APES
Investigation of NW v. SE Owl Pellets
Purpose: To dissect one Barn Owl pellet from the NW U.S. and one Barn Owl from the SE U.S.
To use a dichotomous key to identify the skulls obtained from the raptor pellets.
To study the contributions of various prey to the diets of Barn owls. This will be looked at from the
perspectives of numbers and biomass.
To construct a food web with a barn owl at the highest trophic level, and grass and seeds at the base.
To analyze the Diversity and Stability of Barn owls from the different regions.
BACKGROUND
All organisms, whether dead or alive, are potential sources of food for other organisms. The sequence of
organisms, each of which is a source of food for the next, is called a food chain. It determines how energy and
nutrients move from one organism to another through an ecosystem. Because most species participate in several
different food chains, the organisms in most ecosystems form a complex network of interconnected food chains
called a food web.
Barn owls pellets have been chosen because these owls swallow small birds and rodents whole, and the resulting
pellets generally contain the complete skeleton of its prey. Pellets begin forming within the digestive tract of an owl
as soon as the prey is swallowed. Enzymatic juices break down the body tissues of prey but leave the bony materials
and hair or feathers undigested. Owls have no teeth for grinding and cannot pass whole bones through their
digestive tract safely. Instead, these materials form a bolus (or pellet) that is surrounded by the hair or feathers
of the prey consumed. The pellet is then orally expressed and the owl begins feeding again.
Scientists take advantage of this adaptation by collecting these pellets and examining their contents. Since owls
are not very selective feeders, these pellets can be used to estimate the diversity of available prey. The contents
are also a direct indicator of what an owl has feed on; information that is crucial for species management and
protection.
Materials: 1 Northwest owl pellet, 1 Southeast Owl pellet, 2 dissecting needles, dichotomous key of Skulls of small
mammals found in Barn owl pellets, Owl Prey Chart, Skull Characteristics Sheet, sorting sheet
Procedure:
1. Two owls pellets per group of two. Label a blank sheet of paper with your name and the region of the owl to
help keep them separate.
2. Use dissecting tools and fingers to gently pry apart the pellet. NOTE: The bones you are looking for are
small and easily broken.
3. Set aside any bones found. Try to assemble an animal skeleton.
4. Analyze the bones with the dichotomous key and identify prey. (Undigested beetles and pill bugs are
sometimes found in owl pellets. These are small animals that find expelled raptor pellets and use them as a
food source and nursery for their eggs and larva. Therefore, these organisms should not be included as owl
prey.)
5. Remember to wash hands at the end of the procedure. These pellets have been sterilized, but…
6. Use The Owl Prey Chart to record student observation and class totals. Calculate total biomass.
7. Complete analyzes of the Diet of the Barn Owls,
8. Construct two food webs, one for the NW and one for the SE. The webs should contain a Barn owl at the
highest trophic level, and grass and seeds at the lowest. The intermediate organisms that you show should
include only those prey found by your class.
9. Analyze the Diversity and stability of barn owls.
APES
Investigation of NW v. SE Owl Pellets
Results:
Owl prey Chart, Food Webs
Analysis:
 Questions on Diet of a barn owl and Diversity and Stability.
CONCLUSION
 Briefly summarize your results.
SUGGESTIONS FOR FURTHER INVESTIGATION
 What changes would you make to this lab to advance your studies on this subject matter?
APES
Investigation of NW v. SE Owl Pellets