Biology High School Assessment Student Lab Manual 38 Prince George’s County Public Schools

Ecology: Examining Owl Pellets

Background Owls are raptors, or birds of prey. They catch their prey, including small birds and rodents, and swallow them whole. Enzymatic juices in the owl’s digestive system break down the body tissues of the prey, but leave the bony materials, hair and feathers undigested. Depending on 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 eliminated from the owl’s body. Since owls do not have teeth for grinding and cannot pass whole bones and claws through their digestive tract safely, these indigestible materials form a bolus, or lump, called a pellet. The pellet is composed of fur, feathers, bones, and other undigested parts of the consumed prey. Pellets begin forming within the digestive tract of the owl as soon as the prey is swallowed. The pellets are then coughed up, or regurgitated, and the owl begins feeding once more. Scientists take advantage of this adaptation by collecting these pellets. Owl pellets are dried and either fumigated (treated with chemicals) or sterilized so that their contents can be examined safely. Since owls are not selective feeders, the pellets can 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.

Objectives Identify the species eaten by an owl from the remains found in an owl pellet. Construct a food web containing an owl from a list of species found in owl pellets

Materials Owl pellet Gloves Dissecting tray Lab apron Dissecting needle White paper Forceps Glue or tape (optional) Small metric ruler Procedure Dissecting an Owl Pellet 1. Put on the gloves and lab apron. 2. Place the owl pellet in the dissecting tray. (Optional: soak the pellet in water to help loosen it up). Use

the dissecting needle and forceps to gently pull apart the pellet. Remove the fur and feathers and push them off to the side. Be very careful not to damage the tiny bones.

3. Place all the bones on a sheet of white paper. Pay particular attention to the skulls. 4. Assemble the different bones into the various piles (look at chart) as best you can. Count the number

of each type of bone and record that number in the chart that follows. Refer to the two sets of diagrams on p.35-37 to aid you in identifying the types of skulls and other bones.

Name: _________________________________ Period: ___________ Date:__________________

Biology High School Assessment Student Lab Manual 39 Prince George’s County Public Schools

Number of Bones Skull _____________ Lower jaw _____________ Scapula (shoulder blade) _____________ Hip (pelvic bone) _____________ Upper leg _____________ Lower leg _____________ Rib _____________ Vertebrae (back bone) _____________ Foot _____________ 5. Optional - Glue or tape the assembled skeletons to the paper. Figure 1: Skull Comparisons

Analysis 1. Using the Key to Skulls of Common Barn Owl Prey as reference, record the number of each

identified skull into the proper column in Table 1. Your teacher will also have you record this number on the class sheet (or on the board, or however the data is being gathered). Any other animal remains you find in your pellet will be from a bird, or an insect. List them as “other prey.”

2. Calculate the Total Biomass, for all species shown, by multiplying the following:

Total number in all samples column x Prey biomass = Total Biomass 3. Calculate the Cumulative total biomass by summing all data in the Total biomass column. 4. You can now calculate what percent each prey contributes to the diet of the barn owl. Use the following

to do determine the percentage:

Total biomass of one species X 100 = percentage Cumulative total biomass

5. Follow your teachers instructions for clean up.

Biology High School Assessment Student Lab Manual 40 Prince George’s County Public Schools

Table 1 Prey Occurrence Number found

In sample Prey Biomass (in grams)

Total number In all samples

Total Biomass

% of Diet

Pocket gopher

+++

150 g

Rat

+

150g

Vole

+

40 g

Mice house mouse deer mouse

+++ ++

18 g 25 g

Mole

+

55 g

Shrew

++

4 g

Other Prey Birds Insects

++ ++

15 g 1 g

Totals 100% Key: +++ = common, ++ = occasional, + = rare Construct a Food Web In the space below, construct a food web with the owl at the highest trophic level. (Note: Be sure to include producers and decomposers). The intermediate organisms should include the prey found in your owl pellets in class.

Biology High School Assessment Student Lab Manual 41 Prince George’s County Public Schools

Questions 1. In what way might the formation of owl pellets increase an owl’s chances of survival in an ecosystem?

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2. How can you distinguish between vertebrate and invertebrate material? Between birds and mammals?

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3. How many different trophic levels are represented in your food web? __________________________________________________________________________________

4. How would a sudden decrease in the shrew population affect the barn-owl population?

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5. How would a sudden decrease in the vole population affect the barn-owl population? __________________________________________________________________________________

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6. Assume an owl eats 100 1-gram insects and 1 100-gram rat. Did the insects or rat contribute more to the owl’s diet? How does foraging time affect this outcome?

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7. Is quantity or quality of prey more important? __________________________________________________________________________________ __________________________________________________________________________________

Biology High School Assessment Student Lab Manual 42 Prince George’s County Public Schools

Biology High School Assessment Student Lab Manual 43 Prince George’s County Public Schools

Biology High School Assessment Student Lab Manual 44 Prince George’s County Public Schools