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How to Calculate the Percent of Biomass of Owl Pellets

Owls devour their prey whole. Hard or indigestible parts of the prey such as fur, feathers or bones form into a pellet inside the owl, which it later coughs up. Naturalists dissect owl pellets to discover exactly what a bird has eaten and to calculate the proportion--or percentage biomass--of each type of prey. According to the World Owl Trust, owls are opportunists, preying on almost any creature smaller than themselves. Typical prey include insects and rodents, but some reports suggest the largest owls are capable of taking foxes, deer and even golden eagles.

Things You'll Need

Owl pellet
Small plastic dish
Sheets of white paper
Large needle
Tweezers
Dichotomous key
Skull chart
Prey chart
Calculator

Instructions

Dissection
- 1
  Place the owl pellet in a small plastic dish. Lay sheets of white paper around the dish, to receive the contents of the pellet as you proceed with the dissection.
- 2
  Jab the pellet with a needle to remove the hair or feathers that form its outer surface and to uncover harder items, such as bones, in its center.
- 3
  Use tweezers to gently remove material from the center of the pellet. As far as possible, keep any skulls whole.
- 4
  Lay the remains of each prey animal on its own separate sheet of paper. Count the number of skulls, or skull parts such as jawbones or beaks, to work out how many animals contributed to the formation of the pellet.
- 5
  Consult a dichotomous key and a skull chart to identify the prey animal to which each skull belongs. Hartnell College has a guide to dissecting owl pellets that includes a suitable key and chart. A link to the Hartnell College guide appears in the Resources section of this article.
Calculation
- 6
  Count how many of each type of animal was present in the pellet. Check the average weight of this type of animal on a prey weight chart. A suitable chart is also available in the Hartnell College guide (see Resources).
- 7
  Multiply the number of each type of animal you have discovered by its average weight. For example, the remains of two pocket gophers multiplied by a gopher's average weight of 150g would give a total "prey weight" of 300g. Repeat this calculation to establish the prey weights of all the different types of animal present in the pellet.
- 8
  Add all the individual prey weights together to find the total prey weight of the pellet. This calculation is an estimate of the actual weight, or biomass, of all the creatures the owl consumed as it formed the pellet.
- 9
  Find the percent of biomass for a particular type of animal by dividing its prey weight by the total prey weight of the pellet and then multiplying the result by 100. You may find a calculator helpful. For example, for two pocket gophers (prey weight 300g) in a pellet with a total prey weight of 500g, the steps in the calculation would be: 300 ÷ 500 = 0.6 and then 0.6 x 100 = 60. This would mean pocket gophers made up 60 percent of the biomass of the pellet.

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