Dissection of Barn Owl Pellets
From Florida’s Everglades Agricultural Area
Richard N. Raid
University of Florida
Everglades Research and Education Center
Belle Glade, FL 33430
What are owl pellets?
What do barn owls eat?
The common barn owl (Tyto alba), one
of nature’s most efficient rodent
predators, frequently swallows its prey
whole. They digest all the fleshy parts
of the prey, leaving bones, fur, and
chitinous body remains undigested.
Such remains are regurgitated (spit-up,
to put it bluntly) in the form of a
compressed pellet. Pellets are actually
formed in the stomach, and are coughed
out the beak, usually at the owl’s nest or
favorite roosting perch.
Barn owls feed primarily on small
rodents. These include: rats, mice,
moles, voles, gophers, and shrews. They
will occasionally feed on somewhat
larger prey, such as rabbits or squirrels,
but this is less common. Barn owls will
also feed on prey other than rodents,
such as other birds, bats, lizards, small
snakes, large insects, frogs, toads, and
very infrequently, fish. Barn owls have
voracious appetites and a grown barn
owl may routinely devour ½ to its full
weight in prey per day when food is
abundant. Growing chicks, as seen in
Figure 1 below, may even eat more,
reportedly up to 1 ½ times their weight
in prey per day.
Figure 1. Two sleepy barn owls nearly ready to
fledge sit in the rafters of a barn.
Barn owls typically regurgitate 2-3
pellets per day.
By examining
recognizable bones and other identifiable
remains within a pellet, the barn owl’s
diet may be determined.
Figure 2. Barn owl chicks 4-5 weeks-old can
eat up to 1 ½ times their own weight in prey per
day.
Where were these pellets collected?
The barn owl pellets that you have
received from the South Florida Barn
Owl Program were collected from
roosting and nesting sites within the
Everglades Agricultural Area (EAA).
Located just south of Lake Okeechobee,
the area is primarily agricultural lands,
consisting of sugarcane, vegetables, rice,
and sod. Sugarcane is the main crop of
this area, supporting abundant rodent
populations that live in the fields and on
surrounding ditch banks. It has been
estimated that rodents cause more than
$30 million annually in damage and
yield losses to sugarcane alone.
barn owl program were collected from
these man-made nesting areas. A glance
at a barn owl nesting site reveals that
they are literally “rodent graveyards,
containing tens-of-thousands of rodent
bones that have accumulated over the
years from regurgitated pellets and
uneaten prey.
Figure 4. A barn owl nesting site is literally a
rodent graveyard. Note the bones and fur
beneath these eggs and days-old chicks at a nest.
Are these pellets safe to handle?
Figure 3. Map of south Florida, showing the
700,000-acre Everglades Agricultural Area south
of Lake Okeechobee.
The EAA is a nearly flat, tree-less
landscape and there are few natural
nesting sites available for barn owls. For
this reason, barn owls take readily to
nesting in old farm structures (pump
houses, pole sheds, storage buildings,
abandoned warehouses) and nesting
boxes positioned in and around fields.
Most of the pellets distributed by our
The pellets supplied are wrapped in
aluminum foil and were heat-sterilized
in an autoclave prior to distribution.
This assures that any microbes normally
associated with owls or rodents are
killed and that the pellets are safe to
handle. While many classes prefer to
use disposable latex or plastic gloves
while handling the pellets, this is not
absolutely necessary.
Regardless,
always wash your hands thoroughly
following pellet dissection as an
additional safeguard. NOTE: To avoid
potential health problems, never handle
or dissect owl pellets that have not been
subjected to some form of sterilization.
They could harbor potentially harmful
organisms such as Hanta-virus or
Salmonella.
muskrat skull, 1 southern short-tailed
shrew, and 1 marsh rabbit skull (Figure
1).
What will I find in my pellet?
Figure 5. Barn owl pellets, showing the range in
size and shape. For scale, these grid lines are
spaced by ½ inch.
How do I dissect an owl pellet?
Start by covering work surfaces with
newspaper for easy clean-up afterwards.
Before dissecting the pellet, measure the
length and width of your pellet(s). This
will allow the class to calculate the range
in size and also the average or mean
size. Pellets should be placed in a
shallow
dish
(disposable
plastic
weighing dishes or Petri plates are
good), and moistened with a small
amount of water to facilitate dissection.
Use enough water to moisten the pellet,
not float it. Using forceps and a teasing
needle (toothpicks or even a pencil point
can do), tease the compressed pellet
apart, removing the fur that adheres to
individual bones. Collect the bones in a
small vial or bag for identification using
a bone chart provided.
You will most assuredly find bones and
fur. Past investigations reveal that one
or more skulls are detected in nearly 2 of
every 3 pellets. Interestingly, when
multiple skulls are found in a pellet,
these are most often mice skulls,
presumably because of their smaller prey
size. Additional EAA pellet studies have
turned up the remains of field mice,
Norway rats, water rats, occasional bird
(usually common blackbirds and egrets)
skeletons, frog and lizard remains, and
large beetle exoskeletons. Once in a
while, students may even find an owl
skull, since owlets have been known to
cannibalize their siblings if they become
weakened or die.
A comparison of EAA pellet remains
and commercially purchased pellets
revealed that pellets from the EAA had
fewer but more intact skeletal remains
than commercial pellets.
This is
presumably due to the larger size of the
EAA prey (rats vs. mice, voles and
moles).
Cotton Rat
House
Mouse
Rice Rat
Other
More than 90% of a barn owl’s diet is
made up of small rodents. A recent
study documenting identifiable remains
in 100 owl pellets collected in the EAA
revealed the following: 44 cotton rat
skulls, 41 house mice skulls, 9 rice rats
skull, 4 roof rat skulls, 1 round-tailed
Figure 6. Relative breakdown of identifiable
prey found in 100 barn owl pellets collected
from the EAA.
What do I do with the bones?
Some students may wish to keep their
skeletal remains to document their
findings. Bones should be freed of all
adhering fur and may be cleaned by
soaking them in a 20% bleach solution
(1 part bleach: 4 parts water) for several
minutes, being careful not to splash
bleach on your clothes.
This will
cleanse and bleach the bones, which can
be mounted onto a paper or board for
display when dry. Many classes are able
to re-construct entire rodent skeletons
for display by consolidating the remains
from several pellets. This is similar to
an archaeologist reconstructing a
dinosaur skeleton, only on a smaller
scale. It is important to remember that
while pellets may hold several skulls,
they may not hold the entire remains of
an individual rodent. When prey is
abundant, barn owls have been known to
simply bite off and swallow the heads of
prey, leaving the remainder on the
ground to decay or be scavenged.
Figure 8. A barn owl nesting box situated on a
field ditch bank in the Everglades Agricultural
Area. Barn owls take readily to these boxes,
rearing their young inside.
Supply List for Pellet Dissection
Old newspaper (to cover desk)
Ruler (for pellet measurement)
Small plastic dish (to hold pellet)
Forceps (to tease pellet apart)
Teasing needle (or similar probe)
Water bottle (to moisten pellet)
Small plastic bag (bone storage)
Bone chart (for prey identification)
Figure 7.
Students and their instructor
reconstruct a rodent skeleton from bones found
in owl pellets.
For further information, contact:
Dr. Richard N. Raid
University of Florida, IFAS
Everglades Research & Education Center,
3200 East Palm Beach Rd.,
Belle Glade, FL 33430
rnr@ifas.ufl.edu