ESS 350: Field Research Methods: Techniques in the science and management of wildlife fisheries populations, communities, and habitats
Assessment of Diet Lab
Assignment 6 20 Points
Due 10 November 2016
Quantifying the diet of an avian predator.
Purpose. The purpose of this lab exercise is to provide hands-on experience quantifying the
diets of animals. Students will learn how to process diet samples, how to quantify the
composition of the diet, and practice methods in comparing diets among potentially competing
species.
Introduction. In this lab, we will dissect owl pellets. Owl pellets are the regurgitated remains
of meals of these avian predators. Usually they are comprised of hard parts, fur, etc. that are
resistant to digestion. Pellets can be found on the ground under owl perch sites and are collected
from wild owls for classroom study. According to the supplier, these pellets have been
fumigated and pose no threat to human health.
Methods. You may be surprised at the quantity and diversity of material that you will find in
owl pellets once you start to tease them apart. Use the additional forms provided to keep track of
what you find and to identify what taxonomic group your prey items came from.
IT IS VERY IMPORTANT THAT YOU KEEP TRACK OF DIET ITEMS SEPERATELY FOR
EACH PELLET TYPE. We will compile all the diet information for the class and use that as our
estimate of diet for the species. You will be asked to determine several metrics to describe diet
(percent by number, percent by weight, frequency of occurrence, index of relative importance)
for your report. In addition to the diet metrics, you will use the Schoener’s Index and/or the
Bray-Curtis Index to compare the similarity in diet between our owls and the diet of red-tailed
hawks from the same area. The hawk diet data are provided in the table below. Use the percent
composition by numbers data to calculate the Schoener’s and/or Bray-Curtis Indexes to compare
the diets of our owls and the hawks.
REMEMBER to classify your owl diet items using the same classifications as in the table
below. We need you to add your data to the class table so we can compile all the data needed
for this lab. Use the Owl Pellet Bone Chart to identify what you have. The only classification
difference is to also classify “rodents” as “small” or “large” in your diet analysis so that it
compares with the hawk data below.
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Metric to summarize diet data.
1. Index of Relative Importance (IRI) from Pinkas et al. (1971).
IRI = (% by number + % by weight) * (%frequency of occurrence)
2. The Schoener’s Index (SI)
For your enjoyment, here is the Schoener Index so you may recall the formula and how it is
calculated (pxi = proportion by number of prey item “i” in predator species “x”; pyi = proportion
by number of prey item “i” in predator species “y.”)
𝑺𝑰 = 𝟏𝟎𝟎[(𝟏. 𝟎 − 𝟎. 𝟓(𝚺 |𝒑𝒙𝒊 − 𝒑𝒚𝒊 |)]
An example calculation of SI with simple diet data
Prey item (i)
Predator X
Predator Y
Px,i – Yy.i
|Px,i – Py,i |
red squirrel
0.40
0.20
0.20
0.20
woodrats
0.10
0.20
-0.10
0.10
flying squirrels
0.50
0.60
-0.10
0.10
SUM =
0.40
S.I. = 100 * [1 - 0.5 (0.40)]
= 100 * [1 - 0.20]
= 100 * [0.80] = 80
3. The Bray-Curtis Index (BC)
An example of how to calculate this index is given in the slides. Remember the BC index is the
same as Schoeners, but it takes count data and is a dissimilarity metric.
n
BC 
| ( X
X 2 j ) |
1j
1
n
(X
X 2j)
1j
1
The Red-Tailed Hawk Diet Data
Table 1. Diet composition (% by number) for 41 red-tailed hawks collected during May –
November 1999 from eastern Kentucky. Use this data to make comparisons with our owl data.
Prey
% by No.
Small Rodent
15
Large Rodent
18
Shrew
13
Mole
5
Bird
49
Total
100
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Other Parameters Needed to do % weight calculations.
Table 2. Use following average weights per individual for calculating total weights and percent
by weight for the owl diet data.
Prey Taxon
Large rodent
Gopher
Rat
Small rodent
Vole
Mouse
Shrew
Mole
Bat
Crabs
Bird
Mass (g)
150
150
150
30
40
19
4
55
7
20
15
OWL PELLET DATA SHEET
Date _____________
Investigator ________________
As a class, we need to decide what the best way is to quantify the number of animals represented
in each pellet. Once we do so, we need to record the numbers of animals per pellet for our diet
analysis. We will compile the lab data for the class and send it to each of you via an e-mail
attachment for your work in answering lab questions, etc. You may record your data in the table
below, but for index calculation purposes from the class dataset, we will arrange it in a different
format, on the computer up front.
Owl Pellet Data.
Prey Item
Gopher
Rat
Total large rodent
Vole
Mice
Total small rodent
Shrew
Mole
Bat
Crab
Bird
Small Pellet
Large Pellet
Ave. Mass per Ind. (g)
150
150
150
40
19
30
4
55
7
20
15
Est. Total Biomass (g)
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Lab Report Questions: 20 points Due 10 November 2016
1. Provide a table of the diet composition by %number, %weight, and frequency of
occurrence for the owl data for the whole class. Hint: Make a professional quality table
in a word processor. Before you do this, think about how the table should look (i.e., what
should be the column and row names). Use the tables above herein as examples to go by.
2. What is the dominant prey type for our owls:
a. By percent number? ___________
b. By percent weight? ___________.
c. By frequency of occurrence? ___________
3. Calculate the Index of Relative Importance (IRI) for the owl data. List the IRI value for
each prey type of the owl. Show an example of one calculation.
4. What does the IRI suggest is most important to owls in this area? _______________.
5. What was the difference between the large and small owl pellets in terms of diet content?
Were they very similar or different? In what way? Don’t just guess anecdotally. Calculate
a metric to support your answer. Hint: find the total number of each prey item for small
and large pellets, then proceed with a metric calculation.
6. Hypothetically speaking (i.e., not considering our data, but in general), answer and
discuss the following scenario: what would contribute most to an owl’s diet in a day—
consuming 10 voles each with a mass of 13 g, OR consuming 1 large rodent weighing
115 g? Is quality or quantity of prey more important?
7. Calculate a metric comparing the similarities of the diets of our owls and the hawks.
Show your work. Interpret the meaning of the result.
8. Hypothetically speaking, if the Schoener’s Index or Bray-Curtis similarity Index is high,
say 0.90, what does this tell us about the two avian predator species? Are they competing
for food? How can you tell? Why or why not?
9. Construct a graph in Excel that you believe is the most appropriate for comparing the diet
composition between the owls (class data) and the red-tailed hawk data.
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