Owls pellet by mikeholy


									Biology 41                                                           Name:____________________
West Valley College
                  The Ecology and Natural History of Owl Pellets
          To   become familiar with owl pellets, their origin, as well as their function.
          To   identify prey items in owl pellets
          To   understand how owl pellets can be used in an ecological context.
          To   understand the concept of energy transfer in food webs.
         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
Biology 41                                                                             Owl Pellets
West Valley College

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 Pairs during the exercise. As you complete your investigation of the
pellet you will need to share your data with your table, as well as with the class by entering your
information on a table on the blackboard. Be prepared to discuss your results with the class
towards the end of the lab.

    Owl Pellets (1/pair)
    Dishes and trays
    Dissecting Needles
    Dust Masks
    Plastic bags (small)
    Dissection Scopes
    Identifying Owl Pellet Contents/ Key to Skulls of Common Barn Owl Prey (in handout)
    Skeletal Anatomy of Voles and Shrews (in handout)

A. Describe your pellet in terms of size, color, texture, and surface features. Sketch it!

Biology 41                                                                                  Owl Pellets
West Valley College

   B. Locate the pages at the back of your handout referred to as “Identifying Owl Pellets
Contents”, “Vole Skeletal Anatomy” and “Shrew Skeletal Anatomy”. Spend some time to become
familiar with the different types of bones in terms of their size and shape before you continue.
The time is well spent since you will have to try to identify and recognize them as you dissect the
pellet. Focus on the skulls since they directly help you to identify the animals in the pellet. We
have also provided several containers from previous dissections to give you an idea of what you
can expect to find in your own pellet.

    C. 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.
    The fur easily piles up as you “unwrap” the bones. Try to discard the fur in the provided
plastic bag as you work yourself through the pellet. Do not inhale the fur. If you are allergic
against fur (rodents) you should try to keep your breathing to a minimum as you dissect the
pellet. 

   D. 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.

   E. Describe the remains that you find in your pellet. Provide a brief description of the major
items in the table. Identify the parts to the best of your ability with the aid of the provided keys.

Item #            Brief Description              Source                         Sketch
                                               (Organism)               (do not copy the keys)




Biology 41            Owl Pellets
West Valley College







Biology 41                                                                           Owl Pellets
West Valley College

F. Summarize your findings (type of organisms and the number of each). Transfer the
   information to the board. In addition, answer the following questions:
- How many individual animals did you find?
- How many species were represented?
- Did you only find mammals?
- Which types of bones are most prevalent?

G. Record the total class numbers in the table below. Number of Pellets Dissected:

      Type of Organisms       Total   Averag                     Comments








       Other Prey Items

Base your answers on the findings from your own pellet

1. How much food does an adult Barn Owl consume during a year? Show your calculations.

Biology 41                                                                             Owl Pellets
West Valley College

2. Use the following information to answer the questions below:
    One pair of adult barn owls produces 6 owlets during a mating season.
    Each owlet consumes two mice per day.
                          Show all your calculations for the questions below

     a. How many mice will the adult owls need to bring to the owlets during the two months they
        feed them in the nest?

     b. How many mice will the owl family consume during one year?

3.   Hoo eats Who? Draw a simple food web in the space below 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 the available classroom resources if
     you need to find out more about the ecological roles of the organisms you found in the pellet.

Biology 41                                                                               Owl Pellets
West Valley College

4. What kind of ecological impact does the presence of an owl family have on the surrounding
   populations of rodents and birds?

The 10% rule: Only about ten percent of the energy on 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

5. Calculate the amount of energy transferred to the Barn Owl. How does your calculated
   number compare with the assimilated energy at the level of the producers?

6. There is typically no trophic level above the owls in nature. Why is that? Explain.

    We will review the information in class. Be prepared to contribute to the class discussion.
       In case you are done with the questions you will need to clean up your lab table.
                         Place the bones/fur from your pellet in the trash.


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