Owl Pellets:
an investigation into the natural history of an
amazing creature
Introduction:
Owls are remarkable creatures. Hunting at night, they effect small mammal populations as a top predator
of many forest and field ecosystems. When an owl eats a mouse or other small mammal, it usually
swallows the prey whole. However, the owl’s digestive system cannot process or make use of the bones
and hair, so these waste products are regurgitated and left near the owl’s den. The regurgitated matter is
known as an owl pellet. By dissecting these pellets, we can learn about the owl’s diet, the general
ecology of the habitat the owl comes from, and the anatomy of small mammals.
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listen to owl calls (enature.com) | more owl info here (Cornell Laboratory of Ornithology
check out pictures of owl prey
read more about owl adaptations and owls found in the Chesapeake region
take an owl quiz and see some pictures
Procedure:
Working in pairs, you will dissect one owl pellet. You must work carefully so that you do not lose any of
the bones. Once you have separated the bones from the fur using the available tools, determine how
many skeletons you have by counting the number of skulls. Then sort out the other bones using the bonesorting chart. Identify the skulls/skeletons as accurately as possible, using the various ID tools (you will
be shown how to use them). Reproduce this chart in your report.
Your data:
Species
Number found
Comments/clues used for identification
Your typed lab report should include the following information. Follow the standard format for lab
reports. Click for a grading rubric.
The introduction section should include some background on owl ecology, diet, and eating methods and
references to the article “Nightlife has earned owls an undeserved dark reputation”.
The data/observations section will include the items listed below
1.
2.
A list of the species found, along with evidence for identification. Record the clues that you
used (such as head shape, tooth formula, etc.) Be specific (don’t just write “shape of front
teeth” but describe the shape). Use the resources online to confirm identification.
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you can confirm identification using this skull ID website -- good pictures!
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You might especially want to check out the deer mouse, meadow vole, shrew,
and mole. Also check out a great horned owl
Another great site for checking out prey species
Bird skeleton chart (this file and the others below are in PDF format)
Mole skeleton chart
Rat skeleton chart
Bone identification chart
Skull identification chart
ID resources (handouts from class)
The biomass calculations and % of diet from each type of prey (reproduce the chart in your
report). In your results/analysis section, discuss the significance of the chart results. For
extra credit, create a bar graph that properly depicts this data.
After identifying the prey species, use the following information to determine the biomass of the
prey. Record the number of each type of prey found under “Frequency found”. Calculate
“percent of diet” by dividing the frequency for each prey species by the total number of
prey. Calculate “biomass units” by multiplying the “frequency found” by the conversion
factor. Calculate percent of biomass by dividing biomass units by the total amount of
biomass. Reproduce this chart in your report; ask for help if you are unsure of how to use
it. Clearly label these data as class data. Class data: 2013 | 2012 | 2011 | 2010
Mean
Mass (g)
Conversion
Factor
Redback
Vole
40
2
Meadow
Vole
40
2
Frequency
Found
Percent
of Diet
Biomass
Units
Percent
of
Biomass
House
Mouse
20
1
Whitefooted
Mouse
20
1
Mole
55
2.75
Shrew
5
.25
Norway
Rat
240
12
Small
Bird
40
2
TOTALS
100%
100%
3. Either a photo of the assembled skeleton (clearly labeled) or a mounted skeleton which has
been glued together. The bones should be labeled. If you use a photo, you must prove it is
your work by including an identification label in the photograph. Photos must be printed in
color. Photos can also be inserted into your lab report and submitted electronically.
The analysis/discussion section will include the items listed under 4-7.
4.
1.
Go to the website and input your data (just yours, not the entire class). Our pellets were from
the NW region. Then answer the following questions
a.
b.
c.
Which skull types occur the most often? The least often?
b. Identify at least one regional difference in the data
c. Which key information is missing from the data?
5.
Discuss the significance of the biomass calculations – why did we convert to biomass, rather
than just compare the number of each type of prey?
6.
Answers to the questions from “Owl vs. Owl ”, which describes competition between two
owl species (scroll down for the questions).
7.
You will be assigned a particular biome and owl within North America. Using your
assigned owl and biome, you must construct a food web which the owl is the top
predator. Use figures in your book as food web examples; don’t forget about decomposers
and producers. Your web should include at least four trophic levels and at least 10
connections. Illustrate your food web with pictures if possible. In order to find out which
prey species to include:
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go to www.enature.com, and search for animals – you can use the advanced search
feature to find creatures based on habitat and/or geographic region.
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also try National Geographic's wild world -- zoom into the correct region of the map,
click on it to find more info and pictures on the region you are interested in.
Another good resource is NatureServe, an online encyclopedia of life
Habitat/Owl combinations:
Great Horned Owl
northern hardwoods (New England, upper Great Lakes)
Burrowing Owl
desert
Boreal Owl
boreal forest
Snowy Owl
tundra
Eastern Screech Owl
deciduous forest
Northern Spotted Owl
temperate rainforest (northwestern US)
Barred Owl
wet, swampy forests
Short-eared owl
grasslands
Some more information on creating food webs: The following chart indicates typical diet of
owl prey species. Please note that the prey listed are general; you need to find the particular
species that lives in your assigned habitat. (source:
http://www.lcusd.net/lchs/mewoldsen/Unit_2/Owl_Pellet.HTM)
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. Deer mice sometimes eat their own feces (coprophagy).
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
Food habit studies have revealed that shrews eat beetles, grasshoppers, butterfly
and moth larvae, ichneumonid 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.
Rat
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 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.
Bird
insects; terrestrial non-insect arthropods, seeds, grains, and nuts; fruit
Owl vs. Owl - a reading to accompany the Owl Lab
source: Natural History, March 1999
Answer all questions in complete sentences. Type your answers and include in your report.
1. Biologist Tom Hamer comments, "The barred owl is a generalist". Explain what he means by
giving examples.
2. Explain the two competing theories that explain the migration of barred owls from the eastern
US to the west coast.
3. Why do barred owls usually out-compete spotted owls? Give several reasons.
4. Do barred owls and spotted owls ever mate successfully? What does this say about our
definition of a species?
5. Near the end of the article Rocky Gutierrez comments, "using [the barred owl] as a reason to
stop trying to manage older forest habitat for spotted owls is a serious mistake. We need to be
thinking about lots of other creatures besides spotted owls". Explain what is meant by this
statement (read the last paragraph, and think about it in context of the entire article).
6. Read the online article "Owl War II". Summarize what you learn. Why are owls
endangered? What do logging and barred owls have to do with the problem? What is the current
plan (more here) for protecting spotted owls? You can also use this supplemental info to help
answer this question. This is a complex question that will require a paragraph or more to answer
7. What action did the Fish and Wildlife Service decide to take in 2013?
Extra credit:
Mount the skeleton in a three dimensional reconstruction – like a museum display. See the sample
reconstructions for ideas.
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Your reconstruction should include identification of the species and of the major bones (see
examples). In order to make the identification, use the dental formulas and other diagrams.
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You might want to bleach the bones in order to fully clean them. In order to do this, put the
bones in bleach for 2-3 minutes. Do not leave them in longer – they will become soft.
Some links for additional research:
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Life History of the Barred Owl - Ohio DNR
Owl Pellets
Assembling a Skeleton - Virtual Assembly
Owl Digestion
Owl Pages - Barred Owl Natural History
Barred Owl Natural History
USGS Barred Owl Site - w/ Song
Owling.com - World's Largest Owl Website
Owl Images You Can Use
The Raptor Center
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