Owl Lab | 1

Energy Transfer with Owl Pellets Purpose:

Students will dissect owl pellets to collect and count the contents in order to create food webs and

pyramids of numbers and biomass.

Student Objectives:

Students will learn the term pyramid is a misnomer because of the huge amount energy required by

organisms to maintain homeostasis. Trophic pyramids are stacked boxes best represented by

biomass of each trophic level.

Background Information

The barn owl (Tyto alba) is the most widely distributed species of owl, and one of the most widespread of all birds. The barn owl is

known by many common names which mostly refer to its pale coloring or silent flight. It is nocturnal over most of its range but in

Britain and some Pacific islands, it also hunts by day. Barn owls specialize in hunting animals on the ground and nearly all of their

food consists of small mammals which they locate by sound, their hearing being very acute.

A typical barn owl may measure about 33 to 39 cm (13 to 15 in) in overall length, with a wingspan of some 80 to 95 cm (31 to 37 in).

Adult body mass is also variable with male owls from the Galapagos weighing 260 g (9.2 oz) while male Pacific barn owls average 555

g (19.6 oz).

The owl digestive system has a specialized section that presses the undigested portions together forming a pellet. Since the owls

cannot pass the pellet through their digestive system, they must spit out the pellet. 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.

Materials

Butcher Paper Owl pellets Forceps Probes

Masking tape Rubber gloves Rulers Identification Charts

Procedure

1. This work is to be done in your lab notebook. This includes: all observations, data tables, and questions answered. Please ensure your making a copy that is legible!

2. Spread out and tape down butcher paper to decrease clean-up time. Obtain rest of the materials.

3. Remember to use gloves, but also wash hands at the end of the procedure. These pellets have been sterilized, but remember to use proper laboratory procedures.

4. Use dissecting tools and fingers to gently pry apart the pellet. NOTE: The bones you are looking for are small and easily broken.

5. Set aside any bones found on separate sheet of paper. Repeat until you have completely dismantled the pellet.

6. 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

7. Use the bone identification keys to identify the owl’s prey. Please remember that voles, mice and rats have similar bones, but there is a huge difference in the sizes (see Table 2 for masses). You will most likely not reassemble an entire skeleton. Use the skulls for identification. Mark your results in the table below and on the class chart.

8. Clean up. Wrap ‘waste’ material in butcher paper and throw it away.

9. Wash, dry and return dissection equipment - Scissors, forceps, probes

Owl Lab | 2

Data Analysis

Create Table 1 in your Notebook. Use the assumption that an owl produces an average of 2.5 pellets per day. Please note that a vole

is equivalent to a mouse.

Table 1. Average Number of Prey eaten by Owl in one day (USE THE AVERAGE/DAY CLASS RESULTS)

Prey Number Found

(N)

Number Eaten per Day

(D = 2.5 x N)

Number Eaten per Year

(Y = 365 x D)

Mouse or Vole

Mole

Shrew

Rat/Rabbit

Bird

Total YT =

Create a two-level Numbers Pyramid in your notebook based on the data you calculated. There will be no producers in this

pyramid.

i. The top level of the pyramid is the secondary consumer. This equals 1 because there is only one owl producing the

pellet.

i. The bottom level of the pyramid is the primary consumer. This is the total number of prey eaten per year (YT)

Create Table 2 in your notebook. Note: Number of prey per year (Y) is from Table 1.

Table 2. 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 (ProdM)

Biomass of Producers

(BM = Y x ProdM)

Mouse/Vole 0.05 5.5

Mole 0.115 12.5

Shrew 0.005 0.8

Rat/Rabbit 0.240 15

Bird 0.020 1.9

Total PMT (kg) Total Biomass of

Producers BMT (kg)

Use your calculations from Table 2 to create a three-level Biomass Pyramid in your notebook.

a. The secondary consumer (top-level) is the Barred Owl and its approximate biomass is indicated in the background information.

b. The biomass of all primary consumers (middle-level) is the Total Mass of the Prey (PMT).

c. The biomass of all producers (bottom-level) is the Total Biomass of the Producers (BMT).

Owl Lab | 3

Trophic Pyramid Analysis Questions

Answer the following questions in your notebook.

1. Which pyramid is more accurate in reflecting the energy available and moving through the ecosystem – the numbers pyramid or the biomass pyramid? Explain why.

2. Does the shape of the biomass pyramid illustrate the concept of ecological efficiency of 10%?

3. The prey are eating a tremendous amount of plant matter, and the owl is eating an enormous amount of prey.

a. How much of the producer biomass makes up the prey – calculate this by prey biomass/producer biomass. Do the same for prey and owl biomass.

b. What happened to the lost biomass?

Owl Food Web

Read the information in the following table. Notice the diet items that are plants, and which are animals.

Table 3. Owl prey and their respective diet

Prey Diet

Mouse/

Vole It eats a wide variety of plant and animal matter depending on what is available, including insects and invertebrates such as

earthworms, centipedes, millipedes, sowbugs. They also eat seeds, fruits, flowers, and nuts.

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, moth larvae, 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 fruit are also eaten by some species of shrews.

Rat The rat's diet typically includes seeds, nuts, grains, fruits. They are omnivorous and often eat small birds, mice, small snakes,

and even other rats.

Bird The birds that owls eat typically consume insects; earthworms, centipedes, millipedes, sowbugs, seeds, grains, and nuts; fruit

In your notebook use the information from Table 3 to draw a Food Web that is representative of the class pellets. Remember all arrows go from food to feeder to show the movement of energy.

a. Draw and label the owl as the top consumer.

b. Draw and label all prey found in the feces as the next level of consumer (shrew, etc.).

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 ‘energy’ arrows from food to feeder to show flow of energy (head of the arrow points towards consumer). Many organisms (especially producers) will have several arrows pointing away from them.

f. Repeat for each level towards the owl. Note: some arrows may be drawn sideways.

g. Include the detritivores and decomposers - they may not be mentioned, but they are always present in an ecosystem!

Owl Lab | 4

Food Web Analysis Questions

Answer the following questions in your notebook.

4. Identify the following on your food chain (use colored pencils and a key to show this on the original): a. 3 food chains within your food web. b. Where INTRAspecific competition is occurring c. Where INTERspecific competition is occurring d. Where resource partitioning between competitors is happening

5. In an area where there competition is occurring, explain how owls (as predators) may be keeping this competition at a

minimum.

6. Explain how good weather conditions that increase the production of primary producers would affect the owls.

7. What would happen to the number of prey if the owl was removed due to something like habitat destruction? Why? Follow this line of thought down all the way to the number of producers.

8. In reality, this is a simplified version of the food web. In natural ecosystems there would be other predators that are consuming similar prey. For example, coyote consume a similar diet as owls.

a. Describe what would happen to the food web if coyotes were added. b. Would this create a more or less stable ecosystem?

9. How do your pyramids and food web demonstrate the first and second law of thermodynamics in this particular

ecosystem? - First law of thermodynamics is that energy is not created or destroyed. Energy is conserved in chemical changes. - Second law of thermodynamics is that energy changes from a more ordered state to a less-ordered state. - Each trophic level contains 2-10% of the energy below it.

10. In the trophic pyramid, we placed owls at a level 3. Now that you have examined a more complete food web, what level

should we place owls?