Examining Trophic Relationships and Energy Transfer Using Barn Owl (Tyto alba) Pellets

This is done by using the average mass of a prey group and total number consumed to calculate the biomass of prey.. Using the average masses and the total number of each prey item cons

Examining Trophic Relationships and Energy Transfer

Using Barn Owl (Tyto alba) Pellets

Based on lab from Jim Serach, The Lawrenceville School, Lawrenceville, NJ

Part I

1 The data below from this year’s biology classes indicate the total number of each prey-type

found based on number of skulls or paired mandibles Utilizing the class data, use Excel to

construct a graph that shows the percentage of skulls of each type found.

2 It is also important to determine how much each prey group contributes to the total intake of 

the raptor. This is done by using the average mass of a prey group and total number 

consumed to calculate the biomass of prey. Using the average masses and the total number 

of each prey item consumed, extend your graph from part one by depicting the quantity 

(percent) of biomass consumed in each prey class

(g)

Conversion Factor

Frequency Found

Percent of Diet

Biomass Units

Percent of Biomass

White-footed

Mouse

Part II Energetics Calculations

1 The following data have been entered into the attached (at the end) Summary Data Spreadsheet:

a Number of Prey (D) – What is the total number of this species found?

c Number of Pellets (B21) – How many pellets were dissected and examined?

The spreadsheet will calculate the rest of the important information, such as proportion, number

of prey per pellet, and biomass We will need this information to answer several questions

about energy flow in this food web

2 What is the energy content of the top predator, the barn owl?

Organisms can be thought of as packets of concentrated, stored energy This information is

usually obtained by putting animals into a laboratory instrument called a bomb calorimeter

This device measures the heat released, in kilocalories per gram, when the animal is

completely combusted Since we do not have a barn owl to burn, we need to do some research According to the literature, non-migratory birds of several species have been shown to contain an average of

5.57 Kcal/g (Odum et al 1965) The average mass of a barn owl is 250 g

Calculate the average energy content (Kcal) of the barn owl

3 What is the annual energy requirement of a barn owl?

Organisms require an enormous and constant supply of energy in order to stay alive

Heterotrophs, like the barn owl, obtain their energy by consuming other organisms

According to the literature, non-nesting adult barn owls require 54.8 grams of food, in the form of small mammals, per day (Hamilton & Neill, 1981) We also need to know how much

energy is contained in a gram of small mammal prey According to Fleharty et al (1973), the

average energy content of the mammals upon which the owl feeds is 1.58 Kcal/g

Calculate the predicted the annual energy requirement (Kcal/yr) of the barn owl

4 How many small mammals does one barn owl eat in one year?

This would be easy if we had only one type of prey present in the owl pellet However, many species are eaten Assume that the owl produces 1.4 pellets/day (Higuchi &.Abe, 1997) Calculate the total number of prey eaten in one year

5. What is the total energy content of a year’s worth of owl food (assorted small mammals)?

To calculate this, we need to know the total mass (g) of the prey consumed in a year The

spreadsheet does this for us (Column H) by multiplying the No Prey/Pellet (Column F) by 511 (1.4 pellets/day x 365 days/year) – this gives us the number of that prey eaten per year – by the average mass of that species (Silva and Downing, 1995) The total biomass is calculated (G21)

We also need to know how much energy is contained in a gram of small mammal prey According to the literature, the average energy content of the mammals upon which the owl feeds is 1.58

Kcal/g (Fleharty et al., 1973) This will give us the energy content of the primary consumers

(herbivores) in this food web

Calculate the total energy content (Kcal) in one year of prey of the owl

6 What is the total energy content of a year’s worth of rodent food (grass)?

To do this we will assume that the only food eaten by the herbivores is grasses This is a

reasonable assumption for voles and we will assume other small rodents are like voles The energy content of the grasses in Michigan old field community was determined to average 4.08 Kcal/g (Golley, 1960)

Laboratory and field studies have shown that voles require about 17.5 Kcal/day (Evans, 1973;

Liu, 2003) Let’s also assume, for the sake of simplicity, that this energy requirement is typical for all the prey

a Calculate the amount of grass energy needed to feed a year’s worth of owl prey

b Calculate the grams of grass needed to feed a year’s worth of owl prey.

7 What is the efficiency of energy transfer from producer (grass) to primary consumer

(rodent) to secondary consumer (barn owl)?

According to the Second Law of Thermodynamics, whenever usable energy is transferred or

converted to another form (i.e., work is done), some of that useful energy is lost as heat Food webs are graphic representations of the transfer of energy within an ecosystem

a Sketch a pyramid of biomass and a pyramid of energy for this food web.

b Calculate the efficiency of energy transfer from producer to primary consumer (grass to

rodents)

c Calculate the efficiency of energy transfer from primary consumer to secondary consumer

(rodents to owl)

Dalgarn, M., and R Wilson 1975 Net productivity and ecological efficiency of Andropogon

scoparius growing in an Ohio relict prairie Ohio J Sci., 75(4):194-197.

Evans, D 1973 Seasonal variations in the body composition and nutrition in the vole Microtus

agrestis Jour Anim Ecol., 42(1):1-18.

Fleharty, E., M Krause, and D Stinnett 1973 Body composition, energy content, and lipid

cycles of four species of rodents J Mammal., 54(2):426-438

Golley, F B 1960 Energy dynamics of a food chain in an old-field community Ecol Monogr

30(2): 187-206.

Hamilton, K., and R Neill 1981 Food habits and bioenergetics of a pair of barn owls and

owlets Amer Midl., Nat 106(1):1-9.

Higuchi, A., and M Abe 1997 Estimation of food consumption from pellets cast by captive

Ural owls (Strix uralensis) In: Duncan, J, et al (Eds.) Biology and conservation of owls of

the northern hemisphere: 2nd International Symposium General Technical Report NC-190

St Paul, MN: U.S Dept of Agriculture, Forest Service, North Central Forest Experiment Station

Liu, H., D Wang, and Z Wang 2003 Energy requirements during reproduction of female

Brandt’s voles (Microtus brandti) J Mammal., 84(4):1410-1416.

Odum, E., S Marshall, and T Marples 1965 The caloric content of migrating birds Ecology,

46(6):901-904.

Silva, M., and J Downing 1995 Handbook of Mammalian Body Masses CRC Press, Inc., Boca

Raton, FL.

Swinker, A 2005 Vitamins and minerals in horse feed rations Penn State University Dairy Animal website http://www.das.psu.edu/news/fullstory.cfm?newsID=480

(Viewed 18 October 2006)

Tom-Tov, Y., & D Wool 1997 Do the contents of barn owl pellets accurately represent the

proportions of prey species I the field? The Condor, 99(4):972-976.

Turrentine, J 2001 Small Mammal Diversity in Contrasting High-elevation Habitats in Watauga County, North Carolina (Unpublished)

Yalden, D.W., and P.A Morris 1990 The Analysis of Owl Pellets The Mammal Society, Occ

Publ., No 13.