The Comparison Of Diets Of Owl Essay

, Research Paper

Comparing the Feeding Habits of Barn Owls and Screech Owls

Background

Owls are comprised of two closely related families in the avian order Strigiformes-the barn owls, or Tytonidae, and the typical owls, or Strigidae. Owls are relatively large birds, with a big head and short neck, a hooked beak, talons adapted to seize prey, and soft, dense plumage adapted for swift yet almost silent flight. Owls have large eyes located on the front of their face. The eyes are almost fixed in their sockets, so that the entire head must be rotated or bobbed for the gaze to be shifted and for distance to be assessed.

Owls have excellent hearing and very large ears, although these are covered by feathers and are not readily seen. The ears are placed asymmetrically on the head to aid in detecting the location of distant, almost silent prey. Their sense of hearing is probably also aided by their facial disk, which helps to focus sound waves onto the ears. An owl’s sense of hearing is so acute that it can accurately strike its prey in total darkness, following the squeaks and rustling sounds created by a small mammal in motion.

The sex of an owl is not easy to distinguish, although typically, female owls are larger than males. Owls begin to incubate their eggs as they are laid. When hatching occurs it is sequential, and different-sized young are in the nest at the same time. During years when prey is relatively abundant, all of the young will have enough to eat and may survive. In leaner years, however, only the largest young may be fed adequately.

(Egan, T. (1994). Oregon, Foiling Forecasters, Thrives as it protects owls. New York Times, October 11, pg A1, A19)

Most owls are nocturnal predators, mostly feeding on small mammals and birds, and sometimes on small reptiles, frogs, larger insects, and earthworms. A few specialized owls feed on fish. Owls are known to change their food preference, depending on local or seasonal availability. Most owls do not digest the fur, feathers, or bones of their prey, and they regurgitate these items as pellets, which can be collected at roosts and examined to learn about the feeding habits of these birds.

Barn owls

The barn owls are a distinctive-looking group, with a characteristic like facial disk of stiff, white feathers, dark eyes, long legs that distinguish them from typical owls. All barn owls are nocturnal predators, and their prey consists mostly of small mammals. There are nine species of barn owls (genus Tyto) and two species of closely related grass owls (genus Phodilus).

The most familiar species is the barn owl ( Tyto alba). The barn owl is one of the most widely distributed species of birds, occurring on all continents except Antarctica. The barn owl is the only representative of this family in the Americas, occurring uncommonly through most of the United States and in much of Central and South America. The barn owl nests in cavities in trees and in barns and abandoned buildings, and it hunts at dusk and at night over marshes, prairies, fields, and farmyards.

Screech Owl

The screech-owl( Otus kennicotti) is a relatively familiar species in woodlands of temperate regions. This 8.5″(22 cm) long species occurs in several color phases-grey, red, and brown-and it nests in cavities and sometimes nestboxes.

Owl Pellets

The average owl pellet contains bones and other non-digestible items such as hair and feathers. After the bird has eaten its prey; the prey is digested into the gizzard where grinding occurs. After grinding the bird’s gizzard makes a ball of the indigestible material, is expelled by mouth. An average owl can produce an average of one to two pellets a day.

John Day, Oregon

The John Day area is considered a semi-arid desert region. John Day, Oregon is located at Latitude 44.418427 and Longitude -118.949399, which is 125 miles south of Pendleton, Oregon in Eastern Oregon. A desert is an arid land area, one that loses more water through evaporation, than it gains through precipitation-rain, sleet, or snow. These areas generally receive fewer than 10 in (250 mm) of water annually. Average annual precipitation in the worlds deserts ranges from about 0.4-1 in (10-25 mm) in the driest areas to 10 in (250 mm) in semiarid regions. The term desert can also be defined by comparing the solar energy an area receives with the its annual precipitation

Desert animals

The many animals that have adapted to harsh desert life include insects, arachnids, reptiles, birds, and mammals. Unlike plants, these animals can seek shelter from the scorching sun, burning cold, and strong winds by crawling into underground burrows. Reptiles, whose body temperature is controlled by the temperature of their environment, travel between sunlight and shade to stay cool. Birds, utilize metabolic water, behavioral adaptations, active at day or cool parts of the day, may be nomadic and they may migrate during the harsher seasons.

Just as the seeds of desert annuals can stay dormant for decades, so can those of egg-laying desert animals. Amphibians and freshwater shrimp hatch, mature, mate, and lay their own eggs rapid succession in desert pools created by infrequent storms.

Some small mammals, such as rodents, excrete only concentrated urine and dry feces, and perspire little as a way of conserving body fluids. Desert insects protect themselves from hot dry conditions with a waxy coating, long legs that keep them elevated above the hot ground, and virtually moistureless excretions. (Watkins, T.H. (1995). Desert Extraordinare. Audubon 97: 44-54.)

My question is how do the Western Screech-owls and the Barn-owls diets compare. Both live in John Day region of Eastern Oregon, which make the same variety and amount of prey available to both of the owls. I hypothesis that their diet will be very similar if not the same due to the same variety of prey that live in John Day Oregon. Both of the owls have similar feeding habits that include prey like Vole, mice, birds and occasional snakes. I will compare a study done by Hancock Field Station in John Day Oregon on the types of food that the Barn-owl eats in accordance to what they find in the owl pellets verses what I find in five Western Screech-owl pellets.

When I looked at the bones in the five different Screech owl pellets, I was able to see what kind of animals they were from and thus use these results against the results of the Barn owl test done at Hancock Field Station. Comparing these two sets of data will help me answer my question of how the diets of Screech owls and Barn owls compare.

Materials and Methods

To find out how the Barn owl and the Western Screech-owl’s diets compare I collected owl pellets to find out what the owls are eating. I located Western Screech-owl pellets, but was not able to obtain any Barn owl pellets from Eastern Oregon. In lieu of Barn owl pellets, I located a study done by the Hancock Field station in John Day concerning the eating habits of Barn owls. I then collected five Screech owl pellets to dissect. When I finish dissecting the Western Screech-owl pellets I will hopefully be able to find out the average diet of a Western Screech-owl.

To find out what the owls’ diet using the owl pellet, the pellet must be dissected and the different bones and skulls examined. I used a dental pick and a pair of tweezers to separate the hair and feathers from the bones and skulls. Once all bones were separated from the hair, I identified animals by using size of bone, color of teeth, size of skull, and what kind of material surrounds the bones (i.e. hair or feathers.) Once the bones were identified I determined what type of animal. Using this technique on all five Western Screech-owl pellets I was able to get an indication of what the Western Screech-owl’s.

Results

The results I got were similar to the results the Hancock Field Station found on their experiment with Barn owls. When my study of the owl pellets was complete I had two pellets that were mice, two pellets that were Vole and one pellet which was of a bird. When I studied the remains of the owl in the pellets I identified all the bones that were present and came up with a conclusion.

In pellet one I removed the following:

Number of Bones Type of Bones

1 Skull

2 Pelvis Bones

2 Tibia with Fibula

2 Humerus

5 Lumbar Vertebrae

3 Thoracic Vertebrae

2 Cervical Vertebrae

6 Ribs

1 Xiphisternum

A way you can tell that this animal is a rodent is by the animal’s teeth being orange. Orange teeth are unique to rodents and rodents only. This animal also had a smaller longer and stouter skull, which ruled out Moles, Shrews and Voles.

In pellet two I removed the following:

Number of Bones Type of Bones

1 Skull

1 Tibula with Fibula and Femer

2 Pelvis Bones

1 Radius Ulna

6 Lumber Vertebrae

1 Sacrum

2 Humerus

1 Femur

2 Cervical Vertebrae

3 Thoracic Vertebrae

4 Ribs

1 Radius with ulna

This animal also had orange teeth, but the skull was longer and thinner. This ruled out a mouse and the orange teeth ruled out moles and shrews. The bones were also bigger than a mouse so my conclusion was it was a Vole.

In pellet three I removed the following:

Number of Bones Type of Bones

1 Keel

1 Tarsometatarus with Digits

1 Sternum

1 Tibiotarsus with Fibula and Femur

1 Claricle

1 Tibula

1 Femur

2 Carpus

1 Ulna with Radius

This pellet had no skull but had a talon and a keel, which made it a bird. There were different bones found in this pellet than any other pellet. The different bones included Claricle, Carpus, Tibiotarsus, Tarsometatarus and a Keel. These bones ruled out any other animal other than a bird.

In pellet four I removed the following:

Number of Bones Type of Bones

1 Skull

2 Tibula with Fibula and Metatarsal

1 Pelvis Bones

1 Metacarpals

1 Tail

2 Femur

2 Humerus

2 Radius with Ulna

1 Tibia with Fibula

1 Scapula

4 Lumbar Vertebrae

5 Ribs

1 Thoracic Vertebrae

The animal in this pellet had a thick and long skull with orange teeth. It also had a longer tail with both feet. The orange teeth made it a rodent and the size and shape of the skull ruled out a Mole, Shrew and a Vole. After careful analysis of the bones and the skull I determined that the animal whose bones were in this pellet was a mouse.

In pellet five I removed the following:

Number of Bones Type of Bones

1 Skull

2 Tibia with Fibula

2 Pelvis Bones

2 Humerus

2 Femur

3 Lumbar Vertebrae

1 Sacrum

1 Thoracic Vertebrae

2 Cervical Vertebrae

4 Ribs

1 Radius Ulna

This animal also had orange teeth, but the skull was longer and thinner. This ruled out a mouse and the orange teeth ruled out moles and shrews. The bones were also bigger than a mouse so my conclusion was it was a Vole.

When I finished identifying all the bones I wanted to get a full count of what I found and how many of a cretin type of bone I found. The following chart is all my data combined.

Number of Bones Type of Bones

4 Skull

2 Tibula with Fibula and Metatarsal

7 Pelvis Bones

1 Metacarpals

1 Tail

6 Femur

8 Humerus

4 Radius with Ulna

3 Tibia with Fibula

1 Scapula

12 Lumbar Vertebrae

19 Ribs

10 Thoracic Vertebrae

6 Keel

1 Tarsometatarus with Digits

1 Sternum

1 Tibiotarsus with Fibula and Femur

1 Claricle

1 Tibula

2 Carpus

1 Ulna with Radius

The Hancock Field Station surveyed 1,000 Barn-owl pellets. After dissecting them in the same way I dissected the Western Screech-owl pellets they came up with the following data:

Type of Animal Number of Animals

Vole 562

Mouse 284

Bird 154

(Hancock Field Station (1999) A Study in the Eating Habits of Barn-owls. Not Published)

After close evaluation of this data I came up with the ratio of 4:2:1 with Vole, mouse and bird respectively. After I evaluated the data given to me by Hancock Field Station I compared it to my ratio of 2:2:1 with Vole, mouse and bird respectively. I used my ratio against the ratio I got from Hancock Field Station and used it in a contingency table. A contingency table is similar to a chi-square analysis, but expected data is not required. Data is collected on two attributes of the elements in a study. When I entered my data into a computer I got G = 0.707, p = 1.14, df = 5 which means that the two samples are not statistically different.

Discussion

When I began this experiment I wanted to know if the diet of Barn-owls and Western Screech-owl which both live in similar places. I hypothesized that they would be similar because of the similar eating habits of both Barn-owls and Western Screech-owls. I found that my results supported my hypothesis. The results I got were a 2:2:1 ratio with Vole, mice and birds respectfully. They compared to the test done by Hancock Field station with the Barn owl; their results were a 4:2:1 ratio with Vole, mice and birds respectfully. My results do not fully reflect the eating habits of Screech owls that live in John Day. Due to the lack of owl pellets, I was not able to get a full representation of what Western Screech-owls eat. To fully represent the eating habits of the Screech owl, I would need 200-300 pellets.

This study is important to find out what the owls in John Day Oregon eat. The results I got showed what the Western Screech-owls eat verses what the Barn-owls eat. Scientists need to know this to see if there is enough prey in John Day to support both the Screech and Barn owl population. If there is a lack of prey there would be competition between Barn and Screech owls, and that might lead to a decrease of one type of owl in John Day Oregon. If there was a decrease in the owl population in John Day, it could possibly throw off the whole ecosystem in not only John Day but also all of Eastern Oregon. So knowing what the Barn owl eats verses what the Screech owl eats is imperative in keeping not only the prey population but also the ecosystem in Eastern Oregon where it should be. Ecosystems and communities within a cretin system take hundreds of thousands of years to develop. But if one animal becomes extinct due to the fact that it has to many predators, the ramifications of the animal’s extinction could last for hundreds of thousands of years. So it is important to know how many predators a cretin animal has so you can make sure there is enough prey for the predators. If you run out of prey, the predator starts to die off and a whole collapse of the food web happens and it has a huge effect on the whole ecosystem.

Some weaknesses that occurred in my experiment were a lack of Western Screech-owl pellets. Some other weaknesses might be the accuracy of the Hancock Field station experiment involving the feeding of Barn owls. To fully get an accurate result in this experiment I would need to collect 200-300 pellets from the Barn Owl and the Western Screech-owl. I would need to make sure that the pellets were gathered in the same area during the same time.

I conclude that the feeding habits of the Screech owl and the Barn owl are similar, but not exactly the same. Even though they might have a similar eating habits could not be exactly the same. Different Owls feed on different things according to the owls size, flight speed and eye sight. The Barn Owl might eat more mice and Vole seeing how it nests in a barn and not out in the open. The Western Screech-owl might eat more snakes and other outside animals. The nesting place effects the eating habits of Owls as do other factors. I can conclude that they are similar, but can never prove they are the same. Further experimentation would be needed to get an absolutely accurate result and comparison between the Barn owls feeding habits and the Screech owls feeding habits