1
Name____________________________________
Problem:
Compare and contrast the diets of northwest and southeast owls.
Activities & Point Values:
1.
2.
3.
4.
10 Owl Facts that you Learned from Movie “Dirty Jobs” 10 points
Owl Pellet Webquest 10 points
Activity #1-#4 40 points
Reconstruction of Prey’s Skeleton or Categorization of Bones 15 points
Introduction:
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 eating.
Predatory mammals such as bobcats and wolves have teeth to grind up bones and claws, and, a digestive tract
adapted to pass these ground parts. Owls, on the other hand, do not have teeth for grinding and cannot pass
whole bone and claws through their digestive tract safely. The protein enzymes and strong acids that occur in the
digestive tract of raptors do not digest the entire meal. The relatively weak stomach muscles of the bird form the
undigested fur, bones, feather etc. 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 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.
2
Activity #1: Interpreting Your Data
Now that you are finished with your dissection, it is time to determine what bones and how many of each bones
you have. Using the diagram below, please list the types of bones that you found in the table and how many of
each.
Bone Tally Table:
Northwest Owl
Bone
Southeast Owl
Tally
Number % of Total
Bone
Tally
Number % of Total
3
Activity #2:
Determine what prey is found in each pellet using the dichotomous key and pictures of skulls
below and then tally the prey on the table provided on page 4. If you didn’t have a skull than
use the bone sheet that your teacher has to figure out prey type.
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?
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?
go to 3.
go to 4.
it's a rabbit.
it's a shrew.
it's a mole.
go to 5.
b) the roof of its mouth not extending past the last
molar?
go to 6.
5. a) a skull length of 22 mm or less?
it's a house mouse.
b) a skull length of more than 22 mm?
6. a) flat molars?
b) rounded molars?
it's a rat.
it's a meadow vole.
it's a deer mouse.
4
Figure OPL-2 (Rodent Skulls):
Northwest
Southeast
1. What conclusions can you make about the diet from this activity?
2. List one type of prey found in:
a. Northwest:
b. Southeast:
3. What would happen if the prey you listed for #2 went extinct? Explain your
answer.
5
Activity 3: Determine the prevalence of prey that your owl pellet revealed
Owl Prey Chart
The number of asterisks (⋆) placed next to each prey type under both geographic regions
represents the frequency of occurrence for each prey type as shown in the following
scale:
(⋆⋆⋆⋆) – Very common
(⋆⋆⋆) – Common
(⋆⋆)Occasional
(⋆) – Rare
( ) – Does not occur
NORTHWEST
Prey
Occurrence
Student
SOUTHEAST
Prey
Class
Total
Weight
Total
Biomass
Occurrence
Student
Prey
Class
Total
Weight
Total
Biomass
Pocket Gopher
Thomomys
(⋆⋆⋆)
150g
( )
-----
Sigmodon
()
-----
(⋆⋆⋆⋆)
100g
Oryzomys
()
-----
⋆)
80g
Rattus
(⋆)
150g
(⋆)
150g
(⋆⋆⋆⋆)
40g
(⋆⋆⋆)
40g
Peromyscus
(⋆⋆)
22g
(⋆⋆)
22g
Mus
(⋆⋆⋆)
18g
(⋆⋆⋆⋆)
18g
Rat
Vole
Microtus
Mice
Reithrodontomys
Perognathus
(⋆⋆)
(⋆⋆)
12g
25g
(⋆⋆)
( )
12g
-----
6
Mole
Scapanus
(⋆)
55g
( )
-----
Scalopus
()
-----
(⋆)
55g
Blarina
()
-----
(⋆⋆⋆)
20g
Cryptotis
()
-----
(⋆⋆⋆⋆)
4g
Sorex
(⋆⋆)
4g
Bats
(⋆⋆)
7g
(⋆)
7g
Birds
(⋆⋆)
15g
(⋆⋆)
15g
Shrew
(⋆)
4g
Other Prey
Insects
(⋆⋆)
Crayfish
(⋆)
Small Reptiles
1g
5g
(⋆⋆)
(⋆)
20g
(⋆)
1g
5g
20g
(⋆)
Questions
1.
Which prey type contributed the most by number for each region? Did the same
prey type contribute the most by number for both regions?
2.
Which prey type contributed the most in biomass for each region? Did the same
prey type contribute the most in biomass for both regions?
7
3.
If an owl needs 100g of food per day, how many Sorex will it need to capture? How
many Sigmodon? (Show your work)
4.
Assume an owl eats 50 1g insects and one 100g rat. Which prey contributed the
most to the owl’s diet? Which prey would have probably required less foraging
time?
5.
Is quantity or quality of prey more important? Explain fully.
6.
Which region has the greatest diversity of prey? (Consider data on prey chart)
7.
Which region has the most stability?
8.
Would a crash in the shrew (Blarina, Cryptotis, Sorex) population seriously affect
either region? Why or why not? (Consider data on prey chart)
9.
Would a crash in the vole (Microtus) population seriously affect either region?
Why or why not?
(Consider data on prey chart)
8
Activity #4: Food Web
Using the blank space below, construct two food webs (one for the northwest owl; one for
the southeast owl). In order to make these food webs as accurate as possible, please use
the data that you collected from this lab.
Each food web must contain:
1. 4 Producers
2. 3 Primary Consumers
3. 2 Secondary Consumers
4. 1 Tertiary Consumer (in this case- it will be the owl)
Questions:
1. Explain bio magnifications in terms of one of the food webs that you constructed.
Please cite specific examples from your food web in your answer.
2. What is the difference between top down and bottom up control? In your
explanation, again cite examples from one of your food webs.
9
Activity #5: Reconstruction of Skeleton:
In this activity you can either:
A. Reconstruct the prey of one of your pellets
B. Categorize the bones for both pellets
Using the diagram provided along with the diagram on page 2, please reconstruct
the skeleton of one of your pellets or categorize all the bones. Please see your
teacher for past examples.