Mrs. Melquist
Owl Pellet Lab
Name_________________
Set___ Date__________
Investigating Food Webs with Owl Pellets
Objectives:
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Dissect an owl pellet.
Identify prey species using a dichotomous key.
Construct a rodent skeleton from recovered skeletal prey.
Construct a food web based upon pellet prey analysis.
Interpret owl prey data from graphs.
Background:
Owls & Owl Pellets:
Owls are “predators” and “raptors”. As predators, they seek out other animals – their prey – to
catch and eat. As raptors, owls and other birds, such as hawks and eagles, catch their prey with
the long claws or talons, on their feet. Eagles and hawks hunt during the day and use their sharp
beaks to tear meat from their prey. Owls, on the other hand, hunt at night and swallow their prey
whole.
In addition to seeing as well as other raptors, owls have an unusual hearing ability that allows
them to locate their prey through their movements. Both their hearing and sight capabilities are
due to the presence of facial disks – specially arranged feathers around each eye that form
depression-shaped funnels which channel light and sound.
An owl is a formidable hunter. In addition to visual acuity and excellent hearing, owls possess
other special features. Their beaks are designed not to tear and rip flesh, like other raptors, but to
kill prey by crushing. Additionally, the leading edge an owl’s wings include thousands of tiny
feathers called “serrations”. Together, these serrations function as “vortex dampers” which
channel and dampen wind currents to prevent high-pitched “whistling” usually associated with
flight. This attribute cloaks the owl’s approach, meaning that most prey never hear their hunter.
The Barn Owl is a medium-sized owl whose range extends virtually around the world, with the
exception of deserts and arctic tundra. It nests in a variety of locales ranging from church steeples
and caves to silos and hollow trees. Barn owls typically feed on various rodents; larger mammals,
such as rabbits; insects, including grasshoppers and beetles; and small birds such as blackbirds.
After an owl swallows its prey, head first, digestive enzymes in the glandular stomach attack the
meat for digestion. Bones and hair are not affected by the digestive process and, therefore, must
be eliminated from the owl’s body. Here the muscular stomach plays an intricate role by pressing
undigested hair against bones to form a hair and bone package, called an “owl pellet”. This pellet
is expelled from the owl’s body through the mouth in a process called “regurgitation”.
An owl pellet usually contains the remains of more than one captured prey animal. Scientists take
advantage of this physiological characteristic of regurgitation by studying owl pellets to find out
more about predator-prey relationships.
Who Eats Whom:
All organisms are “biochemical machines” which are powered by energy captured from the sun
through photosynthesis. Ecologists assign every organism in an ecosystem to a “trophic level”
based upon what it eats. The organisms which first capture the sun’s energy are called
“producers” which make energy-storing molecules. Producers include plants, some types of
protests and bacteria. All other organisms in an ecosystem are “consumers”, which obtain the
sun’s energy by consuming producers. A special class of consumers which obtain their energy
from the organic wastes and dead bodies of all other organisms are “decomposers”. A graphic
representations of this “who eats whom” relationship is called a “food chain”: The sum of these
interconnecting networks of food chains is a “food web” – a graphic picture of how energy and
materials move through a community.
In this investigation you will dissect an owl pellet and recover as many prey remains as possible.
You will then use a dichotomous or “two-answer” key to identify prey animals. Then, you will
calculate the “biomass requirements” for the “population” of owls studied by the class and
determine the prey diet of these owls. Finally, you will construct a food web that illustrates te
predator-prey relationship.
Part 1: Dissecting an Owl Pellet
Materials:
Forceps
Newspaper
Plastic Bag
Gloves
Owl Pellet
Probe
Marking Pen
Paper Plate
Objectives:
- In Part 1 you will recover bones from various prey animals. In many cases, a single pellet
from a given owl may contain bones from multiple types of prey.
Procedure:
1. Use a marking pen to label the edge of a paper plate with the names of the students in
your group. Create a labeled “Prey Plate” having pie-shaped areas as illustrated on the
next page:
2. Place the owl pellet in the center of the newspaper cover sheet. Carefully unwrap the
aluminum foil from the pellet.
3. Use a sharp probe and forceps to break apart the owl pellet. Be especially careful to not
damage small bones as you work.
4. As each bone is uncovered, carefully use the sharp probe to help you remove hair and
other debris. Place the bone on the “Prey Plate” in its correct group. Use the Visual Guide
to Owl Pellet Prey as an aid in identifying bone types. Be alert! You may have more than
a single prey organism in your pellet.
Undigested insects, such as beetles, pill bugs and other forms may have invaded the pellet
before it was collected. Do not count them as prey.
5. Carefully place the “Prey Plate” in a safe place for next class. Wash your hands before
leaving the laboratory.
Part 2: Identifying Owl Prey
Materials:
Diagrams, Rodent & Bird Skeletons
Gloves
Marking Pen
Paper Plate
“Prey Plate”
White glue
Forceps
Key to Owl Prey
Newspaper
Ruler
Visual Guide to Owl Prey
Objectives:
- In Part 2 you will use a dichotomous key to identify prey animals removed form the
pellet. You will then assemble half-skeletons of recovered prey. Additionally, you will
determine the “cumulative biomass” of prey that your class “population of owls”
consumed.
Procedure:
1. Use the Key to Owl Prey to identify recovered prey animals in your group’s pellet.
2. Record your group’s prey totals in the “Number of Prey Animals (Group)” column of
Data Table #1. Also report your group’s prey data to your teacher who will tally prey
data for the class and post it on the board.
3. Select bones from the “Prey Plate” (from Part 1) to reconstruct a “half-skeleton” of a
rodent or bird prey animal on a clean paper plate. Use glue to affix the bones to the paper
plate. Leave spaces for missing bones. Use the “Rodent” or “Bird” skeleton diagrams as a
guide during your reconstruction.
4. Record class data for each prey animal in the “Total Number of Prey Animals (Class)”
column of Data Table #1.
5. Finish your calculations in Data Table #1 for the total number of prey animals identified
by the class.
To calculate total biomass by prey animal, multiply the number of prey organisms in the
“Total Number (Class)” column by its corresponding biomass value in the “Prey
Biomass” column.
For example: If 8 mice were counted for the class and the biomass value for each
mouse is 24 grams, then 8 prey animals (mice) X 24g biomass/ mouse = 192 g total
biomass
To calculate the cumulative total biomass, sum all data in the “Total Biomass (Class)”
column. This would include the total biomass for each type of prey for all of the owls in
the population surveyed by the class.
To calculate the “Percent Class Owl Population” data in Data Table #1 divide the “Total
Biomass (Class)” column for each prey species by the “Cumulative Total Biomass” and
multiply your answer by 100 to arrive at the percent age value.
For example: If 8 mice were counted by the class with a total biomass of 192g and the
total cumulative biomass for the class is 2000g then, 192g/2,000g = 0.096 X 100 = 9.6%
From the data, one can conclude that mice contribute approximately 10%, on
average, of the biomass of the diet of the “population” of owls examined by the class.
6. Use the calculated “Percent Barn Owl Population” data to create a pie chart – a graph
which is useful in showing parts or proportions of a whole. Draw lines that approximate
the calculated percentages for each prey organism.
Questions:
1. Based upon the class data, rank the most frequently consumed prey for the class “owl
population”.
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2. If an owl requires approximately 130g of food per day, how many mice would it need to
capture? How many rats?
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3. A predator expends energy when hunting for food. Which is the more “energy expensive”
cuisine, 35 insects at 1g each or one 35g vole?
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4. Try to define the food-getting “strategy” of a predator.
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5. Based upon your data, suppose the shrew population seriously declined. Would it affect
the areas owl population?
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Part 3: Constructing a Food Web
Materials:
Diagram, Food Web
Pencil
Colored Pencils
Objectives:
- In Part 3 you will use information in the table on the following page to determine the
feeding (i.e. trophic) levels for a food web. In this process you will learn “who eats
whom”.
Procedure:
1. Use a pencil to draw arrows that connect predators with their associated prey. Draw the
arrows to point from the prey towards the predator.
2. Use the colored pencils to color each organism with the appropriate color for each trophic
level: