Biology Lessons Part 2: Population Biology


 Lesson 2.2: Owl Pellets

To Ponder
Introduction and Background

AAAS Benchmarks
Exercise 1: Owl Pellet Dissection
Exercise 2: Owl Eating Habits and Neighbors

Grade Level

Prospective and Practicing K-8 Teachers; may be adapted for use in K-12 classes



This lesson takes approximately 2.5 hours.

To Ponder


1. Do you have owls living in your area? If so, do you know what kinds? Have you seen any?

How does the owl catch its prey?


3. What kinds of information do you think scientists can obtain by dissecting owl pellets?
  4. Owls are meat-eaters. Is that true of all birds?



These are some materials you may want for the sample experiments supplied with this lesson.

  • owl pellets (from scientific supply house)
  • dissecting needles
  • petri dishes, top and bottom
  • glue
  • forceps (tweezers)
  • bone identification guides
  • 5" X 8" index cards
  • pan balance
  • saran wrap
  • gloves




To discover what an owl eats.


To learn about an owl's digestive system.


To learn about the value of dissecting the contents of an owl pellet.


To make some inferences about the nature of the community in which the owl lives.



There is a population of Great Horned Owls in the San Diego State University (SDSU) Biological Field Stations. The Great Horned Owl feeds on an extremely wide variety of prey, including waterfowl, rabbits, squirrels, marsh birds, and rodents. Owls eat their prey whole and then regurgitate the indigestible parts of their feast each day. They rarely tear the prey into pieces before eating. Yet owls have weak stomach muscles and weak digestive juices. Because of this, they don't digest their prey completely. Instead, owls form compacted pellets of the "leftovers" (fur and bone) and cough these pellets up about 12 hours after eating.

This is a very neat system for scientists, because it is easy to collect and analyze these regurgitated owl pellets or castings to see what the owls have been eating recently. With owls doing the collecting, the scientist must simply locate the owl roost to obtain the skulls and bones of the small prey species living in the area.


Exercise 1

Owl Pellet Dissection

 To Do    1. 

Obtain two owl pellets. Carefully remove the foil wrapping from each pellet while keeping the pellet intact. Put a small piece of saran wrap on the pan and then weigh each pellet with a pan balance. Record the weights of your two pellets (Table 1).

Table 1. Predicted and observed # of skulls per pellet, Group # _____.
  Weight, grams  Predicted # of Skulls Observed # of Skulls
Pellet 1      
Pellet 2      

    2. Based upon the relative weights of the pellets, predict the number of small animal skulls in your pellets and record your predictions (Table 1). The number of skulls found usually ranges from zero to five. The "observed # of skulls" column will be filled in after you complete your dissection.
    3.  Dissect each pellet separately, using the bone sorting chart in Figure 1 to identify bones. Separate the bones from the fur and other debris in the pellet. Hip bones and upper leg bones with their large ball joints are readily identified. The scapula or shoulder blade, ribs, vertebrae, foot bones, skull bones, feathers and beaks are also recognizable. Dissect the pellet completely and save all bones. Enter the number of skulls found in each pellet in Table 1, above.
    4.  Arrange all the bones from each pellet on a separate 5" X 8" index card and then glue into place. You may choose to show each animal skeleton separately, or organize all bones according to the type of bone (Figures 2 and 3). Use more than one card for each pellet, if necessary.
     5. Label each card, including (a) your group number, (b) whether it is Pellet 1 or Pellet 2, and (c) the weight of the pellet before dissection.

Add the cards to the display on the wall. 


Figure 1. Bone Sorting Chart


Figure 2. Rodent Bones - whole animal skeleton



Figure 3. Rodent Bones - sorted by type



Enter all group findings on the board and record in Table 2. How might you account for differences in the number of skulls found in each pellet?

Table 2. Predicted and observed number of skulls per pellet, all groups.
 Group #

Pellet #

Weight, grams

Predicted # of Skulls

Observed # of Skulls
















































































     8. Is there any correlation between the weight of a pellet and the number of skulls in it? Explain.

Let's look at the data in another way. In Table 3 below, fill in the frequency of skulls per owl pellet. Refer to the numbers in Table 2 and count how many pellets had no skulls, 1 skull, etc.

Table 3. Frequency of Skulls Per Pellet
# Skulls/Pellet   0 1 2 3 4 5 6  7
# Pellets                 


Use the numbers in Table 3 to construct a graph showing the frequency distribution of animal skulls in this sample of owl pellets (Figure 4).

Figure 4. Frequency Distribution of Animal Skulls per Pellet


Use Table 3 and the graph in Figure 4 to complete Table 4, below. Calculate the average number of small animals per owl pellet, based on the number of skulls found and the number of pellets dissected. Identify the range of small animals per pellet, which is the smallest number to the largest number of skulls found in this sample of pellets. Looking at Figure 4, identify the most frequently occurring number of skulls per pellet. This is the mode.

Table 4. No. of skulls found in each pellet
 Total No. of Skulls Found by All Groups  
 Total No. of Pellets Analyzed  
 Average No. Skulls per Pellet  
 Range of No. of Skulls per Pellet  
 Mode No. of Skulls per Pellet  

 Questions   12. 

Can you describe the variation among owl pellets more precisely now?


Exercise 2

Owl Eating Habits and Neighbors

Questions 1. Owl pellets are uniformly dark gray, from 1 inch to 3 inches long and 3/4 to 1 inch in diameter. These pellets can be found where owls roost, usually in open barns or in big trees which offer protection from bright light. They are odorless, but may be "whitewashed" by the owl since they fall beneath the perch. Describe the owl pellets you used. Did they match this description? If not, what were the differences?
2. Owl pellets have been used for scientific study of small mammals and their distribution, as well as in studies of owl behavior patterns. Based upon your data, what kinds of small animal populations do you think lived around each of your owls? (We cannot know if your two pellets came from one owl over time or from two different owls.)
3. Did you find any insects or insect parts in the owl pellet? Would you expect to find any? Why?

The owls compete with one another for their food and they also compete with other populations such as hawks and eagles.

Background 5. Owls, hawks, and eagles are types of raptors. Raptors are animals which 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 which can digest most of the bone material which 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.
Questions 6.

Do you think you would learn as much from dissecting hawk and eagle pellets? Why?


7. Would you expect to find a pellet with no bones on occasion?



Ball State University, Burris Laboratory School. Retrieved from the World Wide Web, May, 1998: Display of owl pellet dissection in young children's classroom.

Key, James (1998). Owl Pellets: A World of Discovery! Carolina Tips, 61 (2). Retrieved from the World Wide Web, May, 1998:

Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc.

Raptor Center, University of Minnesota. Raptor Facts. Retrieved 6/2/98 from the World Wide Web:

The Owl Cam Home Page: This site "shares in the adventures of a pair of Northern Barred Owls ( Strix varia varia )
as they raise their family in a nest box in Eastern Massachusetts".
The Wildlife Rehabilitation Center Home Pages: a comprehensive list of wildlife rehabilitation organizations with web presence.



Section B: Scientific Inquiry

Grade K-2 (Benchmark 3 of 4)

Describing things as accurately as possible is important in science because it enables people to compare their observations with those of others.

Grade 3-5 Benchmark 1 of 4

Scientific investigations may take many different forms, including observing what things are like or what is happening somewhere, collecting specimens for analysis, and doing experiments. Investigations can focus on physical, biological, and social questions.


Section D: Interdependence of Life

Grade K-2 (Benchmark 1 of 2)

Animals eat plants or other animals for food and may also use plants (or even other animals) for shelter and nesting.


Grades 3-5 (Benchmark 2 of 3)

Some source of "energy" is needed for all organisms to stay alive and grow.