Lab 2-2: Owl Pellets, Food Webs, Pyramids, and Bio-magnification
The barred owl inhabits woodlands and swamps in the Eastern part of the United States. This owl
is approximately 43-50 cm (17-20 in) long with a wingspan of 99-110 cm (39-43 in) and has a mass of
about 0.8 kg. Owls are nocturnal raptors that feed on small mammals, birds and reptiles. Owls swallow
their food whole or if too big, may tear it into chunks. Owls are not able to digest hair, bones or feathers.
The owl digestive system has a specialized section that presses the undigested portions together forming a
pellet. Since the owls cannot pass the pellet through their digestive system, they must spit out the pellet.
Scientists use pellets to investigate the diet of raptors such as owls. They can determine what the
raptor ate, how healthy it is, how large it may be, if there are diseases present, and many other things.
Food chains show how each organism gets the food it needs, the energy it uses, and how those nutrients
are passed from organism to organism. Many food chains begin with the sun, but every organism needs
some source of energy. Because everything is connected in a food chain, if an organism on the bottom
level consumes any chemicals or contracts a certain disease, the organisms in the upper levels often face
serious issues. This becomes a major problem when pesticides are thrown into the mix. As we go up the
links on the food chain (trophic levels), the concentration of the pesticide consumed becomes greater. So
animals at the top of a food chain would be at the highest risk for pesticide ingestion.
Let’s look at a simple food chain: Flowers  Insects  Mice  Hawk. The flowers may absorb
some pesticide from the soil and the insect eats the flowers. The pesticide may or may not kill the insect,
but remains in the insect’s body. Dead or alive, the insect is eaten by a mouse who then has a greater
concentration of the pesticide in its digestive system. The only effects may be a simple stomachache or
intestinal problems, but the mouse survives. It may then be eaten by a hawk. Because the concentration of
the pesticide has gotten larger, the hawk may get very sick and die. If the hawk doesn’t die, that pesticide
still remains in its system and can be passed on to chicks or influence the health of any offspring. When
Dichlorodiphenyltrichloroethane (DDT) was used on crops, scientists found that it was entering the food
chain of Bald Eagles. Some of those scientists concluded that the DDT was thinning the egg shells of the
reproducing eagles, causing them to be crushed when the parents attempted to incubate them. Hence, the
use of DDT was banned in the United States.
Materials: owl pellets, dissecting tools, identification keys, magnifying glass, glue, paper
Procedure:
1.
Obtain one owl pellet per group. ALL students are to wear gloves (if
you are allergic to latex, use other gloves) and goggles.
2.
Use dissecting tools and fingers to gently pry apart the pellet. NOTE:
The bones you are looking for are small and easily broken.
3.
Set aside any bones found.
4.
The ‘detective(s)’ will analyze the bones with the identification key
and identify prey. Pay attention to:
a.
Shape and size of the skull
b.
Shape of eye sockets
c.
Length of the snout compared to the rest of the skull
5.
Repeat until you have completely dismantled the pellet.
6.
Try to assemble an animal skeleton on a sheet of paper using glue.
Label the animal skeletons. Also glue any incomplete skeletons to the
paper and label them according to type of prey. (This step is optional – only do this after you
finish all calculations in the next section.)
7.
Put all hair, feathers and waste in garbage (not down the drain!) and wash off dissecting tray.
8.
Remember to wash hands at the end of the procedure. These pellets have been sterilized, but…
Data and Calculations:
Owl Pellet Dissection
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A. Complete Chart #1 below – Please note that a vole is ‘equivalent’ to a mouse
Chart #1. Number of prey eaten by a barred owl per time interval.
A
B
C
D
Length
Number
Number eaten
Number eaten
Prey
(cm)
Found in
per day
per week
1 pellet
(2.5 pellets/day)
(7 days/wk)
Class data
B x 2.5 = C
Cx7=D
6.5 – 9.5
Mouse
12 – 14
Mole
8
Shrew
40
Rat
15
Bird
Total
E
Number eaten
per year
(52 weeks/yr)
D x 52 = E
B. Create and label a Pyramid of Numbers for the owl based on the number eaten per year with one
owl on the top with total annual number of all prey below. *Use the assumption that an owl
produces an average of 2.5 pellets per day. Draw your pyramid of numbers on separate paper.
C. Use the following facts from Chart #2 about the prey to draw a complete food web that is
representative of the pellet your group dissected. Draw your food web on separate paper.
Chart #2. The diets of various barred owl prey.
Prey
Diet
It eats a wide variety of plant and animal matter depending on what is available, including
Mouse insects and other invertebrates, seeds, fruits, flowers, nuts, and other plant products. Deer
mice sometimes eat their own feces (coprophagy).
A mole's diet is mostly insects and other invertebrates, including earthworms, centipedes,
Mole
millipedes, snails, slugs, grubs, ants, sowbugs, termites, beetles, and crickets
Food habit studies have revealed that shrews eat beetles, grasshoppers, butterfly and moth
larvae, ichneumonid wasps, crickets, spiders, snails, earthworms, slugs, centipedes, and
Shrew millipedes. Shrews also eat small birds, mice, small snakes, and even other shrews when the
opportunity presents itself. Seeds, roots, and other vegetable matter are also eaten by some
species of shrews.
The rat's diet typically includes seeds, nuts, grains, vegetables, fruits, meats and invertebrates.
They consume about one-third of their weight in food every 24 hours. Because of their
Rat inability to vomit, rats are very hesitant to try new foods that may be poisonous. They will
take a small nibble and wait to see if they feel sick and, if so, will avoid that food in the
future.
Birds are usually insectivores (eat insects) and frugivores (eat fruit) insects. They also
Bird
sometimes eat terrestrial non-insect arthropods, seeds, grains, and nuts.
Owl Pellet Dissection
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D. Complete Chart #3 below.
Chart #3. Calculations for the biomass eaten by barred owl prey.
F
G
H
I
J
K
Mass of
Number per
Mass of
Amount of food that
Biomass eaten
Mass
Prey in
Prey
year
Prey in
1 kg of this prey by prey in one year
each (g)
one year
one year (g)
eats in one year
(kg)
(kg)
Copy E from Given
1000g = 1
FxG=H
Given below
IxJ=K
Chart #1
below
kg
20
45.6 kg
Mouse
55
365 kg
Mole
5
1168 kg
Shrew
240
12.8 kg
Rat
20
127 kg
Bird
E. Use your data to create a Biomass Pyramid. For the top carnivore level you need to see the
opening paragraph to find the mass of Barred Owl. For the herbivore level use the assumption that
an owl produces an average of 2.5 pellets per day (which you already accounted for in your
pyramid of numbers) and the total Mass of prey (kg). For the producer level, make the same
assumption and then use the total of the Biomass eaten by the prey.
 Total mass of all prey consumed by owl per year (Chart #1) = ___________

Total mass of all producers consumed by prey per year (Chart #3) = __________

Draw your biomass pyramid on separate paper.
Discussion:
Analysis Questions: Answer the following questions in complete sentences, using data from the lab when
necessary.
1. Explain why the shape of your Pyramid of Numbers is triangular (rather than rectangular).
2. What is the barred owl’s niche?
3. Other types of birds form pellets. What would you expect to find in the pellet of a seagull?
4. Owls, hawks, and eagles are types of raptors, animals that have hooked beaks and sharp claws, and
are therefore adapted for seizing prey animals. Hawks and eagles differ from owls in that they eat
their prey animals by tearing them into small pieces, picking out the flesh and avoiding most of
the fur and bones. They also have strong stomachs that can digest most of the bone material that
they might eat. The relatively small amount of indigestible bone and fur that remain will be
compacted by their stomach muscles into a pellet similar to the owl's. Do you think an eagle pellet
would be as useful for dissecting as an owl's? Why or why not?
5. What other facts could be determined from examining pellets?
6. How would pesticide use come into play for the raptor food chain?
7. (Honor credit): What is a major source of error in this lab, specifically in the calculations? Why
does this error exist?
8. (Extra credit): Why do you think there are no organisms that regularly eat owls? In other words
why is the owl a top consumer? Use your data/ diagrams to support your answer.
Owl Pellet Dissection
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