OWL PREDATION LAB

Objectives:

  • Determine the relative frequencies of prey types in the barn owl’s diet at 3 separate locations.
  • Compare the prey frequencies among locations to see if diet changes.
  • Calculate Chi square analysis and recognize its limitations
  • Calculate a percent similarity

Introduction:

When animals harvest food, they have to make conscious or unconscious choices, e.g., choices about when to hunt, what prey to seek, how long to spend choosing a prey individual, etc. In general, there is a strong evolutionary pressure to make choices that increase the efficiency of predation. For example, predators like the tawny owl are more selective about prey when food is abundant,e.g, in the summer, then when it is scarce. Some predators, like backswimmers, adjust their preferences to match the availability of different types of prey; this is called switching (Smith and Smith, 2009). Many predators eat predominately the young, old, and sick individuals in a population. These individuals require less energy to capture.

The Barn owl, Tyto alba, is a predator with a world wide geographic range in temperate and tropical regions (Taylor, 1994). While the fossil record is poor, it seems likely that the species originated in warm savannahs or lightly wooded environments and then expanded its range following the spread of human settlements and farming. While it’s natural nesting location is in hollow trees and cliffs, today it prefers human structures like old buildings, barns, and towers, hence the name. Barn owls predominately feed on small rodents; i.e. mice, shrews, and voles. But they are generalists in that they will eat whatever they can subdue, this includes smaller birds, insects, even bats and frogs. (Taylor, 1994; Roulin, 2002) Barn owls (and most other owls) hunt by using their acute sense of hearing. Their long wings and legs make them well adapted to hunt over open areas like grasslands and farms. Their numbers in the US and around the world have been declining recently due to rodenticides and the decrease of farmland (Taylor,1994 )

The barn owl diet has been studied in more detail and more extensively than that of any other bird of prey. This is probably because of the species widespread distribution and close association with human beings. Also, after digestion the owl regurgitates pellets of undigested fur, bones, and exoskeletons of its preys, which makes it easy to determine exactly what an owl has eaten. Small prey are swallowed whole, whereas larger animals are first torn apart and then eaten. Owls and other carnivorous birds digest the soft organs of their prey fairly rapidly over several hours. The digested material is passed to the small intestine, while the undigested contents, the bone, fur and feathers become trapped in the stomach and are gradually compacted into a “pellet”. The pellet is usually coughed up(regurgitated) within about 12 hours. Because barn owls roost at the same locations every day, owl pellets accumulate underneath the roost. Collectively they provide a history of an owl’s recent diet (Roulin, 2002). By collecting and dissecting the pellets one can identify the prey and analyze different aspects of the predator/prey interaction.

The pellets we have are from 3 different locations along the owl’s pacific coast range. Using the data gathered today we will be able to answer the following questions.

  1. Are there are differences between the prey choice and prey frequency between locations?
  2. What is the overall percent similarity of prey choice between locations?
  3. Is there is a preference for juvenile prey over adult prey?

The pellets have been fumigated and are safe to handle. However, gloves are available for you.

References:

Roulin, A. 2002, Barn Owl, Tyto alba, Birds of Western Palearctic Update 4, pp. 115-138

Smith, T.M. Smith R.L. 2009. Elements of Ecology. 7th Edition. Pearson/Benjamin Cummings, San Francisco, CA.

Taylor, I. 1994. Barn Owls: Predator-Prey Relationships and Conservation, Cambridge University Press, Cambridge.

Methods:

  1. Look around the room at the dissecting scopes set up with identified bones to familiarize yourself with what you will be looking for. Also take note of the difference between and adult and juvenile femur
  1. Obtain 5 (or more) pellets from the location your instructor specifies.
  1. Using your probe and forceps pry apart the first pellet to find bones and exoskeletons. Concentrate on finding skulls, mandibles (jaws), and femurs. You do not need every little bone.
  1. Look at the skulls and mandibles first. Use the bone sorting chart (Figure 1) and the flow chart (Figure 2) to identify the prey group consumed. Mandibles often break into 2 pieces, try not to overestimate the numbers. i.e. a shrew: you may see 1 skull and 2 mandibles –you record only 1 shrew.
  1. Identify each femur as a juvenile or adult (species does not matter). Count gross total on your data sheet. Do not divide by 2 or 4.
  1. Record class totals

Data Sheet: OWL PREDATION

Name:______

Prior to Class:

1. Read and review Chi Square section of your statistics handout (pages 6-9).

2. What null hypothesis(es) will you be testing in this lab?

I. Observed Prey Consumed:

Locations
PREY / Totals
vole
mouse
shrew
mole
rat
bird
insects
other
Totals

Prepare a bar graph of the frequency distribution as a function of prey type for each location.

3. Compare your data from the 3 locations. Do they differ? In what way?

4. Using the formulas from your statistics hand out (pgs 6-9), you can calculate the probabilities of each prey at each location, and the expected values. (this can be done by hand or on Excel)

5. Calculate the Chi square for the entire table ______

6. How many degrees of freedom are there? _____

7. Is the Chi Square number statistically significant (p<.05)? hint: use the table in the statistics hand out.______

8. What does this mean about the prey selection from the different locations? Is your null hypothesis supported?

II. Percent similarity:

  1. Using the data from Part I, calculate the percent of each prey type at each location. i.e. 22 voles out of 43 total prey in Northern California is 0.51 (51%) and 15 voles out of 44 total prey in Washington is 0.34 (34%).

Locations

PREY
vole
mouse
shrew
mole
rat
bird
insects
other
  1. Use the formula [pi-qi]/2 to compare any 2 locations for prey type.

i.e. voles compared between northern California and Washington =[0.51-0.34]/2 =0.085

  1. Sum all of the prey comparisons
  2. Calculate Percent similarity= 1-Sum [pi-qi]/2

Note: You can only compare 2 locations at a time so you will be doing 3 different comparisons

Location x location / Percent similarity

III. Observed Ages of Rodent Prey:

ADULTS: Femur will be rounded at ends

JUVENILES: Femur will be jagged b/c they often break at the “weaker” growth plates.

LOCATION
AGE / Totals
juvenile
adult
Totals
  1. Using the expected probability based on reproduction of rodents (calculated with instructor) Determine the chi square statistic ______
  2. Degrees of freedom?______
  3. Is it statistically significant? (p>0.05)______
  4. What does this mean about the prey selection of barn owls on juvenile vs. adult prey?