Owl Pellet Taphonomy: A Preliminary Study of
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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: rcterry@uchicago.edu
PALAIOS, 2004, V. 19, p. 497–506 clude mammalian carnivores, diurnal birds of prey, and
nocturnal owls (Mellett, 1974; Andrews, 1990). Although
Owls are important contributors to the Tertiary small-ver- members of all raptor families produce pellets (regurgitat-
tebrate fossil record. They concentrate small-vertebrate re- ed oblong masses of the undigested components of a bird’s
mains by producing pellets rich in skeletal material that food, usually consisting of fur, bones, claws, and teeth),
provide a sample of the small-vertebrate fauna of an area. A owl pellets are characterized by survival of a higher pro-
common assumption is that different predators inflict portion of skeletal material than pellets/scat produced by
unique fragmentation and skeletal element representation other predators (both avian and mammalian), and thus
signatures, thus providing a method for identifying a field are thought to provide a more complete sample of the local
assemblage as pellet derived, and possibly identifying the fauna (Mayhew, 1977; Andrews, 1983; Hoffman 1998; Ly-
predator. In addition to the digestive process of pellet for- man, 1994). Owl pellets also can include climate indicators
mation, the taphonomic history of a pellet includes the post- such as pollen (Fernandez-Jalvo et al., 1996; Scott et al.,
regurgitation processes of weathering, disintegration, 1996; Fernandez-Jalvo et al., 1999). Because many owls
transport, and burial, all of which can introduce biases show high roost fidelity (Bent, 1961; Andrews, 1990), pel-
into an assemblage and confound paleoecological interpre- let-derived accumulations also can provide a geohistorical
tation. Analysis of a modern accumulation of small-verte- time series for small-vertebrate communities, and thus
brate remains from Great Horned Owl (Bubo virginianus) contribute information that is critical to our ability to as-
pellets in a temperate forest environment on San Juan Is- sess small-vertebrate community change (on seasonal to
land, Washington, reveals that fragmentation and skeletal- millennial scales), as well as changes in predator behavior
element representation change with residence time on the patterns and local climate change (Avery, 1995; Hadly,
forest floor as pellets disintegrate and skeletal elements be- 1996; Vigne and Valladas, 1996; Fernandez-Jalvo et al.,
come dispersed. Matted hair initially protects the skeletal 1998; Grayson, 2000; Hadly and Maurer, 2001; Avery et
elements. As the pellet breaks down, the bones become dis- al., 2002).
persed, fragmentation of the bones increases (from 99% in- Despite their potential as primary resources for recon-
tact bones in intact pellets to 75% intact bones in fully dis- structing paleoclimates and assessing ecological and en-
persed pellets), and small, fragile skeletal elements are lost, vironmental change, pellet-derived small-vertebrate as-
resulting in a residual concentration of larger, more robust semblages have been underutilized, perhaps because of
skeletal elements. The spatial distribution of skeletal ele- uncertainty about their taphonomic history (Fernandez-
ments below the roosting site follows a right-skewed, bimod- Jalvo et al., 1998). The taphonomic history of a pellet con-
al pattern. Skeletal elements are preserved in the soil to a sists of multiple phases, each with particular biases. First,
depth of three centimeters. Post-regurgitation processes selective hunting behavior of predators, time of predator
have the potential to distort the original faunal and skeletal activity, season, vulnerability of prey, and local climate
composition of pellet-derived assemblages, thus masking conditions will introduce an initial bias into the species
any original predator-specific signatures. Actualistic taph- composition of any predator-derived accumulation (Craig-
onomic studies are necessary in order to understand how head and Craighead, 1956; Andrews, 1990; Denys et al.,
well pellet-derived assemblages capture information on lo- 1996; Saavedra and Simonetti, 1998), although these
cal ecological and environmental conditions. This is a crit- same factors may permit identification of rare elements in
ical question that must be addressed to enable correction for the fauna (Avery et al., 2002). Second, the digestive pro-
such biases before pellet-derived assemblages are used for cess of pellet formation results in characteristic fragmen-
assessment of small-vertebrate community change and pa- tation and bone loss (Andrews, 1990). Owl pellets are
leoenvironmental reconstruction. formed in the stomach because a narrow pyloric opening
near the entrance of the esophagus prohibits large parti-
cles from entering the intestines; the undigested material
INTRODUCTION travels back up the esophagus and is ejected as a pellet
(Bent, 1961; Hoffman, 1988; Andrews, 1990). Third, pellet
Predation has long been recognized as an important history following regurgitation (weathering, transport,
mechanism leading to the concentration of small-verte- disintegration, and burial) has the potential to mask orig-
brate skeletal remains (Mellett, 1974; Dodson and Wexlar, inal species composition and other taphonomic signatures,
1979; Maas, 1985; Andrews, 1990). Modern predators in- thus biasing the fossil record.
Copyright � 2004, SEPM (Society for Sedimentary Geology) 0883-1351/04/0019-0497/$3.00498 TERRY
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; An-
drews, 1990). Other large pellet-producing raptors com-
mon to San Juan Island include the diurnal Bald Eagle
(Haliaeetus leucocephalus), Red-tailed Hawk (Buteo ja-
maicensis), Turkey Vulture (Cathartes aura), Northern
Harrier (Circus cyaneus), and the nocturnal Barn Owl
FIGURE 1—Maps of San Juan Island and Point Caution, Washington (Tyto alba), Western Screech Owl (Otus kennicottii), and
State, USA. (A) San Juan Island (redrawn from Morgan, 1960). Inset
Northern Saw-whet Owl (Aegolius acadicus). Of the noc-
shows location of San Juan Island in the San Juan Archipelago. (B)
Point Caution (redrawn from Friday Harbor quadrangle, USGS 7.5 turnal owls present on San Juan Island, Great Horned
minute series, 1954). 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
Of these three potential sources of bias, only raptor die- the last several years (Britton-Simmons, pers. comm.
tary preferences and patterns of bone destruction due to 2002, Smith et al., 1997).
digestion have received significant attention (e.g., Craig-
head and Craighead, 1956; Dodson and Wexlar, 1979; MATERIALS AND METHODS
Hoffman, 1988; Andrews, 1990; Denys et al., 1996; Saave-
dra and Simonetti, 1998; Trejo and Guthmann, 2003). The analyses included in this paper are restricted to the
Some of these studies have attempted to identify unique roosting tree in the Western Red Cedar grove with the
fragmentation and skeletal element representation sig- largest concentration of skeletal elements around its base.
natures left by different predators (e.g., Dodson and Wex- The surface distribution of skeletal elements around the
lar, 1979; Hoffman, 1988; Andrews, 1990), whereas others roosting tree was mapped using a north/south-oriented
have investigated the microscopic effects of digestion on grid, a handheld compass, tape measure, and six 0.25 m2
bones and teeth (e.g., Rensberger and Krentz, 1988; An- squares (50 cm x 50 cm) subdivided into 10 cm increments.
drews, 1990). Comprehensive study of the post-regurgita- Skeletal elements rarely were isolated, typically occur-
tion processes acting on pellet-derived small-vertebrate ring in discrete (or semi-discrete or dispersed but still spa-
assemblages is noticeably absent (but see Andrews, 1990, tially definable) clusters. Spatially defined occurrences of
for limestone-cave deposits). This study presents an initial bones typically contained at least one skull. Dodson and
investigation into the post-regurgitation taphonomic his- Wexlar (1979) reported that the mean number of pellets
tory of owl pellets in a temperate-forest environment with cast per meal is close to one for Great Horned Owls, thus,
a focus on: (1) the pattern of skeletal-element distribution each assemblage was reasonably interpreted as a modified
that has developed on the forest floor below a roosting site, pellet.
(2) changes in the fragmentation and relative abundance Assemblages were subdivided into three categories: (1)
of skeletal elements, and (3) changes in the taphonomic intact pellets, (2) partially dispersed pellets, and (3) fully
condition of skeletal elements as pellets disintegrate and dispersed pellets (Fig. 2). Pellets were classified based on
skeletal material is dispersed and incorporated into the the presence and condition of the matted-hair matrix of
soil. the pellet. Intact pellets were unbroken and characterized
by smooth, matted fur and an oblong shape. In partially
STUDY AREA dispersed pellets, the surface of the pellet was no longer
smooth and matted, the pellet was broken, and/or skeletal
The Friday Harbor Laboratory of the University of elements had fallen loose from the pellet. In fully dis-
Washington is situated on the eastern side of San Juan Is- persed pellets, spatially definable assemblages of bones
land, Washington, and includes both coastline and tem- were characterized by the absence of matted fur. All pellet
perate forest (Fig. 1). Point Caution, a headland directly material was mapped, and skeletal elements identified
north of the lab, is dominated by Western Red Cedar (Thu- when possible. Each quarter-meter quadrat on the surface
ja plicata) and Douglas Fir (Pseudotsuga menziesii) forests map was assigned a unique coordinate and the distance
that have not been logged for over 100 years (Staude, pers. from the center of the tree to the center of each quadrat
comm., 2002; Guberlet, 1975). Approximately 1.6 km was calculated. Five assemblages of each type were select-
northwest of the lab is a 0.01 km2 grove of Western Red ed randomly and collected by hand. Before collection, the
Cedar [N48�33.615�, W123�0.891�]. In summer 2002, when orientation of each bone with respect to the soil was
this study was undertaken, the floor of the grove was lit- marked on the exposed surface of the bone for all skeletal
tered with more than 1,500 small-vertebrate skeletal ele- elements free from the matted hair of pellets.
ments and more than 20 pellets in various stages of disin- Partial and complete pellets were hydrated overnight
tegration. The largest concentration of skeletal elements and picked apart with forceps under a dissecting scope
occurred around the base of one tree, but at least four oth- (magnification 2x to 6x). All elements were identified, andOWL 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. the phosphorus, nitrogen, and potassium (potash) content Fragmentation is defined for this study as breakage re- were determined using a Lammott Soil Kit. Finally, ex- sulting in loss of information about the function or shape ploratory excavation of six 20-cm2 sites was done using a of a skeletal element. The taphonomic condition of all ele- knife and spoon. Sites were excavated in 1-cm layers to a ments from nine representative assemblages (three from depth of 5 to 7 cm. Each layer was collected and wet sieved each assemblage type) was assessed. Bone modification is (250 �m mesh). used in this study to refer to a combination of chemical dis- Unless otherwise noted, statistical analysis consisted of solution due to digestive modification and secondary the construction of contingency tables analyzed using Chi- weathering processes, following Behrensmeyer (1978), in- square tests. Significance was established at the � � 0.05 cluding the effects of contact with/burial in soil (chemical level. alteration, etching, and pitting) and physical weathering agents operating on the forest floor (exposure to wind, RESULTS rain, temperature change). Bone modification categories used in this study are defined as follows: (1) unmodified Spatial Analysis bone—the inner cancellous bone is unexposed, protected by an intact outer layer of dense compact bone; (2) slightly The surface of the ground surrounding the roost tree modified bone—the cancellous bone is exposed; and (3) ex- slopes �20�–25� to the northeast. The soil beneath the co- tensively modified bone—the outer compact layer is ab- nifer detritus is acidic (pH � 5) and contains trace sent and the inner cancellous bone is exposed and eroded. amounts of phosphorus and nitrogen and a high amount of The slope of the forest floor was mapped using a square- potassium (potash). Pellet-derived material (assemblages) meter grid and a Brunton compass. A layer of mobile co- is found on all sides of the roost tree. The total number of nifer (cedar) detritus covers the soil. The orientations of assemblages decreases with distance from the roost tree; a skeletal elements from fully dispersed pellets were record- histogram of assemblages versus distance shows a con- ed to address the importance of hydrodynamic transport. cave, right-skewed distribution. This pattern is similar The pH of the soil beneath the conifer detritus, as well as when all assemblages are pooled, as well as when assem-
500 TERRY
FIGURE 4—Number of bones with distance from the tree. The distri-
bution of bones follows a bimodal pattern with a peak in abundance
at approximately 2 m. A two-tailed binomial test for proportions indi-
cates significance (p K 0.0001; n � 69).
tained in pellets would increase the sample size, and
would reinforce the pattern described.
Long bones were separated into two categories: (1) ro-
bust elements (humerus, pelvis, femur), and (2) fragile el-
ements (ulna, radius, tibia). The distribution patterns of
FIGURE 3—Number of assemblages pooled and subdivided by type robust and fragile elements were analyzed separately us-
with distance from the tree. (A) Total number of assemblages. (B)
Assemblages subdivided by type. The number of assemblages de-
ing the binomial statistical method described above. Both
creases following a right-skewed, concave trend. Intact pellets peak Figure 5 and the disparate sample sizes (see below) indi-
in abundance approximately 2 m from the tree and do not persist at cate that robust elements are present in higher numbers
distance. Results are not significant due to the small sample size, than fragile elements throughout the entire distribution,
which violates the assumptions of the Chi-square test (p � 0.2620; n and persist farther from the tree. The total numbers of ro-
� 52). bust and fragile skeletal elements in each area were sig-
nificantly different at � � 0.05 (Robust: p K 0.0001; n �
59; Fragile: p � 0.0001; n � 10). There is no evidence (such
blages are split on the basis of type (Fig. 3). Pellet abun- as preferred orientations of bones or runneling of the soil
dance peaks approximately 2 m from the tree, and intact surface and/or mobile conifer detritus) that would indicate
pellets do not persist at distance from the tree, whereas hydrodynamic transport at the site.
partially and fully dispersed pellets do persist. The above
pattern is not significant (p � 0.2620; n � 52, the number Taphonomic Analysis
of assemblages), perhaps due to the relatively small sam-
Relative proportions of all skeletal elements from 15 as-
ple size for the number of quadrats; expected values of
semblages were calculated for each assemblage type (in-
20% of the quadrats were fewer than five, a violation of the
tact pellets, partially dispersed pellets, fully dispersed pel-
assumptions of the Chi-square test.
lets; Fig. 6). The relative proportions of different skeletal
The total number of bones exposed on the surface dis-
elements differ significantly between assemblage types (P
plays a bimodal distribution with distance from the tree
K 0.0001, n �672). Fully dispersed pellets are dominated
(Fig. 4). Bones appear to be concentrated in peaks between
by mandibles (18%), followed closely by vertebrae (15%),
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 ac-
count was performed to determine significance of the dis-
tribution. 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 ex-
pected 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. Skel-
etal elements in pellets and disarticulating pellets are not FIGURE 5—Number of robust and fragile skeletal elements with dis-
tance from the tree. Robust elements are present in higher numbers
included because not all pellets were collected and dissect- throughout the distribution than fragile elements, and persist farther
ed. Figure 3 shows that pellets are found closer to the tree from the tree. A two-tailed binomial test for proportions indicates sig-
than dispersed assemblages. Including the bones con- nificance (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 ele-
ments. Results are significant (p K 0.0001; n � 351).
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 ele-
ments 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 con-
tained 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 sig-
nificantly different (p K 0.0001; n � 351).
Different types of skeletal elements display unique
breakage patterns. Skulls, for example, exhibit a charac-
teristic breakage pattern of the cranium in which the pos-
terior 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 sig-
nificant (p � 0.0151; n � 13), the small sample size vio-
lates 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
FIGURE 6—Relative proportions of skeletal elements with assem- an animal (Swindler, 1998), can provide clues about the
blage type; proportions differ significantly among assemblage types age structure of a prey assemblage. Approximately 63% of
(p K 0.0001; n � 672); Sk � skull; Ma � mandible; Sc � scapula;
V � vertebra; H � humerus; U � ulna; Ra � radius; P � pelvis; F �
all long bones (humerus, femur, tibia) from 15 assemblag-
femur; T � tibia/fibula; Ri � rib; CTP � manus and pes elements. (A) es are missing epiphyses (Fig. 9). Significance of this pat-
Intact pellets. (B) Partially dispersed pellets. Note (A) and (B) are dom- tern was established using an unpaired t-test (p � 0.0098;
inated by vertebrae, ribs, and manus and pes elements. (C) Fully dis- n � 46). This suggests a potential age bias towards youn-
persed pellets, which are dominated by more robust skeletal elements.502 TERRY
FIGURE 9—Epiphyseal presence and absence in humeri and femora
of Microtus townsendi; scale bar in millimeters. (A) Humerus with prox-
imal epiphysis missing. (B) Humerus with proximal epiphysis present.
(C) Femur with distal epiphysis missing. (D) Femur with distal epiph-
ysis present.
ger individuals in the prey assemblage. No significant dif-
ference was found for the frequency of epiphysis loss be-
tween the different types of skeletal elements (p �
0.0758).
Bone-modification results pooled for all elements by as-
semblage 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. Con-
versely, about 25% of skeletal elements from fully dis-
persed pellets and almost 20% of skeletal elements from
intact pellets show evidence of extensive bone modifica-
tion.
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 differ-
ence in terms of bone modification between elements pre-
sent 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 modifi-
cation than the shafts (p K 0.0001; n � 43; Fig. 11).
Skeletal elements from fully dispersed pellets are scat-
tered 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 ful-
ly dispersed pellets is shown in Figure 12. Seventy percent
FIGURE 8—Fragmentation patterns of skulls; scale bar in millimeters. of surfaces exposed to the air showed little to no bone mod-
(A) Dorsal view of three skulls (Microtus townsendii). Two exhibit a
common posterior breakage pattern. (B) Ventral view of the same
ification, whereas only 50% of surfaces exposed to the soil
skulls. (C) Frequency of posterior skull breakage. All skulls recovered showed little to no bone modification. However, almost
from intact pellets were unbroken. Results are significant (p � 0.0151; 20% of surfaces exposed to the soil show extensive bone
n � 13), however, small sample size violates the assumptions of the modification, while only 5% of surfaces exposed to the air
Chi-square test. 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 incorporatedOWL PELLET TAPHONOMY 503
FIGURE 11—Bone modification compared for the ends and shafts of
longbones. The ends of longbones show a significantly higher fre-
quency of modification than do the shafts (p K 0.0001; n � 43).
son and Wexlar, 1979; Hoffman, 1988; Andrews, 1990). In
his paper on pellet-derived fragmentation patterns, Hoff-
man (1988) briefly acknowledged that post-pellet diage-
netic processes might confound the taphonomic signatures
of small-vertebrate assemblages, making definitive pred-
ator identification difficult. However, the number of actu-
FIGURE 10—Bone modification (A) Examples of different degrees of alistic studies of the post-regurgitation history of pellets is
bone modification in pelves of Microtus townsendii; N � no bone mod-
ification; M � extensive bone modification (cancellous bone is ex-
limited (but see Andrews, 1990). The results from this
posed on the shaft and eroded on the distal ilium); scale in millimeters. study illustrate the importance of understanding how
(B) Bone modification by assemblage type. Bones from fully dispersed post-regurgitation processes can bias small-vertebrate as-
pellets show the highest frequency of no modification. Results are semblages over time in a temperate-forest environment.
significant (p � 0.0116; n � 191). 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 as-
semblage, which should then affect paleoecological inter-
pretation (Behrensmeyer, 1993). Thus, before a pellet-de-
rived small-vertebrate assemblage can be used for paleo-
ecological 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 pel-
let-derived assemblages (Dodson and Wexlar, 1979; Hoff-
man, 1988; Saavedra and Simonetti, 1998; Andrews, 1990;
Kusmer, 1990). Relative abundance of skeletal material FIGURE 12—Bone modification of elements from fully dispersed pel-
and characteristic breakage patterns produced in labora- lets, compared for surfaces exposed to air and soil. Surfaces in con-
tory settings also have been proposed as methods to facil- tact with the soil show a significantly higher frequency of extensive
itate predator identification from field assemblages (Dod- bone modification (p � 0.0449; n � 241).504 TERRY 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 el- servations of Dodson and Wexlar (1979), while broken ements contained inside (Andrews, 1990). Assuming the skulls were found in partially dispersed pellets and fully different pellet types designated in this study (intact pel- dispersed pellets on the forest floor (Fig. 8). This suggests lets, partially dispersed pellets, and fully dispersed pel- that skulls become fragmented more frequently during lets) represent steps in the disintegration process over post-regurgitation processes such as scavenging; and that time, both the relative proportion of skeletal elements pre- caution should be used if employing skull breakage as a sent in an assemblage and the degree of fragmentation tool for predator identification. changed as a result of time exposed on the forest floor. As Based on this study, as pellets disintegrate, the skeletal the pellets broke down, bones became dispersed, and frag- composition of the assemblage shifts from a high propor- mentation increased (Fig. 7). Evidence for transport and tion of small, fragile elements to a high proportion of larg- dispersal by hydrodynamic processes is lacking, thus, the er, more robust elements, because the fragile elements are increased dispersal and fragmentation of skeletal materi- preferentially lost (Figs. 5, 6). Skeletal elements from fully al must be due to other factors, such as scavenging. dispersed pellets (which potentially have spent the most Dodson and Wexlar (1979) observed a high frequency of time in contact with the forest floor) show the highest fre- posterior breakage in skulls recovered from Great Horned quency of no bone modification. Elements from intact pel- Owl pellets in a laboratory setting and presented this as a lets, however, show lower proportions of no modification possible predator identification tool. However, they also than bones from dispersed pellets and high frequencies of reported feeding observations for Great Horned Owls, in both slight and extensive bone modification (Fig. 10). A which the owls swallow mice whole and headfirst, making possible explanation is that the matted hair in a pellet ini- no attempt to extract the brain of the animal. In compari- tially shields the smaller and fragile bone fractions (which son, the skulls recovered from intact pellets in this study are perhaps more intensely modified by digestion). Upon primarily were unbroken, consistent with the feeding ob- pellet disintegration, these smaller, fragile, extensively
OWL PELLET TAPHONOMY 505
modified elements then become exposed to physical- and regurgitation processes such as scavenging. Thus, frag-
chemical-weathering processes on the forest floor. Ar- mentation is not a reliable indicator for predator identifi-
mour-Chelu and Andrews (1994) documented significant cation.
burial and lateral transport of small skeletal elements due (4) The frequency of intense bone modification is highest
to earthworm bioturbation. This, coupled with potential in pellets and lowest in dispersed assemblages. This likely
effects of preferential destruction due to higher modifica- reflects preferential destruction of smaller, more fragile
tion, would result in a residual concentration of the larger, skeletal elements, which may be modified more intensely
robust, skeletal elements that were less extensively modi- due to digestive processes, upon being released from the
fied by digestion. Of the bones that do remain on the forest pellet.
floor, it should be noted that the surface in contact with (5) The taphonomic history of pellet-derived small-ver-
the soil shows a higher degree of bone modification than tebrate assemblages is more complex than commonly ac-
the surface that is exposed to the air (Fig. 12). This pref- knowledged. Since post-regurgitation processes distort
erential damage is consistent with a variety of actualistic the original skeletal composition of pellet-derived assem-
studies of weathering of larger bones (e.g., Behrensmeyer blages, actualistic studies are necessary in order to under-
and Hill, 1980). stand and correct for this bias, leading to more accurate
The distinctive characteristics of fragmentation and assessments of small-vertebrate community change and
skeletal element representation thus change over time. paleoenvironmental reconstruction.
This presents a problem if these characteristics are to be
used to determine whether an assemblage is predator-de- ACKNOWLEDGEMENTS
rived, and to identify the predator. Alternate methods for
identifying an assemblage as pellet-derived deserve more This work was completed as part of the 2002 Marine Ta-
attention. As expected, in this study, the density of skele- phonomy course at the Friday Harbor Laboratory of the
tal elements decreases with distance from the tree. The University of Washington. I would like to give special
right-skewed, bimodal distribution of skeletal elements thanks to Michael LaBarbera and Michal Kowalewski for
seen below the roost site, with a distribution radius of in- their enthusiasm, patience, and invaluable assistance and
tact pellets approximately 2 m from the tree (Figs. 3, 4), is their willingness to let me work on terrestrial vertebrates
most likely the result of a reflecting boundary (the tree in the context of a marine invertebrate taphonomy course.
trunk) and preferred roosting position of the owl. As pel- I would also like to thank David Eberth, Karen Chin, and
lets are regurgitated and tumble through the dense Peter Andrews for their helpful reviews of this manu-
branches to the ground below, scattering off the branches script, as well as Susan Kidwell, Matthew Kosnik, and
and the trunk would yield a peak in abundance at a mod- Mark Terry for their initial comments on the paper. Final-
erate distance from the trunk. If surface patterns also are ly, much thanks is also due to Katie LaBarbera for her
preserved at depth (especially in environments more con- field assistance.
ducive to preservation than acidic, moist, coastal-forest
soils), they could be powerful indicators that an assem-
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