Investigation of NW vs. SE Owl Pellets
You have been provided pellets produced by the barn Owl (Tyto alba). Barn Owl pellets have been chosen because these owls swallow small birds and rodents whole, and the resulting pellets generally contain the complete skeletons of these 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 in the prey but leave the bony materials and hair or feathers undigested. Depending upon the prey eaten, the undigested portions may include beaks, claws, scales, or insect exoskeletons. This type of material has little nutritional value and must be passed from the body.
Predatory mammals such as bobcats and wolves have teeth to grind up bones and claws, and, a digestive tract adapted to pass these ground parts. Owls, on the other hand, do not have teeth for grinding and cannot pass whole bone and claws through their digestive tract safely. Instead, these materials from a bolus (or pellet) which is surrounded by the hair or feathers of the prey Consumed, The pellet is then orally expelled and the owl begins. feeding once more.
Scientists take advantage of this adaptation by collecting these pellets and examining their contents, Since owls are not very selective feeders, these pellets be used to estimate the diversity of available prey. The contents are also a direct indicator of what an owl has fed on; information that is crucial for species management and protection.
Data and questions you need to address are highlighted in violet. Additional information on Barn Owls and other owls can be found here.Using the dissecting needle provided, begin to loosen the hair of the owl pellet. As bones are uncovered, carefully remove them and place them to the side of your sheet of paper. Extra care should be taken to keep skulls intact and associated with their respective mandibles. If you prefer you can label a small sheet to hold the bones of each prey Item that you extract.
Attempt to keep the bones from each prey item separate from one another. After you have removed all bones, you can begin to identify the prey. Soak the bones in diluted bleach to whiten them. Keep the bones from each prey item separate from one another. After the bones have been cleaned, Arrange them on a piece of cardboard to reconstruct the skeleton (skull at the top; disjointed mandible on either side of the skull; then the cervical vertebrae, ribs, etc). Use white glue to attach the bones to the cardboard. Label with scientific name, source (Northeast or Southwest). Choose one of the prey items and then search the internet or other resources to determine the types of food your animal eats, who eats it, lifespan, reproductive potential, behavior, and range.
Using a Dichotomous Key
Procedure:
1. Coin edge smooth go to 2 Coin edge grooved go to 3 2. Coin copper in color Penny Coin silver in color Nickel 3. Picture of Roosevelt on front Dime Picture of Washington on front Quarter
Use the Dichotomous Key and Skull
Characteristics chart to identify the skulls of small mammals found in your pellets and
record the number of each type on a worksheet labeled "Pellet Contents". If
birds, insects, etc. are found, then no further identification is needed.
A skull chart to help you with some of the terminology is here: 
Note: Undigested beetles and pillbugs are sometimes found in owl
pellets. These ore small animals that find expelled raptor pellets and use them as a food
source and nursery for their eggs, and/or larvae. Therefore, these organisms should nor be
included as owl prey.
NOTE: After pressing the skull chart key (above) use your
browser's forward and back keys to flip between this page and the skull chart.
Start Here!! 
1. No gap (diastema) between incisors and cheek teeth: Go to Order Insectivora (Moles and shrews); go to key below.
2. Gap (diastema) between incisors and cheek teeth: Go to Order Rodentia (Rats, voles, and mice); go to key below.
Order insectivora (Moles and shrews)
| 1. | Zygomatic arch complete, skull flat and broad | (Moles) Go to 2 |
| Zygomatic arch not complete, skull not flat and broad | (Shrews) Go to 3 | |
| 2. | Upper teeth 10 on each side: mandibular teeth 8 each | (SE Mole) Scalopus |
| Upper teeth 10 on each side: mandibuIar teeth 11 on each side | (NW Mole) Scapanus | |
| 3. | Skull 1.0 to 1.5cm total length; upper teeth 9 on each side | (Least Shrew) Cryptotis |
| Skull greater than 1.5cm total length; upper teeth 10 on each side | Go to 4 | |
| 4. | Skull robust; second and third teeth of same size and larger than fourth and fifth teeth which are also of the same size; sixth tooth minute and hidden from lateral view | (Short Tailed Shrew) Blarina |
| Skull delicate; second through fifth teeth not distinctly paired by size but almost uniform; sixth minute but clearly visible from lateral view | (Shrew) Sorex |
Order rodentia (Rats, voles, and mice)
| 1. | Infraorbital canal present | Go to 2 |
| Infraorbital canal not present | Go to 3 | |
| 2. | Upper incisors distinctly grooved | (Pocket Mouse) Perognathus |
| Upper incisors not distinctly grooved | (Pocket Gopher) Thomomys | |
| 3. | Skull flat and broad; cheek teeth acutely angled and may appear as one continuous tooth | (Pine Vole) Microtus |
| Skull generally rounded; cheek teeth lobed or rounded and easily distinguished individually | Go to 4 | |
| 4. | Upper incisors distinctly grooved | (Harvest Mouse) Reithrodontomys |
| Upper incisors not distinctly grooved | Go to 5 | |
| 5. | Posterior edge of palate ending well beyond last cheek teeth | Go to 6 |
| Posterior edge of palate ending even with or slightly beyond last cheek teeth | Go to 8 | |
| 6. | Upper incisors notched; anterior palatine foramina extend well beyond anterior edge of cheek teeth | (House Mouse) Mus |
| Upper incisors not notched; anterior palatine foramina do not extend well beyond anterior edge of cheek teeth -------------- | Go to 7 | |
| 7. | Posterior palatine foramina obvious and located just beyond last cheek teeth | (Rice Rat) Oryzomys |
| Posterior palatine formina minute and located between second pair of cheek teeth | Rat) Rattus | |
| 8. | Zygomatic plate undercut and having a distinct dorsal protrusion; second and third pair of cheek teeth sigmoid or "S" Shaped | (Cotton Rat) Sigmodon |
| Zygomatic plate not undercut; cheek teeth not sigmoid | (Deer Mouse) Peromyscus |
Diet of a Barn Owl
Procedure: There are many genera of prey that occur in
both the Northwest and Southeast, as well as several that a re exclusive to one of these
areas. The following chart contains 14 mammalian prey types that should account for
96-100% of the prey found in your investigations. Any other prey will be composed of
birds, bats, insects, crayfish, and small reptiles. There are occasional and too diverge
to address in detail. These prey are listed as "other prey" at the bottom of the
worksheet. Your data should be e-mailed to me so I can make the class data available on
the net. Put the data in an Excel spreadsheet following the format in the Owl Prey Chart
shown below. Data not following this format will be returned. When all the data are in I
will post the class data for the remainder of the analysis.
The number of asterisks [*) placed next to each prey type under both geographic regions represents the frequency of occurrence for each prey type as shown in the following scale:
(****) - Very common
(***) - Common
(**) - Occasional
(*} - Rare
( )- Does not occur
Owl Prey Chart
After the owl pellets have been dissected, send your instructor the class totals for each type of prey in both geographic regions. You can get the current class totals HERE. Use these data (not simply your own resutls) for the analyses. After these have been determined, record these in the 'Total" column of your worksheet. Use these numbers to multiply the "prey weight" for each prey. For example, if you record "5" in the Total column for Thomomys, multiply by the prey weight of 150g: 5 x 150g = 750g. This is the total weight or biomass that the prey contributed to the diet of the Northwest Barn Owl. Thomomys does not occur in the Southeast, so this column space is filled with a line (---). This should be done for each prey type found for both geographic regions. Your should also add up all the types of prey in a certain category/ for example, all of the rats. This should include Sigmodon, Oryzomys, and Rattus.
Questions
Which type of prey contributed the
most by number for each region? Did the same prey type contribute the most by number for
both regions?
Which prey type contributed the most
In biomass for each region? Did this same prey type contribute the most in biomass for
both regions?
If an owl needs 100g of food per day,
how many Sorex does it need to capture? How many Sigmodon?
Assume an owl eats 100 1g insects and
one 100g rat. Which prey contributed the most to the owl's diet (explain)?
Is quantity or quality of prey more
important (why)?
Constructing a Food Web
Procedure: A food web is a
relationship diagram showing organisms arranged by energy flow from organisms at lower
trophic (feeding) levels to higher trophic levels. Energy passes from one organism to
another on a higher level through the: consumption of that lower organism. With the Barn
Owl, for instance, energy is passed from grass and seeds to insects which are in turn
eaten by birds, who are finally consumed by the owl. There arc many different combinations
in how energy is passed from one trophic level to another. Several examples are shown in
the diagram below.
There are usually three trophic levels found in a food web. The first is a primary consumer, which consumes photosynthetic products such as grass. Secondary consumers on the next trophic level are carnivores (meat-eaters) that also eat herbivores (plant-eaters). Tertiary consumers a carnivores that eat other carnivores/ and are usually found at the top of the food chain.
Construct two food webs- 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. One food web should be constructed for the Northwest and another for the Southeast. Here Is one example of a food web:
Based on the class data, perform a diversity analyses based on the class data for each region. Discuss your results.
Perform Diversity
Calculations
Diversity (and thus stability) increases as one nears the equator. For example, Alaskan owls have a very limited selection of prey types and feed almost exclusively on Lemmings (Lemmus). Therefore a decline in the lemming population will seriously affect the resident owls. Owls near the equator have a large diversity of prey from which to choose and typically feed on 15-20 prey types ranging in size from insects to opossums (Dedelphis), While a particular species of owl may feed more heavily on one of these prey than the others, a decline in any one of these prey will not likely have a major impact on overall prey availability. This principle of greater diversity toward the equator can be seen (but with less dramatic differences by comparing pellets from the Northwestern U.S. (Washington, Oregon) to those from the Southeastern U.S. (Mississippi, Alabama, Louisiana).
Questions