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A Field Experiment in Avian Taphonomy

Author(s): K. Jeffrey BickartSource: Journal of Vertebrate Paleontology, Vol. 4, No. 4 (Dec., 1984), pp. 525-535Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate Paleontology

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they rested on bare soil, leaf litter, live plants, or twigs,although as disarticulation reached an advanced stageonly bones resting on bare soil continued to be gluedto the ground. Subsequent floods, which on severaloccasions washed over the floodplain and submergedthe cages, failed in most cases to change the positionsof the bones by more than several centimeters. Onlythe most severe storm of the season had a significant

effect, moving some of the bones of three of the spec-imens to one edge of their cages, but leaving otherbones of the same birds, and all bones of the other

three, little moved from their original positions (Fig.4). Because the mesh size of the cages was large, andbecause deposited mud was found on carcasses after

floods, it is reasonable to assume that the cages didnot significantly impede water flow around the car-casses.

Other Movements of Carcasses and Bones

Figure 4 shows the positions of the bones of four ofthe caged rock doves after five months. Many of thebones moved outward from the

originalcarcass loca-

tion and changed orientation. Some of these changesin position and orientation occurred with the mostsevere floods during the wet early summer. Bones not

resting on bare soil, and thus not stuck down, weremore susceptible to displacement than bones stuck tothe ground; plant growth in four of the cages probablymoved some bones, but not significantly.

After completion of disarticulation, there was little

change in bone orientation and distance from the orig-inal location of the carcass. During the winter of 198 1-

82, long periods of coverage by snow and ice alter-

nating with briefer periods of partial or complete thawalso had little effect on bone position. Bones were fro-zen into the positions they had attained prior to thesnows. Substantial movement occurred only with ma-

jor thaws, when the concomitant floods moved someof the bones nearer to the cage edges.

Near the end of the one-year observation period,one of the cages of the five adult rock doves was de-

stroyed and the contents disrupted, probably by hu-mans. Some of the bones of two of the other four adult

specimens had moved into contact with the cage edgessince my examination at the beginning of March, and

undoubtedly would have gone further. Nevertheless,most of the bones of these specimens remained in thecenters of the cages. The positions of the bones of the

other two specimens were as before, and not in contactwith the cage edges. Approximately half of the bonesof two of these four specimens became partially buriedin the soil and also fixed in place by vegetation.

One gull, #15, was carried by floodwaters from thebank into the stream, about 1.6 m downstream and 1

m out from the bank. The gull was exposed in thestream for several days. Sediment carried by streamwaters after several storms then buried the gull quickly.The carcass was completely articulated and well cov-ered by feathers when it was buried, with the exception

of the right tibiotarsus, tarsometatarsus, and pha-

langes, which separated from the rest of the carcassand were buried a short distance away. As the waterlevel of the stream dropped during the dry part of the

summer, the mud covering the carcass became ex-

posed. I excavated the carcass 3.5 months after burial.I noted the following changes which occurred afterburial (see Fig. 5): (1) almost all feathers and all soft

tissue disappeared; (2) the left carpometacarpus andthe first phalanx of the second digit moved about 10cm from the distal ends of the left radius and ulna; (3)the phalanx subsequently disarticulated from the car-

pometacarpus; (4) both humeri disarticulated from the

coracoids and scapulae, and those bones from the ster-

num; (5) the ribs and the cervical vertebrae disartic-ulated and became separated or transported away; (6)the plunge of the left tarsometatarsus changed from

approximately zero to 900; (7) the left femur disartic-

ulated from the pelvis, the plunge became vertical, andit rotated almost 1800 around its vertical axis; (8) a

scapula also plunged at 900, and the right humerus,

radius, and ulna plunged at about 300. I observed onthe mud over the carcass racoon and dog tracks which

were as deep as the top level of the bones, and nu-

merous living plant roots in the mud surrounding thebones. Thus the changes noted above could have beencaused by trampling or growth of plants; the formerhas been suggested as a burial process (Behrensmeyerand Dechant-Boaz, 1980; Laporte and Behrensmeyer,1980).

Decay and DisarticulationThe timing of decay and disarticulation showed great

individual variation. For six individuals time to com-

pletedisarticulation

rangedfrom 13 days for the one

juvenile rock dove to about six months for one of theadults. The times for the four adult birds between the

extremes ranged from 65 to 100 days (Table 2).The disarticulation of the skeletal elements also

showed individual variation, within an overall distinct

sequence (Table 2, Fig. 6). Figure 6 indicates: (1) early

(i.e., with respect to the 75 days over which most ofthe disarticulation occurred) disarticulation of ribs from

the sternum; (2) early disarticulation of hind limb joints;(3) late disarticulation of vertebrae; (4) late disarticu-lation of the pectoral girdle; (5) a tendency for proximal

wing joints to disarticulate before distal wing joints;

(6) completion of leg disarticulation before wing dis-

articulation. For each of the joints or groups of jointsshown in Figure 6, I calculated the time at which 75%of the joints in the sample had disarticulated. Theseare: vertebrae 34 days; sternum-ribs 25 days; stemrnum-coracoid 35 days; coracoid-scapula 55 days; coracoid,scapula-humerus 20 days; humerus, radius, ulna 45

days; manus 65 days; hind limb 45 days. These figuresmay be useful in estimating length of time before burialof a carcass (but see discussion of the stream-buried

gull, below). Observation of the sequence of disartic-ulation was difficult because feathers obscured the bones

528 JVP4(4), December1984

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a b30 30-

20 20 _ __3

cPi

10 10-

CM 10 20 CM 10 20 30

30

20-20

.-

Vi I

CM 10 20 CM 10 20 30 40

FIGURE 4. Drawings of four of the five caged adult rock doves, showing disarticulation and movement of bones outwardfrom the original locations of carcasses (dotted outlines) in centers of cages. Solid lines next to bones indicate contact of boneand cage edge. Dotted lines cutting across bones indicate burial of undrawn part of bone. Drawing made after five months. a,#1. b, #6 -carcass originally in location at left; severe flood moved carcass, with exception of stuck skull, to location at right,from which bones then scattered outward. c, #5. d, #2.

JVP 4(4), December 1984 529





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25

20

15

1 0

5

C M 1 0 1 5 2 0 2 5 land urfaceI I I I I

tarsus O[5humerus I i --1

femur radius cerv. vert.ulna

530 JVP4(4), December 1984





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TABLE 2. Times of disarticulation of skeletal joints of six caged rock doves. Specimens indicated by numbers 1-6. Wherethe same number is noted for a joint twice, it signifies a different disarticulation time for the two sides; where only one numberis noted for a bilaterally symmetrical joint, it signifies the same disarticulation time for the two sides. Note that those jointsof #3 not on diagram by 110 days were still articulated at that time. The times of disarticulation not on the table for other

specimens were not noted.

Time in days

0- 6- 11- 16- 21- 26- 31- 36- 41- 46- 51- 56- 61- 66- 81- 96- 106-

5 10 15 20 25 30 35 40 45 50 55 60 65 70 85 101 1101. Skull-lower mandible 6 52. Skull-atlas 2 3 1 5 63. Cervical vert-cerv

vert 6 1 54. Cerv vert-thoracic

vert 2 65. Thoracic vert-pelvis 6 1 26. Pelvis-caudal vert I 1 2 67. Thoracic vert-ribs 1 2, 68. Synsacrum-ilia and

ischia 2, 4 6 1, 59. Sternum-ribs 2 1, 5 6

10. Sternum-coracoid 4, 5 1, 2 3 1 6

11. Coracoid-scapula 4 6 6 2, 5 212. Coracoid and scap-

ula-humerus 2, 4, 6 5 113. Humerus-radius and

ulna 4 6 1 5 214. Radius proximal-

ulna proximal 4 2, 6 3, 5 3 215. Radius distal-

ulna distal 4 2, 6 3, 5 3 216. Radius and ulna-car-

pometacarpus 4 6 1 2, 5 2 6 317. Carpometacarpus-

digit 2 4 1 2, 5 2 318. Digit 2, phalanx

1-digit 2,phalanx 2 4 5 2 6 1 119. Pelvis-femur 4 2, 6 1, 6 620. Femur-tibiotarsus 4 1 2 2 5 321. Tibiotarsus-tarso-

metatarsus 4 1 1 2 6 2, 5 3 322. Tarsometatarsus-

phalanges 4 2 6 1 6 2

and made the exact time of disarticulation of a jointuncertain. Thus the patterns shown in Figure 6 shouldbe considered only probable patterns.

Damage to Bones by CarnivoresI recovered from the site a small number of bones

damaged by racoons and foxes. They were similar in

appearance to mammal bones damaged by carnivores

(Haynes, 1980; Gifford, 1981; Shipman, 1981). The

scraps all show green fractures typical of fresh bone

(D. C. Fisher, pers. comm.) (Fig. 7). The fracture sur-faces consist of smooth edges interrupted by jags. Frac-tures of spongy bone (Fig. 7, middle) look on a gross

level rougher or "fuzzier" than fractures of compactbone (Fig. 7, top, bottom), but when magnified (Fig.8, middle) the green fracture pattern is evident. Tooth

punctures are present on two specimens (Fig. 7, top;Fig. 8, top). Of the 19 long bones recovered, 16, or

84%, were damaged by removal of one or both (onespecimen) of the articular ends. This has been observed

4--

FIGURE 5. Drawings of gull (#15) washed into stream and buried by sediment. Drawings made following excavation 3.5

months after burial. Top: plan view. Bottom: schematic projection of bone positions onto plane perpendicular to land surface;some bones omitted for clarity.






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- =

•i•

FIGURE 8. Enlargementof bones shown in Figure 7, insame order. Scalebarequals 5 mm.

ably applicableto areas with: (a) enough rainfall to

keep the floodplain well vegetated and wet enough for

potential scavengers to have their dens elsewhere; (b)

large numbers of scavengers; and (c) low sedimentation

rates. Changes in any of these variables might lead to

different results. If gluing is dependent not only on

fluids from the decaying carcass but also on a wet sub-

strate, it might not occur, for example, on the loose

sand of a beach or in an area with low precipitation.

Gluing does not last if the bones are not in contact

with the soil, and so may be seasonally dependent, not

happening in autumn, for example, when a dead bird's

bones would rest on a bed of fallen leaves. Carcass size

might also affect the results. I found remains of oneout of the six uncaged rock doves, but remains of fourout of 16, or 1/4, f the ring-billed and herring gulls, a

higher proportion of the largerbirds. This suggests that

scavengers were more likely to completely carry awayfrom the site or completely consume at the site thesmaller birds, thus leaving at the site fewer remains ofthem.

Hill (1979) concluded from both theoretical and em-pirical considerations that there is an effective limit tothe scattering of bones by random processes (e.g., dis-

articulation, kicking by passing animals) and that theseform "the prevailing process upon which the effects ofmore specific processes my be imposed" (p. 272). One

specific process is disturbance by scavengers. I ob-served an apparent limit to the scattering of bird bonesin the absence of vertebrate scavengers; this may, es-

pecially when combined with observations of bone

damage, facilitate recognition of degree of scavengingand predation.

Knowledge of disarticulation sequences may also do

this. For example, if a fossil, semi-articulated skeletonis found to be in a state of disarticulation not consistentwith modern observations of undisturbed carcasses

(e.g., wings disarticulated, legs not), disturbance byscavengers should be considered as one possible ex-

planation. Just as ".... disarticulation pattern ... mayillustrate those features that are unique to various hu-

I" , ;~~

:'I

FIGURE 9. Tarsometatarsusof a juvenile rock dove,showingdamagethat occurredon articularends of most ofthe long bones; cause uncertain.Note holes in middle andbottom trochleae.Scale barsequal 5 mm.






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man butchery patterns" (Hill, 1980:134), it may illus-trate those features unique to non-human scavengingpatterns.

From the changes in articulation and positions ofthe bones of the buried gull it is clear, however, thatsuch characteristics of a skeleton do not necessarilybecome permanent after burial. These observationscontrast with Hill's (1979:261) statement that disar-

ticulation "provides an estimate of the interval be-tween death and burial," and with Toots' (1965:38)statement that "Partial burial has the effect of fixingthe buried part in the position and state of articulation

they had at the time of burial ... ." Disarticulation

should, then, be used only cautiously as an indicationof duration of subaerial exposure and condition of thebones when buried.

The characteristics of bones damaged by predatorsand scavengers have received much attention fromworkers on mammals (e.g., Brain, 1980; Binford, 198 1;Gifford, 1981). Ability to distinguish damage by pred-ators and scavengers from damage by other processesmay enable one to make inference about presence orabsence of predators and scavengers, and intensity of

predation and scavenging, among other things. Theobservations presented in this study are, however, basedon only a few specimens, and further work should bedone.

If weathering can be recognized on fossil bones, one

may be able to estimate length of time of subaerial

exposure of bones. I observed no weathering of thebones of the adult rock doves after one year. In other

environments, however, bird bone may break down

very quickly. C. G. Spies (pers. comm.) has observedcarcasses reduced from fresh to chalky, crumbly bones

within three weeks. Those carcasses were found onislands in Oneida Lake, New York, among vegetationfour to five feet high, closely packed, with the watertable a few inches below the ground surface and hu-

midity near the ground probably near 100%. Clearlythere is a need for much more work, including com-

parative work with birds and mammals in the sameenvironment. The results of such work would have

implications for the contention that bird fossils arerarer than mammal fossils because bird bones are moredelicate than mammal bones.

An actual example from the avian fossil record mayilluminate how modern taphonomic observations may

help solve problems in avian paleoecology. The lacus-trine deposits of the Big Sandy Formation of Arizona,mentioned in the Introduction above and under studyby me, contain some of the richest known concentra-tions of fossil birds. Most of the fossils are of ducks,geese, and swans; other water-associated types such as

storks, rails, flamingoes, and shorebirds are present inlesser numbers, and there are a very few land birds,primarily diurnal raptors. Articulated skeletons or partsof skeletons are common, and the material is on thewhole densely packed. The mammals known from thesame deposits as the birds include carnivores of at least

five families; they include a fox, Vulpes stenognathus,and other canids, and a procyonid, Bassariscus sp.Among the important questions are: How does thisfossil assemblage reflect the species composition andrelative abundances of species in the original com-

munity? Were ducks really the most common group?How does the species diversity at this site in the earlyPliocene compare with species diversity in lacustrine

habitats today? Knowledge of how modern carnivorescan by scavenging change the composition of an as-

semblage of carcasses on a land surface will help toreconstruct the original bird community. Observationsof degree of disarticulation, and of any damage to thebones from the carnivores will also help, as discussedabove. The practical problem arises in this denselypacked assemblage of determining what bones go towhat individuals; modern observations of disarticu-lation and scattering will help to solve this, too. In

addition, as Hill (1980:134) has noted, ".... disartic-ulation pattern ... may also explain aspects of differ-ential representation of skeletal parts in fossil accu-

mulations"; the majority of the bird fossils from theBig Sandy Formation are wing elements-a very cu-rious feature of the assemblage, which needs to be

explained.

CONCLUSIONS

This study presents some of the first observationson avian taphonomy and provides a base for further

studies, with the goal of understanding the nature andevolution of ancient bird communities. The main re-sults and conclusions of this study are: (1) eliminationof bird remains from a potential fossil record may bedue to

primaryremoval

at death rather than to theirsupposed fragility--the common explanation-andlarge predators and scavengers are important agentsdecreasing the probability of preservation of skeletalremains in the environment of death; (2) adherence ofcarcasses to the ground increases preservation poten-tial in an environment of light scavenging and highdepositional rates, and decreases preservation potentialin an environment of heavy scavenging and low de-

positional rates; (3) there is a distinctive pattern ofdisarticulation for the avian skeleton, and there is alimit to the scattering of bird bones in the absence of

scavengers; (4) carnivore damage to bird bones ap-

pears, initially,to be similar to that observed for mam-

mal bones; (5) the above observations on disarticula-

tion, scattering, and damage to bones can provideinformation on presence, absence, and extent of scav-

enging and predation; (6) disarticulation and scatteringof skeletons can continue after burial.

Acknowledgements-I am indebted to many people fortheir help with this study. E. R. Meyer helped me toobtain rock dove specimens and suggested the methodfor constructing a string-trailing device. C. G. Spiessupplied the carcasses of gulls, identified plants at the

site, and contributed numerous suggestions. T. A. and

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F. R. Bickart provided financial support without which

my work would not have been possible. George Junne

generously photographed the damaged bones. The fol-

lowing people read and commented on one or more

drafts of the manuscript, and their suggestions have

improved it immeasurably: C. Badgley, R. T. Bakker,A. K. Behrensmeyer, P. Dodson, D. C. Fisher, P. D.

Gingerich, D. W. Steadman, and M. G. Wolman. I am

particularly grateful to Catherine Badgley for manyhours of conversation on paleoecology in general and

taphonomy in particular, and to Robert T. Bakker for

the stimulating discussions which formed such an im-

portant part of my undergraduate studies at The Johns

Hopkins University.

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