Owl Pellet Food Webs
AUSTRALIAN CURRICULUM ALIGNMENT:
- Ecosystems consist of communities of interdependent organisms and abiotic components of the environment matter and energy flow through these systems
- Exploring interactions between organisms such as predator/prey, parasites, competitors, pollinators and disease
Owls are birds of prey that hunt during night-time, and frequently swallow their catch whole. When they are not able to swallow their prey whole, it is ingested in large pieces. After it is swallowed, the prey travels down to the Owl’s stomach. Their unique stomach physiology includes the Proventriculus (the upper portion) and a Ventriculus or Gizzard (lower portion). During the first stage of digestion, the acids and enzymes within the Proventriculus partially digest the prey. The remains then travel to the Gizzard, whereby the indigestible parts of the prey; such as hair, bones, teeth, and feathers, are compressed to form a matted pellet. The pellet then re-enters the Proventriculus to remain there until the Owl regurgitates it as a pellet. Common barn Owls typically produce 1 or 2 of these pellets a day. Owls are nocturnal hunters and survive on a diet of rodents, such as Mice, Rat and Voles. Due to their hunting efficiency and ability to reduce rodent populations, some farmers intentionally attract Owls to inhabit their land by constructing artificial nests.
In this practical, students study barn Owl pellets to develop a food web model. Food web models map the trophic (feeding) networks between species. Like food chains, food webs map predator prey relationships; however, food webs show energy relationships among plants and organisms. Owl pellets are perfect to study Owl diet, prey region and habitat. Their pellets can be used to determine relative numbers of small animals found in an Owl’s feeding area. For example, a study on 200 pellets from a barn Owl which produced 454 mammal skulls showed 225 Voles, 179 Mice, 20 Shrews, and 20 Rat. Students conduct a similar study by dissecting an owl pellet and identifying the origin of the bones located within it. After counting the number and variety of prey found within their pellets, students develop a food web and calculate the average diet of barn Owls using the class data. Using the food web and simple calculations, students track energy flow and biomass in Owls to explore the concepts of matter and energy conservation, interdependence between organisms within ecosystems, and predator/prey relationships.
Barn Owl Pellet Dissection
Unwrap your Owl pellet and place it on a clean tray. Note the colour, size, and texture of the pellet.
- Carefully separate the bones from the fur using wooden probes and dissecting forceps.
- Identify and categorise the bones based on type of bone and the animal it came from. Use a bone sorting chart to do this.
- Record this information in Table 1.
- Use the table below and the results of your Owl dissection, to draw a simple food web for the barn Owl. Ensure you label the trophic levels and class of consumer. For plants, write the sun down as the energy source.
Calculating Barn Owl Diet
- An average barn Owl’s diet is approximately 50% Voles, 40% Mice, 5% Rat, and 5% Shrews. Barn Owls typically regurgitate 2 pellets a day with each pellet generally containing 4 skulls. Based on the results of your dissection, determine the type and quantity of animals the Owl consumed over a 24-hour period. Gather data from another group to represent the second pellet. Using only whole numbers, record the figures in the data table in Table 2.
- To continue to fill out the table, calculate the following:
Total mass of food consumed by the Owl daily.
- Percentage of mass each type of consumed prey contributes to the Owl’s total daily consumption.
- Based on the assumption the Owl weighs 550g, calculate the percentage of the Owl’s body weight it consumed.
- Using the consumption requirements column in the primary diet of barn Owl’s table, identify the mass of food each of the Owl’s prey need to eat daily.
- Total mass of food consumed at every trophic level outlined in the food web.
OBSERVATIONS AND RESULTS
Table 1: Owl Pellet Dissection Results
Individual results will vary. Below is an example of expected results.
Barn Owl Food Web
Based on the Owl pellet dissection results, the simple food web should resemble the figure below, with the trophic levels and type of consumer labelled. The sun should be included as the energy source for the plants.
Table 2: Barn Owl Diet Calculations
The below results are based on an Owl that consumed 50% Voles, 40% Mice, 5% Rat, and 5% Shrews. The figures below are to be used as a guide only as the quantities and calculations will depend on the individual animals.
Total mass of food consumed by the Owl daily: 547 g
- Percentage of mass each type of consumed prey contributes to the Owl’s total daily consumption: See table above.
- The percentage of the Owl’s body weight it consumed, based on the assumption the Owl weighs 550 g: (547 g / 550 g) × 100 = 99.5%
- Mass of food each of the Owl’s prey need to eat daily: See table above.
- Total mass of food consumed at every trophic level outlined in the food web: Primary and secondary consumers require roughly 156 g of food daily (assuming Shrews are not included in the food web). The Owl requires approximately 547 g of food daily.
- Ask students to consider how the pattern of food mass consumed for the primary and secondary consumer trophic level can be explained using the laws of conservation of matter and the conservation of energy.
- Ask students to consider how would the food web be affected if an Owl were only able to consume rats. Discuss whether this would alter the web’s amount of trophic levels and the percentage of initial energy transference to the Owl.
- Continue to check student calculations throughout the activity. You may wish to provide students with a formula sheet for percentages and converting percentages into decimal numbers.
- 45 mins
- Apron required
- Safety Gloves Required
- Safety Goggles Required
- Southern Biological Owl Pellets have been sterilised, and therefore are safe to use. They are not subject to any restrictions on use or disposal.