Owl Pellet Dissection and Ecological Pyramid Construction
Adapted from labs by Mark Little and Mark Ewoldsen
OBJECTIVES: Upon completion of this lab activity, you will
1. Apply mathematical routines to quantities that describe interactions among living systems and their
environment, which result in the movement of matter and energy.
2. Use visual representations to analyze situations to illustrate how interactions among living systems and with
their environment result in the movement of matter and energy.
3. Predict how changes in free energy availability affect organisms, populations and ecosystems.
4. Refine scientific models and questions about the effect of complex biotic and abiotic interactions on all
biological systems, from cells and organisms to populations, communities and ecosystems.
5. Design a plan for collecting data to show that all biological systems (cells, organisms, populations, communities
and ecosystems) are affected by complex biotic and abiotic interactions.
6. Analyze data to identify possible patterns and relationships between a biotic or abiotic factor and a biological
system (cells, organisms, populations, communities or ecosystems).
INTRODUCTION
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 to form a pellet.
Owl pellets are masses of bone, teeth, hair, feathers and exoskeletons of various animals
preyed upon by raptors, or birds of prey. Pellets are produced and regurgitated not only by
owls, but by hawks, eagles and other raptors that swallow their prey whole of in small pieces.
Owls feed early in the evening and regurgitate a single pellet approximately 20 hours after
Image from
eating. Predatory mammals such as bobcats and wolves have teeth to grind up bones and
http://en.wikipedia.org/wik
claws,
and a digestive tract adapted to pass these ground parts. Owls, on the other hand, do
i/File:Tyto_alba_not have teeth for grinding and cannot pass whole bone and claws through their digestive
British_Wildlife_Centre,_Su
tract safely. The protein enzymes and strong acids that occur in the digestive tract of raptors
rrey,_England-8a_(1).jpg
do not digest the entire meal. The relatively weak stomach muscles of the bird form the
undigested fur, bones, and feathers into a bolus (or wet slimy pellets). Depending upon the prey eaten, the undigested
portions may include beaks, claws, scales, or insect exoskeletons. This type of material has little nutritional value and
must be passed from the body. In this process even the most fragile bones are usually preserved unbroken.
Scientists take advantage of this adaptation by collecting these pellets and examining their contents. Since owls are not
very selective feeders, these pellets can be used to estimate the diversity of available prey. The contents are also a
direct indicator of what an owl has fed on—information that is crucial for species management and protection.
The owl pellets that you will be examining in this lab have been collected and fumigated from common barn owls (Tyto
alba). Owl pellets themselves are ecosystems, providing food and shelter for communities that 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. The
larvae metamorphose near the surface of a pellet in cocoons made of fur.
The bones that are found in the pellet can be identified using pictorial ‘keys’. This information can be used to determine
a food web and the approximate number and biomass of prey ingested in one year. This data will be used to create
number and biomass pyramids.
Owl Pellet Lab 2012
Adapted from labs by Mark Little and Mark Ewoldsen
Page 1
Today you will dissect an owl pellet, gather data about the organisms the owl has consumed, and then use this
information to create the following:
 A food web
 A pyramid of numbers
 A pyramid of biomass
You will also
Materials Needed:
Paper plate
Electronic balance
Large weigh dishes
Owl pellets
Gloves
Small weigh trays
Forceps
Rulers
Water bottle
Dissection needles
Bone Identification Chart
Plastic bags (if needed)
SAFETY CONSIDERATION: While the pellets have been fumigated and sterilized, it is
important that you wear gloves while performing the lab. Be sure to wash your hands
thoroughly once the lab is complete.
Procedure
1. Gather all materials from the supply area. You will be dissecting your pellet on a paper plate for easy clean up.
2. Carefully remove the foil from around your pellet, and using a weigh dish, determine the mass of your pellet.
Record this in the data chart provided.
3. Use forceps and fingers to gently pry apart the pellet. It may help to moisten the pellet using a water bottle to
make this process a bit faster. NOTE: The bones you are looking for are small and easily broken so be cautious
while you are trying to take the pellet apart.
4. Set aside any bones found in a small weigh tray. Repeat until you have completely dismantled the pellet.
5. Separate the skull bones using the following criteria:
a. Shape and size of the skull
b. Shape of eye sockets
c. Length of the snout compared to the rest of the skull
6. One partner should use the dichotomous key on page 3 to identify the bird’s prey.
7. The other partner should use the bone identification chart to identify what types of bones are found in the
pellet.
8. Clean your work area. Carefully fold the waste material into the paper plate and throw it in the trash. Clean off
any dissecting equipment used and return it to the supply area.
Data Analysis
CREATING A PYRAMID OF NUMBERS:
1. Fill in Table 3. Use the assumption that an owl produces an average of 2.5 pellets per day. Assume that a mouse
and a vole are equal to one another.
2. Now use the data processed above to complete Table 4: Biomass of Prey and Producers.
3. Create a two-level Numbers Pyramid. There will be no producers in this pyramid.
a. The top level of the pyramid is the secondary consumer: This level equals 1 because there is only one owl
producing the pellet.
b. The bottom level of the pyramid is the primary consumer. This is the total number of prey eaten per
year (YT).
c. In your lab notebook, draw the pyramid. Make sure to indicate the correct numerical relationship
between top and bottom level. Be sure to draw the shape that most biology textbooks use to illustrate
food pyramids.
Owl Pellet Lab 2012
Adapted from labs by Mark Little and Mark Ewoldsen
Page 2
CREATING A PYRAMID OF BIOMASS:
4.
Use your calculations from Table 4 to create a three-level Biomass Pyramid:
a. Use the Number of Prey (Y) value you calculated from Table 3.
b. The secondary consumer (top-level) is the owl and its approximate biomass is 0.55 kg .
c. The biomass of all primary consumers (middle-level) is the Total Mass of the Prey (PMT).
d. The biomass of all producers (bottom-level) is the Total Biomass of the Producers (BMT).
CREATING A FOOD WEB:
5. In your lab notebook, draw a food web that is representative of YOUR pellet. Use the information from Table 5 to
complete this portion of the activity. You can represent organisms simply by placing their name in a box or circle
that is connected to the other organisms in your web.
a. Draw and label the owl as the top consumer.
b. Draw and label all prey found in the pellet as the next level of consumer (shrew).
c. Draw and label any consumers eaten by the prey (crickets eaten by shrew).
d. Draw and label any producers eaten by prey or any producers eaten by consumers eaten by prey (seeds,
fruit and grass eaten by crickets; seeds and roots eaten by shrew).
e. Draw arrows from organism consumed towards consumer (head of the arrow points towards consumer)
to represent energy flow between organisms.
f. You will also draw arrows from each producer towards each primary consumer that eats that producer.
g. Repeat for each level towards the owl. Note: some arrows may be drawn sideways. Also, some
organisms will have multiple arrows either going toward them or away from them.
CONCLUSION QUESTIONS: Answer these using complete sentences in your lab notebook.
1. Look at your pyramid of numbers. Note that producers were not included in this pyramid. How could the
number of producers be determined?
2. Now look at your pyramid of biomass. How does the shape of this pyramid illustrate the concept of ecological
efficiency?
3. Between the pyramid of numbers and the pyramid of biomass, explain which of the two pyramids best
represents the concept of ecological efficiency.
4. What is the relationship between the biomass of the prey and the biomass of the producers they consume?
5. Explain why the mass of producers consumed by prey animals changes as the latitude of the pellet collection
location changes. How would the mass of producers consumed by prey animals change with each season?
6. What component of ecosystems is missing from your pyramid of biomass that helps it to maintain its shape?
7. Examine your food web. Why do the arrows point in the direction of the consumers? Why do the arrows
ultimately point to the top of the food web?
8. Using your food web, determine the ultimate source of energy for all ecosystems.
9. How would the organisms in the food web (and thus, the ecosystem) be affected if the following events
occurred:
a. A drought sharply reduces the number of grain plants available for mice to eat.
b. The rat population doubles in size.
c. There is abundant rainfall during the year in the location where the pellet was collected.
d. A defoliant known to be toxic to small animals is applied to a field of weeds by a developer in a suburban
area in order to clear land for a new shopping center.
10. The first law of thermodynamics can be summarized by stating that energy cannot be created or destroyed, but
only changed from one form to another. Explain how your data support this statement.
11. The Law of Conservation of Mass states that the amount of matter in a closed system remains constant over
time. The Earth is a closed system. Explain how your data support this statement.
Owl Pellet Lab 2012
Adapted from labs by Mark Little and Mark Ewoldsen
Page 3
DATA COLLECTION:
Table 1: Information about your owl pellet
Information about the Owl Pellet
Pellet mass (g)
Pellet length (cm)
Pellet width (cm)
Number of bones found
Biomass (g)
Table 2: Information about the bones collected
Bone Type
Tally
% of
Number Total
Bone Type
Tally
% of
Number Total
Table 3: Number of Prey eaten by Owl
Prey
Number Found
(N)
Number Eaten per Day
(D = 2.5 x N)
Mouse or Vole
Mole
Shrew
Rat
Bird
Total YT =
Owl Pellet Lab 2012
Adapted from labs by Mark Little and Mark Ewoldsen
Page 4
Number Eaten per Year
(Y = 365 x D)
Table 4: Biomass of Prey and Producers
Prey
Number of
Prey/year (Y)
Mass (kg)
(M)
Mass of Prey (PM = Y x M)
Mass (kg) of Producers
eaten by Prey (Prod)
Mouse
0.020
45.6
Mole
0.055
365
Shrew
0.005
1168
Rat
0.240
12.8
Bird
0.020
127
Biomass of Producers
(BM = PM x Prod)
Total Biomass of
Total PMT (kg)
Producers BMT (kg)
Table 5: Owl prey and their respective diet
Prey
Diet
Mouse
It eats a wide variety of plant and animal matter depending on what is available, including insects and other invertebrates,
seeds, fruits, flowers, nuts, and other plant products.
Mole
A mole's diet is mostly insects and other invertebrates, including earthworms, centipedes, millipedes, snails, slugs, grubs,
ants, sowbugs, termites, beetles, and crickets
Shrew
Rat
Bird
Food habit studies have revealed that shrews eat beetles, grasshoppers, butterfly and moth larvae, wasps, crickets,
spiders, snails, earthworms, slugs, centipedes, and 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 onethird of their weight in food every 24 hours. Because of their 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.
insects; terrestrial non-insect arthropods, seeds, grains, and nuts; fruit
Owl Pellet Lab 2012
Adapted from labs by Mark Little and Mark Ewoldsen
Page 5
DICHOTOMOUS KEY
Use this to identify what organisms are present in the pellet.
Does the animal have....
Then...
1. a) 3 or fewer teeth on each side of its upper jaw?
go to 2.
b) 4 or more teeth on each side of its upper jaw?
go to 3.
c) It has no teeth, long beak…
It is a bird
2. a) 2 biting teeth on its upper jaw?
b) 4 biting teeth on its upper jaw?
3. a) a skull length of 23 mm or less and brown teeth?
b) a skull length of more than 23mm and approx. 44 teeth
4. a) the roof of its mouth extending past the last molar?
b) the roof of its mouth not extending past the last molar?
5. a) a skull length of 22 mm or less?
b) a skull length of more than 22 mm?
6. a) flat molars?
b) rounded molars?
Owl Pellet Lab 2012
Adapted from labs by Mark Little and Mark Ewoldsen
Page 6
go to 4.
it's a rabbit.
it's a shrew.
it's a mole.
go to 5.
go to 6.
it's a house mouse.
it's a rat.
it's a meadow vole.
it's a deer mouse.
Owl Pellet Lab 2012
Adapted from labs by Mark Little and Mark Ewoldsen
Page 7