RESEARCH REPORTS
497
Owl Pellet Taphonomy: A Preliminary Study of the
Post-Regurgitation Taphonomic History of Pellets in a
Temperate Forest
REBECCA C. TERRY
Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637,
Email: [email protected]
Predation has long been recognized as an important
mechanism leading to the concentration of small-vertebrate skeletal remains (Mellett, 1974; Dodson and Wexlar,
1979; Maas, 1985; Andrews, 1990). Modern predators in-
clude mammalian carnivores, diurnal birds of prey, and
nocturnal owls (Mellett, 1974; Andrews, 1990). Although
members of all raptor families produce pellets (regurgitated oblong masses of the undigested components of a bird’s
food, usually consisting of fur, bones, claws, and teeth),
owl pellets are characterized by survival of a higher proportion of skeletal material than pellets/scat produced by
other predators (both avian and mammalian), and thus
are thought to provide a more complete sample of the local
fauna (Mayhew, 1977; Andrews, 1983; Hoffman 1998; Lyman, 1994). Owl pellets also can include climate indicators
such as pollen (Fernandez-Jalvo et al., 1996; Scott et al.,
1996; Fernandez-Jalvo et al., 1999). Because many owls
show high roost fidelity (Bent, 1961; Andrews, 1990), pellet-derived accumulations also can provide a geohistorical
time series for small-vertebrate communities, and thus
contribute information that is critical to our ability to assess small-vertebrate community change (on seasonal to
millennial scales), as well as changes in predator behavior
patterns and local climate change (Avery, 1995; Hadly,
1996; Vigne and Valladas, 1996; Fernandez-Jalvo et al.,
1998; Grayson, 2000; Hadly and Maurer, 2001; Avery et
al., 2002).
Despite their potential as primary resources for reconstructing paleoclimates and assessing ecological and environmental change, pellet-derived small-vertebrate assemblages have been underutilized, perhaps because of
uncertainty about their taphonomic history (FernandezJalvo et al., 1998). The taphonomic history of a pellet consists of multiple phases, each with particular biases. First,
selective hunting behavior of predators, time of predator
activity, season, vulnerability of prey, and local climate
conditions will introduce an initial bias into the species
composition of any predator-derived accumulation (Craighead and Craighead, 1956; Andrews, 1990; Denys et al.,
1996; Saavedra and Simonetti, 1998), although these
same factors may permit identification of rare elements in
the fauna (Avery et al., 2002). Second, the digestive process of pellet formation results in characteristic fragmentation and bone loss (Andrews, 1990). Owl pellets are
formed in the stomach because a narrow pyloric opening
near the entrance of the esophagus prohibits large particles from entering the intestines; the undigested material
travels back up the esophagus and is ejected as a pellet
(Bent, 1961; Hoffman, 1988; Andrews, 1990). Third, pellet
history following regurgitation (weathering, transport,
disintegration, and burial) has the potential to mask original species composition and other taphonomic signatures,
thus biasing the fossil record.
Copyright � 2004, SEPM (Society for Sedimentary Geology)
0883-1351/04/0019-0497/$3.00
PALAIOS, 2004, V. 19, p. 497–506
Owls are important contributors to the Tertiary small-vertebrate fossil record. They concentrate small-vertebrate remains by producing pellets rich in skeletal material that
provide a sample of the small-vertebrate fauna of an area. A
common assumption is that different predators inflict
unique fragmentation and skeletal element representation
signatures, thus providing a method for identifying a field
assemblage as pellet derived, and possibly identifying the
predator. In addition to the digestive process of pellet formation, the taphonomic history of a pellet includes the postregurgitation processes of weathering, disintegration,
transport, and burial, all of which can introduce biases
into an assemblage and confound paleoecological interpretation. Analysis of a modern accumulation of small-vertebrate remains from Great Horned Owl (Bubo virginianus)
pellets in a temperate forest environment on San Juan Island, Washington, reveals that fragmentation and skeletalelement representation change with residence time on the
forest floor as pellets disintegrate and skeletal elements become dispersed. Matted hair initially protects the skeletal
elements. As the pellet breaks down, the bones become dispersed, fragmentation of the bones increases (from 99% intact bones in intact pellets to 75% intact bones in fully dispersed pellets), and small, fragile skeletal elements are lost,
resulting in a residual concentration of larger, more robust
skeletal elements. The spatial distribution of skeletal elements below the roosting site follows a right-skewed, bimodal pattern. Skeletal elements are preserved in the soil to a
depth of three centimeters. Post-regurgitation processes
have the potential to distort the original faunal and skeletal
composition of pellet-derived assemblages, thus masking
any original predator-specific signatures. Actualistic taphonomic studies are necessary in order to understand how
well pellet-derived assemblages capture information on local ecological and environmental conditions. This is a critical question that must be addressed to enable correction for
such biases before pellet-derived assemblages are used for
assessment of small-vertebrate community change and paleoenvironmental reconstruction.
INTRODUCTION
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FIGURE 1—Maps of San Juan Island and Point Caution, Washington
State, USA. (A) San Juan Island (redrawn from Morgan, 1960). Inset
shows location of San Juan Island in the San Juan Archipelago. (B)
Point Caution (redrawn from Friday Harbor quadrangle, USGS 7.5
minute series, 1954).
Of these three potential sources of bias, only raptor dietary preferences and patterns of bone destruction due to
digestion have received significant attention (e.g., Craighead and Craighead, 1956; Dodson and Wexlar, 1979;
Hoffman, 1988; Andrews, 1990; Denys et al., 1996; Saavedra and Simonetti, 1998; Trejo and Guthmann, 2003).
Some of these studies have attempted to identify unique
fragmentation and skeletal element representation signatures left by different predators (e.g., Dodson and Wexlar, 1979; Hoffman, 1988; Andrews, 1990), whereas others
have investigated the microscopic effects of digestion on
bones and teeth (e.g., Rensberger and Krentz, 1988; Andrews, 1990). Comprehensive study of the post-regurgitation processes acting on pellet-derived small-vertebrate
assemblages is noticeably absent (but see Andrews, 1990,
for limestone-cave deposits). This study presents an initial
investigation into the post-regurgitation taphonomic history of owl pellets in a temperate-forest environment with
a focus on: (1) the pattern of skeletal-element distribution
that has developed on the forest floor below a roosting site,
(2) changes in the fragmentation and relative abundance
of skeletal elements, and (3) changes in the taphonomic
condition of skeletal elements as pellets disintegrate and
skeletal material is dispersed and incorporated into the
soil.
STUDY AREA
The Friday Harbor Laboratory of the University of
Washington is situated on the eastern side of San Juan Island, Washington, and includes both coastline and temperate forest (Fig. 1). Point Caution, a headland directly
north of the lab, is dominated by Western Red Cedar (Thuja plicata) and Douglas Fir (Pseudotsuga menziesii) forests
that have not been logged for over 100 years (Staude, pers.
comm., 2002; Guberlet, 1975). Approximately 1.6 km
northwest of the lab is a 0.01 km2 grove of Western Red
Cedar [N48�33.615�, W123�0.891�]. In summer 2002, when
this study was undertaken, the floor of the grove was littered with more than 1,500 small-vertebrate skeletal elements and more than 20 pellets in various stages of disintegration. The largest concentration of skeletal elements
occurred around the base of one tree, but at least four oth-
er trees in the vicinity also showed separate pellet-derived
bone accumulations around their bases.
The high skeletal content of the pellets as well as pellet
size (average size: 5.6 cm x 3.8 cm x 2.4 cm) and the size
and taxonomic composition of the prey—primarily the
nocturnal Townsend’s Vole (Microtus townsendii), with a
shrew (Sorex sp.) and a mole (Scapanus sp.) also present
(Glass and Thies, 1997)—suggest that the pellets are from
a Great Horned Owl (Bubo virginianus) (Bent, 1961; Andrews, 1990). Other large pellet-producing raptors common to San Juan Island include the diurnal Bald Eagle
(Haliaeetus leucocephalus), Red-tailed Hawk (Buteo jamaicensis), Turkey Vulture (Cathartes aura), Northern
Harrier (Circus cyaneus), and the nocturnal Barn Owl
(Tyto alba), Western Screech Owl (Otus kennicottii), and
Northern Saw-whet Owl (Aegolius acadicus). Of the nocturnal owls present on San Juan Island, Great Horned
Owls are the largest, most common, are frequently found
in dense forest habitat, and are the only owls to have been
seen and heard around the Friday Harbor Laboratory in
the last several years (Britton-Simmons, pers. comm.
2002, Smith et al., 1997).
MATERIALS AND METHODS
The analyses included in this paper are restricted to the
roosting tree in the Western Red Cedar grove with the
largest concentration of skeletal elements around its base.
The surface distribution of skeletal elements around the
roosting tree was mapped using a north/south-oriented
grid, a handheld compass, tape measure, and six 0.25 m2
squares (50 cm x 50 cm) subdivided into 10 cm increments.
Skeletal elements rarely were isolated, typically occurring in discrete (or semi-discrete or dispersed but still spatially definable) clusters. Spatially defined occurrences of
bones typically contained at least one skull. Dodson and
Wexlar (1979) reported that the mean number of pellets
cast per meal is close to one for Great Horned Owls, thus,
each assemblage was reasonably interpreted as a modified
pellet.
Assemblages were subdivided into three categories: (1)
intact pellets, (2) partially dispersed pellets, and (3) fully
dispersed pellets (Fig. 2). Pellets were classified based on
the presence and condition of the matted-hair matrix of
the pellet. Intact pellets were unbroken and characterized
by smooth, matted fur and an oblong shape. In partially
dispersed pellets, the surface of the pellet was no longer
smooth and matted, the pellet was broken, and/or skeletal
elements had fallen loose from the pellet. In fully dispersed pellets, spatially definable assemblages of bones
were characterized by the absence of matted fur. All pellet
material was mapped, and skeletal elements identified
when possible. Each quarter-meter quadrat on the surface
map was assigned a unique coordinate and the distance
from the center of the tree to the center of each quadrat
was calculated. Five assemblages of each type were selected randomly and collected by hand. Before collection, the
orientation of each bone with respect to the soil was
marked on the exposed surface of the bone for all skeletal
elements free from the matted hair of pellets.
Partial and complete pellets were hydrated overnight
and picked apart with forceps under a dissecting scope
(magnification 2x to 6x). All elements were identified, and
OWL PELLET TAPHONOMY
499
FIGURE 2—Pellet classification scheme. Pellets were classified based on the presence and condition of the matted hair matrix. (A) Intact
pellet. (B) Partially dispersed pellet. (C, D) Fully dispersed pellets.
bone fragmentation and modification states recorded.
Fragmentation is defined for this study as breakage resulting in loss of information about the function or shape
of a skeletal element. The taphonomic condition of all elements from nine representative assemblages (three from
each assemblage type) was assessed. Bone modification is
used in this study to refer to a combination of chemical dissolution due to digestive modification and secondary
weathering processes, following Behrensmeyer (1978), including the effects of contact with/burial in soil (chemical
alteration, etching, and pitting) and physical weathering
agents operating on the forest floor (exposure to wind,
rain, temperature change). Bone modification categories
used in this study are defined as follows: (1) unmodified
bone—the inner cancellous bone is unexposed, protected
by an intact outer layer of dense compact bone; (2) slightly
modified bone—the cancellous bone is exposed; and (3) extensively modified bone—the outer compact layer is absent and the inner cancellous bone is exposed and eroded.
The slope of the forest floor was mapped using a squaremeter grid and a Brunton compass. A layer of mobile conifer (cedar) detritus covers the soil. The orientations of
skeletal elements from fully dispersed pellets were recorded to address the importance of hydrodynamic transport.
The pH of the soil beneath the conifer detritus, as well as
the phosphorus, nitrogen, and potassium (potash) content
were determined using a Lammott Soil Kit. Finally, exploratory excavation of six 20-cm2 sites was done using a
knife and spoon. Sites were excavated in 1-cm layers to a
depth of 5 to 7 cm. Each layer was collected and wet sieved
(250 �m mesh).
Unless otherwise noted, statistical analysis consisted of
the construction of contingency tables analyzed using Chisquare tests. Significance was established at the � � 0.05
level.
RESULTS
Spatial Analysis
The surface of the ground surrounding the roost tree
slopes �20�–25� to the northeast. The soil beneath the conifer detritus is acidic (pH � 5) and contains trace
amounts of phosphorus and nitrogen and a high amount of
potassium (potash). Pellet-derived material (assemblages)
is found on all sides of the roost tree. The total number of
assemblages decreases with distance from the roost tree; a
histogram of assemblages versus distance shows a concave, right-skewed distribution. This pattern is similar
when all assemblages are pooled, as well as when assem-
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FIGURE 4—Number of bones with distance from the tree. The distribution of bones follows a bimodal pattern with a peak in abundance
at approximately 2 m. A two-tailed binomial test for proportions indicates significance (p K 0.0001; n � 69).
FIGURE 3—Number of assemblages pooled and subdivided by type
with distance from the tree. (A) Total number of assemblages. (B)
Assemblages subdivided by type. The number of assemblages decreases following a right-skewed, concave trend. Intact pellets peak
in abundance approximately 2 m from the tree and do not persist at
distance. Results are not significant due to the small sample size,
which violates the assumptions of the Chi-square test (p � 0.2620; n
� 52).
blages are split on the basis of type (Fig. 3). Pellet abundance peaks approximately 2 m from the tree, and intact
pellets do not persist at distance from the tree, whereas
partially and fully dispersed pellets do persist. The above
pattern is not significant (p � 0.2620; n � 52, the number
of assemblages), perhaps due to the relatively small sample size for the number of quadrats; expected values of
20% of the quadrats were fewer than five, a violation of the
assumptions of the Chi-square test.
The total number of bones exposed on the surface displays a bimodal distribution with distance from the tree
(Fig. 4). Bones appear to be concentrated in peaks between
0 and 0.5 m from the tree, and again between 1.5 and 2.5
m from the tree. The 5-m measured distribution radius
around the tree was divided at 2.5 m, creating an inner bin
that contained 25% of the total area, and an outer bin that
contained 75% of the total area. Skeletal elements were
counted for each of the two zones, and a binomial test for
proportions that took the disproportionate areas into account was performed to determine significance of the distribution. Despite the fact that the inner zone represents
only 25% of the total area of the site, 59 bones (85.5% of the
total number of bones) were found in the inner zone. This
pattern is significantly different from what would be expected if elements were distributed randomly between the
two zones at a 25:75 ratio (p K 0.0001; n � 69). Note that
these data include only bones visible on the surface. Skeletal elements in pellets and disarticulating pellets are not
included because not all pellets were collected and dissected. Figure 3 shows that pellets are found closer to the tree
than dispersed assemblages. Including the bones con-
tained in pellets would increase the sample size, and
would reinforce the pattern described.
Long bones were separated into two categories: (1) robust elements (humerus, pelvis, femur), and (2) fragile elements (ulna, radius, tibia). The distribution patterns of
robust and fragile elements were analyzed separately using the binomial statistical method described above. Both
Figure 5 and the disparate sample sizes (see below) indicate that robust elements are present in higher numbers
than fragile elements throughout the entire distribution,
and persist farther from the tree. The total numbers of robust and fragile skeletal elements in each area were significantly different at � � 0.05 (Robust: p K 0.0001; n �
59; Fragile: p � 0.0001; n � 10). There is no evidence (such
as preferred orientations of bones or runneling of the soil
surface and/or mobile conifer detritus) that would indicate
hydrodynamic transport at the site.
Taphonomic Analysis
Relative proportions of all skeletal elements from 15 assemblages were calculated for each assemblage type (intact pellets, partially dispersed pellets, fully dispersed pellets; Fig. 6). The relative proportions of different skeletal
elements differ significantly between assemblage types (P
K 0.0001, n �672). Fully dispersed pellets are dominated
by mandibles (18%), followed closely by vertebrae (15%),
FIGURE 5—Number of robust and fragile skeletal elements with distance from the tree. Robust elements are present in higher numbers
throughout the distribution than fragile elements, and persist farther
from the tree. A two-tailed binomial test for proportions indicates significance (Robust: p K 0.0001; n � 59; Fragile: p K 0.0001; n � 10).
OWL PELLET TAPHONOMY
501
FIGURE 7—Fragmentation frequency with assemblage type. Fully
dispersed pellets contain the highest proportion of fragmented elements. Results are significant (p K 0.0001; n � 351).
FIGURE 6—Relative proportions of skeletal elements with assemblage type; proportions differ significantly among assemblage types
(p K 0.0001; n � 672); Sk � skull; Ma � mandible; Sc � scapula;
V � vertebra; H � humerus; U � ulna; Ra � radius; P � pelvis; F �
femur; T � tibia/fibula; Ri � rib; CTP � manus and pes elements. (A)
Intact pellets. (B) Partially dispersed pellets. Note (A) and (B) are dominated by vertebrae, ribs, and manus and pes elements. (C) Fully dispersed pellets, which are dominated by more robust skeletal elements.
skulls (11%), humeri (11%), femora (11%), tibiae (11%),
and ribs (11%). Pes and manus elements are not present.
Partially dispersed pellets and intact pellets show a much
greater apparent similarity than either assemblage type
shows to fully dispersed pellets. Partially dispersed and
intact pellets have a much lower proportion of robust long
bones and mandibles, instead consisting primarily of elements such as manus and pes (25–44%), ribs (24%), and
vertebrae (20–24%).
Fragmentation was calculated for nine assemblages
and pooled by assemblage type (Fig. 7). Intact pellets contained the highest proportion of whole elements (99%) and
the lowest proportion of fragmented elements (1.0%). At
the other extreme, fragmentation frequency was highest
in fully dispersed pellets (26%). Fragmentation frequency
among the different assemblage types was found to be significantly different (p K 0.0001; n � 351).
Different types of skeletal elements display unique
breakage patterns. Skulls, for example, exhibit a characteristic breakage pattern of the cranium in which the posterior half of the skull is completely absent (Fig. 8A, B).
Skulls that exhibit this type of breakage are found most
frequently in fully dispersed pellets (23%) and partially
dispersed pellets (30%; Fig. 8C). All skulls found in intact
pellets were unbroken. While results were found to be significant (p � 0.0151; n � 13), the small sample size violates assumptions of the test, making statistical analysis
inconclusive but consistent with the observed pattern.
The epiphyseal ends of long bones, which represent
growth centers that become fused during the adult life of
an animal (Swindler, 1998), can provide clues about the
age structure of a prey assemblage. Approximately 63% of
all long bones (humerus, femur, tibia) from 15 assemblages are missing epiphyses (Fig. 9). Significance of this pattern was established using an unpaired t-test (p � 0.0098;
n � 46). This suggests a potential age bias towards youn-
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FIGURE 9—Epiphyseal presence and absence in humeri and femora
of Microtus townsendi; scale bar in millimeters. (A) Humerus with proximal epiphysis missing. (B) Humerus with proximal epiphysis present.
(C) Femur with distal epiphysis missing. (D) Femur with distal epiphysis present.
FIGURE 8—Fragmentation patterns of skulls; scale bar in millimeters.
(A) Dorsal view of three skulls (Microtus townsendii). Two exhibit a
common posterior breakage pattern. (B) Ventral view of the same
skulls. (C) Frequency of posterior skull breakage. All skulls recovered
from intact pellets were unbroken. Results are significant (p � 0.0151;
n � 13), however, small sample size violates the assumptions of the
Chi-square test.
ger individuals in the prey assemblage. No significant difference was found for the frequency of epiphysis loss between the different types of skeletal elements (p �
0.0758).
Bone-modification results pooled for all elements by assemblage type are presented in Figure 10. The differences
are significant (p � 0.0116; n � 191) and suggest that for
fully dispersed pellets, more than 60% of the bones exhibit
no bone modification, whereas for intact pellets, only
about 45% of the bones show no bone modification. Conversely, about 25% of skeletal elements from fully dispersed pellets and almost 20% of skeletal elements from
intact pellets show evidence of extensive bone modification.
Bone modification results for skeletal elements found in
fully dispersed pellets were compared with results for the
same skeletal elements from intact pellets. Results were
not significant (p � 0.2022; n � 101), indicating no difference in terms of bone modification between elements present in both assemblage types.
Different skeletal elements show different degrees of
modification, as do the shaft and ends of a single long
bone. For all long bones (with and without epiphyses) for
all assemblage types pooled together, the ends of long
bones show a significantly higher degree of bone modification than the shafts (p K 0.0001; n � 43; Fig. 11).
Skeletal elements from fully dispersed pellets are scattered on the forest floor, with one side of the bone exposed
to air, and the other side exposed to twig litter or soil. Bone
modification for each surface of all bones derived from fully dispersed pellets is shown in Figure 12. Seventy percent
of surfaces exposed to the air showed little to no bone modification, whereas only 50% of surfaces exposed to the soil
showed little to no bone modification. However, almost
20% of surfaces exposed to the soil show extensive bone
modification, while only 5% of surfaces exposed to the air
show extensive bone modification (p � 0.0449; n � 241).
Excavation Results
Preliminary excavation of three 20-cm squares to
depths of 5 to 7 cm yielded skeletal elements incorporated
OWL PELLET TAPHONOMY
503
FIGURE 11—Bone modification compared for the ends and shafts of
longbones. The ends of longbones show a significantly higher frequency of modification than do the shafts (p K 0.0001; n � 43).
FIGURE 10—Bone modification (A) Examples of different degrees of
bone modification in pelves of Microtus townsendii; N � no bone modification; M � extensive bone modification (cancellous bone is exposed on the shaft and eroded on the distal ilium); scale in millimeters.
(B) Bone modification by assemblage type. Bones from fully dispersed
pellets show the highest frequency of no modification. Results are
significant (p � 0.0116; n � 191).
son and Wexlar, 1979; Hoffman, 1988; Andrews, 1990). In
his paper on pellet-derived fragmentation patterns, Hoffman (1988) briefly acknowledged that post-pellet diagenetic processes might confound the taphonomic signatures
of small-vertebrate assemblages, making definitive predator identification difficult. However, the number of actualistic studies of the post-regurgitation history of pellets is
limited (but see Andrews, 1990). The results from this
study illustrate the importance of understanding how
post-regurgitation processes can bias small-vertebrate assemblages over time in a temperate-forest environment.
Once a pellet has been regurgitated and is exposed to
physical and chemical weathering processes on the forest
into the soil to a depth of 2 cm below the soil surface (2
skulls, 4 mandibles, 1 pelvis, 1 femur, 1 tibia, 1 vertebra;
Fig. 13). All skeletal elements recovered from the soil show
extensive bone modification.
DISCUSSION
Skeletal material can be concentrated by both physical
(fluvial sorting, wind deflation) and biological (predation)
processes. These different concentrating mechanisms
have the potential to introduce different biases into an assemblage, which should then affect paleoecological interpretation (Behrensmeyer, 1993). Thus, before a pellet-derived small-vertebrate assemblage can be used for paleoecological reconstruction, there must be a reliable way to
recognize that it is indeed pellet-derived. Previous studies,
primarily conducted in laboratory settings, have focused
on fragmentation patterns as the means of identifying pellet-derived assemblages (Dodson and Wexlar, 1979; Hoffman, 1988; Saavedra and Simonetti, 1998; Andrews, 1990;
Kusmer, 1990). Relative abundance of skeletal material
and characteristic breakage patterns produced in laboratory settings also have been proposed as methods to facilitate predator identification from field assemblages (Dod-
FIGURE 12—Bone modification of elements from fully dispersed pellets, compared for surfaces exposed to air and soil. Surfaces in contact with the soil show a significantly higher frequency of extensive
bone modification (p � 0.0449; n � 241).
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FIGURE 13—Skeletal elements (Microtus townsendii) recovered from the subsurface; scale in millimeters. (A) Skull from 1 cm below the soil
surface. (B) Mandible from 2 cm below the soil surface. (C) Mandible and pelvis from 1 cm below the soil surface. (D) Mandibles from the top
of the soil surface below the leaf litter.
floor, the matted hair in the pellet protects the skeletal elements contained inside (Andrews, 1990). Assuming the
different pellet types designated in this study (intact pellets, partially dispersed pellets, and fully dispersed pellets) represent steps in the disintegration process over
time, both the relative proportion of skeletal elements present in an assemblage and the degree of fragmentation
changed as a result of time exposed on the forest floor. As
the pellets broke down, bones became dispersed, and fragmentation increased (Fig. 7). Evidence for transport and
dispersal by hydrodynamic processes is lacking, thus, the
increased dispersal and fragmentation of skeletal material must be due to other factors, such as scavenging.
Dodson and Wexlar (1979) observed a high frequency of
posterior breakage in skulls recovered from Great Horned
Owl pellets in a laboratory setting and presented this as a
possible predator identification tool. However, they also
reported feeding observations for Great Horned Owls, in
which the owls swallow mice whole and headfirst, making
no attempt to extract the brain of the animal. In comparison, the skulls recovered from intact pellets in this study
primarily were unbroken, consistent with the feeding ob-
servations of Dodson and Wexlar (1979), while broken
skulls were found in partially dispersed pellets and fully
dispersed pellets on the forest floor (Fig. 8). This suggests
that skulls become fragmented more frequently during
post-regurgitation processes such as scavenging; and that
caution should be used if employing skull breakage as a
tool for predator identification.
Based on this study, as pellets disintegrate, the skeletal
composition of the assemblage shifts from a high proportion of small, fragile elements to a high proportion of larger, more robust elements, because the fragile elements are
preferentially lost (Figs. 5, 6). Skeletal elements from fully
dispersed pellets (which potentially have spent the most
time in contact with the forest floor) show the highest frequency of no bone modification. Elements from intact pellets, however, show lower proportions of no modification
than bones from dispersed pellets and high frequencies of
both slight and extensive bone modification (Fig. 10). A
possible explanation is that the matted hair in a pellet initially shields the smaller and fragile bone fractions (which
are perhaps more intensely modified by digestion). Upon
pellet disintegration, these smaller, fragile, extensively
OWL PELLET TAPHONOMY
modified elements then become exposed to physical- and
chemical-weathering processes on the forest floor. Armour-Chelu and Andrews (1994) documented significant
burial and lateral transport of small skeletal elements due
to earthworm bioturbation. This, coupled with potential
effects of preferential destruction due to higher modification, would result in a residual concentration of the larger,
robust, skeletal elements that were less extensively modified by digestion. Of the bones that do remain on the forest
floor, it should be noted that the surface in contact with
the soil shows a higher degree of bone modification than
the surface that is exposed to the air (Fig. 12). This preferential damage is consistent with a variety of actualistic
studies of weathering of larger bones (e.g., Behrensmeyer
and Hill, 1980).
The distinctive characteristics of fragmentation and
skeletal element representation thus change over time.
This presents a problem if these characteristics are to be
used to determine whether an assemblage is predator-derived, and to identify the predator. Alternate methods for
identifying an assemblage as pellet-derived deserve more
attention. As expected, in this study, the density of skeletal elements decreases with distance from the tree. The
right-skewed, bimodal distribution of skeletal elements
seen below the roost site, with a distribution radius of intact pellets approximately 2 m from the tree (Figs. 3, 4), is
most likely the result of a reflecting boundary (the tree
trunk) and preferred roosting position of the owl. As pellets are regurgitated and tumble through the dense
branches to the ground below, scattering off the branches
and the trunk would yield a peak in abundance at a moderate distance from the trunk. If surface patterns also are
preserved at depth (especially in environments more conducive to preservation than acidic, moist, coastal-forest
soils), they could be powerful indicators that an assemblage is pellet-derived. Alternative techniques for identifying whether an assemblage is predator-derived that deserve further study include microscopic investigation of
chemical etching, pitting, and dissolution that occurs with
digestion (and taxonomic differences in preservation potential). Microscopic analyses also are needed to distinguish the effects of digestion from secondary chemical dissolution that occurs as bones come in contact with the soil.
Finally, actualistic and microscopic studies also are needed to better understand the confounding effects of scavenging on dispersal, skeletal-element representation, and
fragmentation patterns. These analyses are beyond the
scope of this work.
CONCLUSIONS
(1) Pellets and skeletal elements become dispersed with
distance from the tree. The pattern of dispersal of skeletal
elements follows a right-skewed bimodal distribution.
(2) The relative proportions of skeletal elements change
as pellets disintegrate, thus this is not a reliable criterion
for identifying the pellet origin of and predator responsible
for (sub)fossil assemblages. Intact pellets are dominated
by small fragile elements, which are preferentially lost
with pellet disintegration over time.
(3) Frequency of fragmentation increases as pellets become dispersed, and individual breakage patterns (such as
loss of the cranium in skulls) can be incurred due to post-
505
regurgitation processes such as scavenging. Thus, fragmentation is not a reliable indicator for predator identification.
(4) The frequency of intense bone modification is highest
in pellets and lowest in dispersed assemblages. This likely
reflects preferential destruction of smaller, more fragile
skeletal elements, which may be modified more intensely
due to digestive processes, upon being released from the
pellet.
(5) The taphonomic history of pellet-derived small-vertebrate assemblages is more complex than commonly acknowledged. Since post-regurgitation processes distort
the original skeletal composition of pellet-derived assemblages, actualistic studies are necessary in order to understand and correct for this bias, leading to more accurate
assessments of small-vertebrate community change and
paleoenvironmental reconstruction.
ACKNOWLEDGEMENTS
This work was completed as part of the 2002 Marine Taphonomy course at the Friday Harbor Laboratory of the
University of Washington. I would like to give special
thanks to Michael LaBarbera and Michal Kowalewski for
their enthusiasm, patience, and invaluable assistance and
their willingness to let me work on terrestrial vertebrates
in the context of a marine invertebrate taphonomy course.
I would also like to thank David Eberth, Karen Chin, and
Peter Andrews for their helpful reviews of this manuscript, as well as Susan Kidwell, Matthew Kosnik, and
Mark Terry for their initial comments on the paper. Finally, much thanks is also due to Katie LaBarbera for her
field assistance.
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ACCEPTED FEBRUARY 20, 2004