Lab: The Ecology and Natural History of Owl Pellets
BACKGROUND INFORMATION
Introduction: Owl pellets represent the remains of meals of owls. Depending on
the diet of the owl, the pellets contain a mixture of undigested bones, fur and
feathers from mice, voles, rats and birds. Occasionally one can find remains of
amphibians, reptiles and exoskeletons from insects. Pellets are common among the
raptors and other birds such as herons, sandpipers and kingfishers. Most raptors
tear the flesh away from the bones of the prey item. Fortunately, most owls swallow
their prey whole and as a consequence the complete remains can be found in the
pellets. Some owl species actually decapitate their prey before swallowing the rest of
the body. As a consequence the skulls of the prey items are missing from such
pellets.
The Barn Owl: The majority of pellets used in the classroom have an origin
from the cosmopolitan Barn Owl (Tyto alba). Found in open grasslands and
farmlands, these monogamous nocturnal hunters lay about 5-11 eggs per
season. The hatchlings are fed for about two months. The species is in decline
due to loss of habitat as well as use of rodent poison.
Ecology: The owls feed at a rapid rate. It is common that an adult consumes
several rats and/or mice each night. During the nesting season (about two months)
the foraging rate increases to accommodate the young owls (5-11) in the nest. Each
owlet consumes several mice each night. As a consequence the impact of the owls
on the local prey population can be enormous. It is important to consider the role of
predators in maintaining the stability within a community of organisms. Historically
barn owls were often used to control the rodent populations in agricultural areas.
The content in the pellets is a direct reflection of the local population of available
prey items. These are readily identified due to the presence of the bones (especially
the skull). Continuous collection of pellets over a longer time period can provide
important ecological information about the feeding habits of the owls, as well as the
population dynamics of the prey items. Pellets from Barn owls are easily collected
since they typically regurgitate them at their nesting site.
Origin of the Pellets - Digestion: Swallowing the prey whole provides a
considerable challenge for the digestive system of the owl. A Barn Owl swallowing a
rat is equivalent to a human consuming a 15lb hamburger in one big gulp. The
digestive systems of birds are quite capable at mechanically and chemically breaking
down a large number of food items. In some respects the gizzard of birds is the
equivalent of the molars of a mammal. The hard and indigestible parts of the meal
remain in the gizzards as the softer parts continue into the intestinal tract. Due to
muscular contraction the hard parts get compressed together with fur into a pellet.
The pellet varies in consistency and size depending on the species. The soft fur (or
feathers) wrapped around the bones eases the passage of the remains through the
esophagus (see drawing). The pellets are regurgitated at a rate of about one to two
pellets every 24 hours (approximately 20 hours after feeding).
Logistics: Work in groups of 2 or 3 as assigned by your instructor. As you complete
your investigation of the pellet, answer the questions on your answer sheet. You
will need to share your data with the class by entering your information on a table
on the board. Be prepared to discuss your results with the class towards the end of
the lab or tomorrow.
Materials:
 Owl Pellet (1/group)
 Answer sheets (1/student)
 Dishes and trays
 Forceps and probes
 Dissection Scopes
 Owl Pellet Bone Chart
Owl Pellets Lab
Name_________________________________Per____
If you have to finish for homework, a copy of the BACKGROUND INFORMATION will be posted on the website.
1. Describe your pellet in terms of size, color, texture, and surface features. Sketch it.
2. Place the pellet in the tray/dish. Carefully tease apart the contents of the pellet by using the dissecting
needles. Take your time! The bones are small and heavily coated with fur. As the pellet unfolds you will need
to try to remove the fur from the bones.
3. Clean the parts of the remains as you remove them from the pellet. Transfer them to a separate plastic dish.
Use the dissection scope to study the parts in more detail if necessary.
4. List the remains that you find in the pellet. Sketch the bone (don’t just copy the bone chart’s picture!) and list
the source (organism) the bone came from. Because the vertebrae are hard to classify as certain
organisms, do not include them in your list.
Sketch
Source/ Organism
5. Answer the following questions:
a. How many species were represented?
b. How many individual animals did you find?
c.
Which types of bones are most prevalent?
d. How many pellets does the average adult barn owl produce per day? (see the background info sheet)
6. Analysis Questions- Base your answers on the findings from your own pellet
a. How much food does an adult barn owl consume during a year? Show your calculations.
b. Use the following information to answer the questions below: (Show your calculations!)
 One pair of adult barn owls produces 6 owlets during a mating season.
 Each owlet consumes two mice per day.
1. How many mice will the adult owls need to bring the owlets during the two months they
feed them in the nest?
2. How many mice will the owl family consume during one year?
c.
What kind of ecological impact does the presence of an owl family have on the surrounding
populations of rodents and birds?
d. The 10% rule: Only about ten percent of the energy in a trophic level actually moves on to the next
trophic level as one organism consumes another. The remainder is lost as heat, waste or movement.
Assume the following:
 The Food Chain: Grass-> Grasshopper -> Shrew -> Barn Owl
 The Barn owl is a tertiary consumer
 The grass assimilated 10,000 calories of energy via photosynthesis
Calculate the amount of energy transferred to the Barn Owl. (Show your calculations!) How
does your calculated number compare with the assimilated energy at the level of the
producers?
e. There is typically no trophic level above the owls in nature. Why is that? Explain.
f.
Where does the energy in food chains come from, originally? What happens to the energy
when a top consumer dies?
g.
Hoo eats who? (he he!) Draw a simple food web on a separate page (stapled to this page!) that
includes all the prey items discovered in the owl pellets. Feel free to include other organisms to make
the web complete. Label the organisms (at least 15) and include their role as a producer and
consumer (primary/ secondary/ tertiary and quaternary). Remember that an organism can act at many
trophic levels in the food web. Feel free to use available classroom and/or web resources if you need
to find out more about the ecological roles of the organisms you found in the pellet.