College of Education & Human Services

Owl Pellet Dissection

INTRODUCTION: In this activity, you will learn about owls, predator/prey relationships, the use of dichotomous keys, animal adaptations (i.e. behavioral and physical), and information regarding owl habitats. You will spend time dissecting and examining the contents of an owl pellet.

CONCEPTS: (1) Owls have acquired physical and behavioral adaptations to be successful in their environments. (2) Dissecting owl pellets provides information about owls’ habits and their role in nature.

OWL ADAPTATIONS: Owls are a type of carnivorous bird called raptors. Other raptors include hawks, eagles, falcons, and ospreys. Most raptors hunt small mammals, birds, amphibians, reptiles, and larger invertebrates. All raptors possess similar adaptations that allow them to hunt: they are strong and agile fliers; they have keen eyesight to locate prey, strong legs and talons for capturing prey, and hooked bills to tear flesh. Although owls share these characteristics, they have adapted to their nocturnal lifestyle. Other species of raptors are active primarily during the day so they have not evolved these specialized adaptations to be active at night.

OWL ADAPTATIONS: Owls are a type of carnivorous bird called raptors. Other raptors include hawks, eagles, falcons, and ospreys. Most raptors hunt small mammals, birds, amphibians, reptiles, and larger invertebrates. All raptors possess similar adaptations that allow them to hunt: they are strong and agile fliers; they have keen eyesight to locate prey, strong legs and talons for capturing prey, and hooked bills to tear flesh. Although owls share these characteristics, they have adapted to their nocturnal lifestyle. Other species of raptors are active primarily during the day so they have not evolved these specialized adaptations to be active at night. 

(1) Eyesight: The size and structure of owls' eyes allow for amazing visual ability. Owls' eyes are so large that there is little room for muscles inside their eye sockets needed to move their eyes. To compensate, owls are able to turn their heads 270 degrees. Larger eyes allow for more light to enter the pupil, which is the round center of the eye. The owl "sees" when light contacts rod and cone cells at the back of the eye. Rod cells gather light and are needed when trying to see in low light conditions, but they do not distinguish color. Cone cells allow for color vision. Since most owls hunt at night, their eyes contain mostly rod cells; therefore, they can only see in black and white. Owls focus on prey with binocular (two-eyed) vision, which means that each eye sees an object from a slightly different angle. The two views overlap, triangulating the object in the owl’s sight, and allowing them to judge exact distance. Their sight is so focused that an owl could read a newspaper a mile away by the light of a candle (if it could only read). Although owls can see distant objects clearly, they cannot see close objects well. Owls will bob and turn their heads to change their viewpoint when focusing on close objects. This behavior makes near objects stand out from the background so they can judge the object’s location more accurately. 

(2) Hearing: Owls have large, oval-shaped ear openings bordered in the front with a fleshy, raised flap called an operculum. This is similar in function to a hand cupped over the front of a person’s ear to capture and amplify sounds from the rear. Many owls, such as the Barn Owl, have facial feathers shaped similar to a satellite dish that collect and direct even the lowest intensity sound waves down the ear channel into the ear. The unusual positioning of owls’ ears also allows them to locate sounds. One ear is higher and points slightly more forward than the other. This asymmetrical positioning of the ears produces a difference in the time that it takes for a sound to reach each ear. Owls can detect the exact distance and location of sounds by analyzing the time difference between sounds. For example, if a sound arrives to the higher ear first, the owl knows that it came from above. 

(3) Silent Flight: Not only can owls see and hear their prey, their prey cannot hear them coming. The owl has a lightweight body and a large wing area, which allows it to fly using a slow, controlled wing-beat. Soft, comb-like serrations on the wing feathers and frayed edges on the tips of the tail feathers break up the air flow and muffle sound, enabling nearly silent flight. Most other bird species have a smooth, knifelike edge on their wings, which creates noisy friction as they cut through the air. Many owl species also have feathers extending down their legs, which muffle sound during flight. Show the students the wing and the talon. Point out the serrated feather ends. 

(4) Talons and Beak: Talons are used for capturing and grasping prey. The toes of an owl extend to the sides to prevent small mammals from moving sideways and escaping. Many owl species have feathers extending down their legs, which insulate their feet during winter. A strong, sharp, curved beak allows owls to tear up prey if it is too large to swallow. Owls normally kill their prey by grasping it with their talons and then biting its neck. 

(5) Behavior: Most owl species will migrate if the winter is harsh enough, but they will continually return to the same area. Owls develop an intimate knowledge of their territory and hunting ground through their methodical behavior. They will use the same trees as lookout perches to search for and ambush prey. 

(6) Diet: Owls hunt small mammals, birds, and invertebrates like insects and worms, and most owls will swallow their prey whole. However, some owls have a varied diet. The Great Horned Owl eats rodents, pheasants, quail, small birds, fish, amphibians and reptiles.

DISSECTING OWL PELLETS: Examining owl pellets provides important information to ornithologists (i.e. those who study birds) regarding owl adaptations and habits. Scientists can determine what owls are eating at various times and places by finding and dissecting owl pellets. Owl pellets also provide information about the approximate numbers of prey animals in the owl's feeding area, which is valuable information to scientists studying animal populations. Pellets are often found at the base of a perching tree. Owls can digest only the soft muscles and organs of their prey. The bones, teeth, fur, feathers, scales, or insect skeletons are too dense and cannot be converted into energy.

 The harder parts may also puncture an owl's soft, curved intestines if passed through its digestive tract. Instead, the waste material is formed into a pellet by the gizzard muscles and passed back up the esophagus to be cast out (thrown up) about twelve hours later. Although other birds, like eagles and hawks, also regurgitate pellets, owls are more efficient at it and they regurgitate more frequently. Owls swallow their prey whole, ingesting the entire skeleton. Other raptors selectively tear at their prey, eating only the soft digestible parts and leaving the indigestible bones. Also, unlike other birds, owls do not have a crop, which is an organ that holds food until the stomach is ready to receive it. Other species of raptors do not need to regurgitate pellets as frequently as owls because some of the food remains in the crop, preventing food from passing up through the mouth for several hours. Although pellets are waste material, they provide food and shelter for other organisms. Pellets sometimes provide homes for clothes moths, carpet beetles, and fungi. You may find droppings, cocoons or exoskeletons from these animals in the pellets.

PROCEDURE: (1) Use a needle, tweezers or sticks to dissect an owl pellet. The pellets have been “scientifically sanitized” or autoclaved so they are free from bacteria and diseases. Nevertheless, wash your hands carefully after working with the pellets. (2) Carefully separate the bones from the soft material in the pellet. 

DATA COLLECTION: (1) Read about the Barn Owl and Introduction to Owl Pellets in the “Student Guide”. (2) Use the supplemental handouts and posters provided to assist you in sorting and skull identification of the prey as well as to identify as many bones and skulls as possible. (3) Examine each skull to determine if it has teeth. If there are no teeth, it is a bird skull. If teeth are present, read the statements on the key to determine which statement is more correct about the skull. (4) Once you have identified the bones and skulls, record the data and complete the questions that follow.