplants) = Erodents
EPlants = Erodents /0.1
Example: EPlants = Erodents /0.1= 3300 kcalories/day/0.1 = 33,000 kcalories/day
Your calculations of EPlants = Erodents /0.1
4. Calculate the calories of sunlight needed to produce the plants. Assume that plants are not very
efficient in converting sunlight into stored chemical potential energy, such as cellulose and sugars.
This efficiency varies from 1.5% for most crop plants to sugarcane’s 8%, which is exceptionally
high. Assume a 1.5% efficiency for converting sunlight into plant material energy, the grass and seed
energy that is consumed by the rodents:
EPlants =0.015 × (ESunlight)
(ESunlight) = EPlants/0.015
Example: (ESunlight) = 33,000 kcalories/day / 0.015 = 2,200,000 kcalories/day
Your calculations of (ESunlight) = EPlants/0.015
5. Assume the amount of sunlight falling on western Washington state where the owl pellets were
collected is 5 kilowatt-hours/(square meter-day) = 4,300,000 calories/(square meter-day)= 4,300 kilocalories/(square meter-day). How many square meters of land must be dedicated to grow the grass
and seeds to feed the mice to feed the owl?
Example calculation: Using the numbers generated by assuming 165 kcal/pellet, or 330 kcal/day
(ESunlight) = 2,200,000 Kcal/day
Area = .ESunlight/ Insolation = (2,200,000 kcal/day)/(4,300 kcal/(square meter-day))
= 512. square meters,
So taking the square root: √512 = a square about 23 meters on each side is needed to feed the owl.
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Substitute your values:
Area = ESunlight/ Insolation = ESunlight/(4,300 kcal/(square meter-day)) =
6. Sketch a hypothetical food chain, with the barn owl at the top and its prey species below. Complete this food web by hypothesizing about possible sources of food available to the prey species.
7. Discuss the difference in the stability of highly webbed food chains and linear food chains. Give
examples of each. Do you suppose the western Washington barn owl food chain is webbed or linear?
8. The top predator in a food chain, whether it is an owl, a human, or a bald eagle, often has a higher
concentration of biologically magnifiable pesticides or carcinogens in their bodies, such as DDT or
polychlorinated biphenyls, than the ambient concentrations existing in the air or water.
a. Explain the mechanism of pesticide biomagnification. Use DDT as an example.
b. If you were to analyze (perform a necropsy on) a barn owl’s flesh, where would you expect to find
the highest concentrations of such non-polar organic compounds, such as DDT and PCB’s?
9. Why are food chains short? Explain why there is no super predator that specializes in eating barn
owls.
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10. Define the following terms:
a. Calorie b. Kilocalorie -
c. Trophic position -
d. Herbivore -
e. Insectivore -
f. Lagomorph -
g. Diastema -
h. Webbed food chain -
i. Biomagnification -
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KEY TO ANIMALS OF WESTERN WASHINGTON STATE OWL PELLETS ©
(Used with the permission of Ms. Kim Gaussoin, Pellets Inc., http://www.pelletsinc.com, Bellingham, Washington. Modified from Clyde M. Senger, Western Washington University, October 1980)
For additional skull images refer to the University of Texas Digital Morphology website, http://www.
digimorph.org/, where skull slices are shown in a movie format.
1.
A. Teeth or tooth sockets present; skull without prominent ridge of bone forming an eye socket: skull of relatively thick bone ............Mammals, Class Mammalia.....................Go to 2
B. Lacking teeth and tooth sockets; skull with prominent ridge of bone forming a complete eye socket; skull of 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 ever growing (Figure 8)................................................................................Go to 3
B. Tooth row continuous (Figures 25 - 27) Shrews and Moles, Order Insectivora
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 5
B. With two upper incisors on each side, the 2nd much smaller than the 1st, no special
coloration on front surface of incisors, 1st 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 .............................Go to 4
Go to 10
4.
A. Skull over 45 mm in length................ Eastern Cottontail, Sylvilagus floridanus (Figure 13)
5.
6. A. 3 cheek teeth...............................................................................................................Go to 6
B. 4 cheek teeth, 1st cheek tooth the largest; two loops with a small central connection.
.......................(Figures 15-18)...........................Northern Pocket Gopher, Thomomys talpoides
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. (Figure 14)
......................................................................... Rats, Genus Rattus (R. rattus or R. norvegicus)
7.
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) field mice or voles......
Voles, Genus Microtus......................................................................................................Go to 8
B. Skull 40 - 42mm ......................................... Pika, probably Ochotona princeps (Figure 29)
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 7
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8.
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 9
A. Skull over 24 mm in length; rows of cheek teeth 5-7 mm long; last upper teeth with 4
vertical ridges on inside. (Figures 9, 10 and 11) 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 top surface of the teeth have not yet been worn away), most
commonly encountered kind of vole. ...............Townsend Meadow Vole, Microtus townsendii
B. Skull under 24 mm; rows of cheek teeth 4-5 mm long; last upper teeth with only 3
vertical ridges on inside (Figure 12) (terminal loop does not have a pointed inner projection.
..................................................................................Creeping Meadow Vole, Microtus oregoni
9.
A. Each 1st upper cheek tooth with a main cusp followed by a grove 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 1st cheek teeth (Figures 19, 20); for lower jaw,
combined length of last two cheek teeth greater than length of 1st cheek tooth, length
of cheek teeth row 3.25-3.75mm..............White-footed Deer Mouse, Peromyscus maniculatus
..
10.
B. Each 1st 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 1st cheek teeth (Figure 21); lower
jaw with combined length of last two cheek teeth less than length of 1st, length of cheek
teeth row 3-3.25mm......................................................................House Mouse, Mus musculus
A. Tips of teeth reddish; upper and lower 1st incisors elongated and projecting well ahead of
bone and with additional points behind main point; zygomatic arch not present (Figures 24
and 25)...............................Shrews, Family Soricidae ..................................................Go to 11
B. Teeth all white or with grayish stain; 1st incisor with a single, broad rounded cusp;
zygomatic arch present or its former presence indicated by broken stubs (Figure 22)
...........................................Moles, Family Talpidae......................................................Go to 12
11.
A. Length of skull about 15 mm; length of each tooth row about 7 mm; length of rows of
tooth socket in lower jaw 4 -5 mm......
..................................................(Figure 25, 26, 27) probably Wandering Shrew, Sorex vagrans
B. Length of skull about 22 mm; length of teeth rows about 10 mm; length of row of tooth
sockets in lower jaw 5-6 mm..................Water Shrew, probably Bendire Shrew, Sorex bendiri
12.
A. Length of skull abut 22 mm; length of rows of teeth about 10 mm .................
...............................................................................Figure 24, Shrew Mole, Neurotrichus gibbsi
B. Length of skull over 25 mm; length of rows of teeth over 12 mm ..........................Go to 13
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13.
A. Length of skull less than 40 mm, about 35 mm; length of rows of teeth about 15 mm
.................................................................................................. Coast Moles, Scapanus orarius
B. Length of skull over 40 mm; length of rows of teeth about 18 mm........................................
........................................................................................Townsend Mole, Scapanus townsendii
C. Length of skull over 40 mm, jaw length 20 mm, nterorbital width 8.9 mm .......................................... (Figures 22 and 23) probably short tailed weasel, Mustela erminea
Figure 8. (Left) Townsend vole
skull drawings. The gap between
the molars and incisors (d) is the
diastema. All black and white
drawings are part of the key by
Clyde M. Senger courtesy of
Pellets Inc., Bellingham, Washington.
Figure 9. Townsend vole skull. All mice and
vole skulls are less than 32 mm long.
Figure 10. Townsend vole palate. Voles have
chewing teeth with worn, smooth, flat surfaces that
form a pattern of loops and triangles.
Figure 11. The Townsend vole last 2 upper
teeth have 4 vertical ridges on the inside.
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Figure 12. The Townsend
vole on the left has four
vertical ridges on the
inside, while the creeping
meadow vole on the right
has three ridges.
Figure 13. (Left)
Cottontail rabbit skull.
Figure 14. (Below) Rat
skulls are over 32 mm
long, but have only three
teeth on each side.
Figure 15. (Above) Pocket gopher palate with
four teeth on each side. The first tooth on each
side is composed of two connected loops.
Figure 16. Pocket gophers skulls are over 32
mm.
Figure 17. Pocket gopher right jaw.
Figure 18. The pocket gopher’s lower jaw, with
the first tooth pulled up to illustrate that is is
composed of two connected loops.
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Figure 20. Close up of first premolar of whitefooted deer mouse skull.
Figure 19. The white footed deer mouse. Incisive foramina terminating at about the level of
the 1st cheek teeth where the red line is.
Figure 22. Weasel skulls are rarely found in
owl pellets. Weasels are in the order Carnivora, and have teeth that are used to shear
meat and crush bone. The teeth are continuous with no diastema. The front teeth are
missing on this skull.
Figure 21. House mouse. Incisive foramina
terminating well behind front of 1st cheek
teeth. The first tooth length, L1, is greater than
the last two teeth, L2,3.
Figure 23. (Right) The last upper premolar
and the first lower molar are modified in all
carnivores, including weasels. These teeth
are called carnassials and have evolved to
slice and chew flesh, operating like a pair of
scissors.
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Figure 24. Shrew mole skull.
Figure 25. Shrews are insectivores. They consume amounts equal to their own weight each day.
Figure 26. Lower
jaw of a wandering
shrew. Note the continuous red-tipped
teeth. The jaw is on
a dime to give an
idea of its extremely
small size.
Figure 27. Wandering shrew jaw and skull. The reddish tipped teeth are distinctive of shrews.
North American short-tailed shrew, Blarina brevicauda, secretes venom from salivary glands in its
lower jaw to paralyze prey as large as mice twice its own size. Chemists have isolated soricidin
from the saliva. It exhibits paralytic properties, making it potentially useful in treating migraines,
myofacial pain, neuromuscular diseases, and even wrinkles.
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Figure 28. Townsend vole bones rearticulated.
Figure 29. Pikas are lagomorphs, as are cottontail rabbits, with two upper incisors on each side. \
Phpikas very rarely show up in owl pellets. They prefer alpine habitat, and may be placed on the
endangered species list soon because global warming has heated their alpine home, shrinking the territory available to them. This little rabbit has short ears since long ears would freeze at mountaintop
altitude, and has extremely dense fur. Overheating is a problem for the pika in the summer.
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