Scavenging of migratory bird carcasses in the Sonoran Desert

Abstract

In this study we report avian and mammalian scavengers foraging on migratory bird carcasses in the Sonoran Desert. We used remote cameras to monitor carcasses we found along a power line right-of-way (n = 25). We documented four species scavenging 10 carcasses (kit fox, Vulpes macrotis, n = 4; coyote, Canis latrans, n = 3; common raven, Corvus corax, n = 2, and greater roadrunner, Geococcyx californianus, n = 1) and recorded coyote tracks at three additional carcasses. Neither remote cameras nor tracks indicated the scavenger species of the remaining carcasses. Our data suggest migrant birds might provide an important food source for resident scavengers, particularly in desert habitats where food can be scarce. Our study also supports prior assertions that failure to account for removal of carcasses by scavengers might cause errors in estimates of mortality.

Full text

THE SOUTHWESTERN NATURALIST 59(4): 542–547 DECEMBER 2014
SCAVENGING OF MIGRATORY BIRD CARCASSES IN THE SONORAN
DESERT
ANDREW M. ROGERS,MICHELLE R. GIBSON,TYLER POCKETTE,JESSICA L. ALEXANDER,AND JAMES F. DWYER
*
EDM International, Inc., 4001 Automation Way, Fort Collins, CO 805252 (AMR, MRG, TP, JLA, JFD)
Present address of MRG: School of Biological and Biomedical Sciences, University of Durham, South Road, Durham DH1 3LE, U.K.
*Correspondent: jdwyer@edmlink.com
ABSTRACT—In this study we report avian and mammalian scavengers foraging on migratory bird carcasses in
the Sonoran Desert. We used remote cameras to monitor carcasses we found along a power line right-of-way
(n =25). We documented four species scavenging 10 carcasses (kit fox, Vulpes macrotis,n=4; coyote, Canis
latrans,n=3; common raven, Corvus corax,n=2, and greater roadrunner, Geococcyx californianus,n=1) and
recorded coyote tracks at three additional carcasses. Neither remote cameras nor tracks indicated the
scavenger species of the remaining carcasses. Our data suggest migrant birds might provide an important food
source for resident scavengers, particularly in desert habitats where food can be scarce. Our study also
supports prior assertions that failure to account for removal of carcasses by scavengers might cause errors in
estimates of mortality.
RESUMEN—En este estudio reportamos observaciones de mam´ıferos y aves carro˜
neros aliment´andose de
cad´averes de aves migratorias en el desierto de Sonora. Usamos c´amaras remotas para monitorear cad´averes
(n =25) que encontramos en la zona de restricci ´on a lo largo de un cable de luz. Se registraron cuatro
especies consumiendo carro˜
na de 10 cad´averes (zorra norte˜
na, Vulpes macrotis,n=4; coyote, Canis latrans,n=
3; cuervo com ´
un, Corvus corax,n=2, y correcaminos norte ˜
no, Geococcyx californianus,n=1), y se registraron
huellas de coyote en 3 cad´averes adicionales. Ni c´amaras remotas ni huellas indicaron la especie carro˜
nera
para el resto de los cad´averes. Nuestros datos sugieren que las aves migratorias son potencialmente una fuente
importante de alimento para las especies de carro ˜
neros residentes, particularmente en los h´abitats des´erticos
donde la comida puede ser escasa. Nuestro estudio tambi´en apoya previas declaraciones de que no considerar
la remoci´on de cad´averes por animales carro ˜
neros puede ocasionar errores en las estimaciones de la
mortalidad.
Studies of avian migration typically focus on when,
where, why, and how birds migrate (Bowlin et al., 2010).
Survival during migration is of interest, particularly
because it is often lower than survival during sedentary
periods. However, because dead migrants are rarely
found, details describing the timing or location of
mortality during migration are scarce (Sillet and Holmes,
2002). Where avian survival during migration has been
investigated, mortality has been attributed to exhaustion
while traversing ecological barriers (Newton, 2008),
collision with anthropogenic structures (Longcore et al.,
2013; Sporer et al., 2013), loss of stopover habitat
(Sutherland, 1996; Schwarzer et al., 2012), and predation
(Schmaljohann and Dierschke, 2005).
Deserts are an important ecological barrier for avian
migrants moving between Europe and Africa (Newton,
2008; Strandberg et al., 2010) and between North
America and South America (Patten et al., 2003).
Migratory birds frequently die while traversing deserts
during migration, but almost no information exists
describing the fate of their carcasses. Avian collisions
with anthropogenic structures also have been widely
reported (Longcore et al., 2013; Sporer et al., 2013), but
there is little understanding of the interactions between
ecological barriers and anthropogenic structures. The
ecological role dead migrants might play in the commu-
nities surrounding the anthropogenic structures has not
been investigated.
We hypothesized that the carcasses of migratory birds
would be an important prey source to desert scavengers
and predicted that if migratory bird carcasses could be
found and monitored, desert scavengers would be shown
consuming these carcasses. Direct observations of scaven-
gers might not provide an unbiased estimator of scavenger
activity because some scavenger species could be more
difficult to detect. For example, some of the potential
scavengers in our study were hunted and were cryptically
colored (e.g., coyote, Canis latrans) while others were
protected and were boldly colored (e.g., common raven,
Corvus corax). Remote cameras can provide useful infor-
mation when human observers might affect behaviors of
interest (Dwyer and Doloughan, 2013). In this study we

used direct observations and remote cameras to provide
the first documentation of residents scavenging the
carcasses of migratory birds in the Sonoran Desert of
southern California.
MATERIALS AND METHODS—Study Area—We conducted our
study along a power line right-of-way between El Centro,
California (3284703100
N, 11583304700
W) and Ocotillo, California
(3284401900
N, 11585903900
W) on land owned by the U.S. Bureau
of Land Management. We selected our survey area because large
numbers of migrants breeding in North America and wintering
in Central and South America pass through this portion of the
Sonoran Desert (Patten et al., 2003), because collisions with
power lines are of management interest (Ponce et al., 2010;
Barrientos et al., 2012; Sporer et al., 2013), and because desert
passages can be particularly difficult for migrant passerines
(Newton, 2008; Strandberg et al., 2010). Our study area was
entirely within the Sonoran Desert with elevations from 1–300 m
above sea level (El Centro is below sea level). Vegetation was
sparse, dominated by creosote bush (Larrea tridentate) and
ocotillo (Fouquieria splendens) and, to a lesser extent, Opuntia
species of cholla and prickly pear cactus, indigo bush
(Psorothamnus species), and occasional mesquite trees (Prosopis
species). Rainfall averaged <13 cm per year (Western Regional
Climate Center, 2013) and summer temperatures regularly
reached 408C.
Data Collection—Between 15 March 2013 and 15 June 2013,
we walked transects daily through our study area in search of
migratory bird carcasses. We defined our survey period to
coincide with the peak of avian migration through our study
area (Patten et al., 2003). Each transect was 500 m long and 50
transects were surveyed 14–16 times. Transects were completed
by three observers walking parallel to one another along a power
line right-of-way. Each observer surveyed a 25-m wide portion of
the total transect, with each observer slightly overlapping the
areas surveyed by the adjacent observer so that total transect
width was 65 m. Each observer zig-zagged back and forth within
their area of responsibility within each transect (as in Faanes,
1987; Barrientos et al., 2012), and walked slowly at about 3–4
km/hr (2 mph; as in Murphy et al., 2009).
We began transects at local sunrise and continued until
approximately 6 h after sunrise. While walking transects, we
recorded observations of foraging behaviors of potential
scavengers; for example, a common raven (Corvus corax)in
flight with nothing in the feet or beak, dropping to the ground,
and then immediately flying up again in possession of a
migratory bird carcass. When this occurred, we followed the
common raven to identify the scavenged animal to species level,
if possible, or to family or order otherwise. We also recorded the
locations of the migratory bird carcasses we encountered. To
minimize the possibility that our presence might influence
scavenger activity, we did not collect, mark, move, or remove any
migratory bird carcass we encountered. We recorded carcass
locations with a WAAS-enabled GPSmap 62s receiver (Garmin
International, Olathe, KS) and used the global positioning
system device to return to carcass locations. We used direct
observations and remote cameras to document scavenging of
migratory bird carcasses.
We also used remote cameras to document scavenging
events. We used three remote cameras, one HC500 (Reconyx,
Inc., Holmen, WI) and two Bushnell Trophy Cams (Bushnell
Corporation, Cody, KS). Each camera was programmed to
capture three, eight-megapixel photographs at 5-s intervals each
time the camera was triggered. We initially used high-sensitivity
settings on the cameras but, during presurvey trials, found that
at these settings the cameras exhausted their memories and
power supplies recording wind-driven movements of vegetation.
Thus, we used medium-sensitively settings throughout the study
to balance oversensitivity to vegetation movement with under-
sensitivity to scavengers.
The cameras recorded color photographs illuminated via
ambient light during the day, and black and white photographs
illuminated via infrared at night, allowing 24-h continuous
observation of carcasses. At each carcass we monitored, we
placed one remote camera under a nearby creosote bush.
Creosote bushes were common in the study area and provided
visual cover, reducing the likelihood that cameras would be
noticed by potential scavengers or people. Creosote bushes
could have obscured the infrared sensors of the camera. To be
sure each camera had a clear view of each monitored carcass, we
laced any branches that would obscure the carcass behind
adjacent branches outside the view of the camera. We wrapped
each camera in burlap and the branches of creosote bushes to
further break up the boxy shapes of the cameras. We then
revisited each monitored carcass every 24–48 h. The substrate in
the study area was a mix of sand and gravel. If the carcass was
absent, we recorded any animal tracks within 5 m, if present, and
retrieved the camera. Because the cameras were triggered based
partially on detection of body heat, being in an already warm
environment could have decreased the likelihood of cameras
triggering when scavengers were present. Recording tracks
enabled us to evaluate scavenging in cases where the camera
did not capture an image of the scavenger.
RESULTS—While walking transects, we recorded 26
instances of a common raven in flight dropping to the
ground with nothing in its feet or beak and then
immediately flying up with a migratory bird carcass in
its beak. Scavenged birds identified to species were two
black-headed grosbeaks (Pheucticus melanocephalus), one
black-throated gray warbler (Setophaga nigrescens), one
orange-crowned warbler (Vermivora celata), one white-
winged dove (Zenaida asiatica), and two yellow warblers
(Setophaga petechia). Five warblers could not be identified
to species (Family Parulidae) and 14 birds could be
identified only as passerines (Order Passeriformes), based
on size, as the raven departed with the carcass in its beak.
These scavenging events occurred primarily during the
morning (mean –SE =0757 h –20 min; min =0543 h,
max =1138 h) with two scavenging events observed in
March, 14 in April, 10 in May, and none in June after
young ravens had fledged and family groups of ravens
moved away from the power line corridor.
We used remote cameras to monitor the carcasses of 21
passerines and four nonpasserines (n=25, Table 1).
Cameras recorded kit fox (Vulpes macrotis), coyote,
common raven, and greater roadrunner (Geococcyx
californianus) scavenging 10 of the carcasses (Fig. 1).
These scavenging events occurred primarily during the
night (n=6), with fewer recorded during the early
December 2014 Rogers et al.—Scavenging of migratory bird carcasses 543

morning before 0630 (n=2) or later in the day (n=2).
Coyote tracks indicated the scavenger species at three
carcasses where cameras failed, but we could not identify
the time of day these carcasses were scavenged. Seven
carcasses were scavenged, but neither cameras nor tracks
indicated the scavenger species. Combining these 20
events, one scavenging event occurred in March, seven in
April, 10 in May, and two in June. Five carcasses were not
scavenged during the monitoring period.
Combining both types of scavenging events (direct
observations and remote cameras), carcasses scavenged
by birds were consistently taken during the day (26/26
documented via direct observation; 3/3 documented via
remote cameras; 100%) and carcasses scavenged by
mammals were taken primarily at night (6/7 documented
via remote cameras; 86%; this excludes three carcasses
where the mammalian scavenger was identified by tracks,
seven carcasses where the scavenger was not identified at
all, and five carcasses which were not scavenged).
Combining all carcasses, we recorded three events in 10
days of monitoring in March, 22 events in 22 days of
monitoring in April, 23 events in 23 days of monitoring in
May, and three events in 10 days of monitoring in June.
Migratory bird carcasses were more likely to be present in
April and May (v
2
=9.11, df =3, P=0.028).
DISCUSSION—Most of the carcasses we found occurred
during the peak of spring migration in our study area
(Patten et al., 2003). Little is known about the fate of the
carcasses of migratory birds that die during migration.
Our data suggest migratory birds might provide an
important food source for resident scavengers in desert
habitats where food resources can be rare. Common
ravens are regularly reported as facultative scavengers
(Boarman and Heinrich, 1999; Matley et al., 2012), and
our observations of common ravens flying directly from
scavenging sites to nest sites (TP, pers. obs.) suggests the
common ravens in our study area used scavenged
carcasses to provision nestlings. Prior studies of the diets
of kit foxes indicated that a low proportion (6.9%) of
fecal scats collected in the Chihuahuan Desert contained
avian remains (Moehrenschlager et al., 2007). In nonde-
sert habitat near Bakersfield, California, White et al.
(1985) found slightly higher values, with 8.6% of kit fox
scats containing avian remains. Prior studies of the diets
of coyotes in the Sonoran Desert also indicated that a low
proportion (2.6%) of fecal scats collected in fall
contained avian remains (Hern´andez et al., 1994),
though scats collected year-round had higher proportions
of birds (7.4%; McKinney and Smith, 2007). Birds were
also relatively rare in the diets of coyotes in the
TABLE 1—Remote cameras documented scavenging of migrant bird carcasses (n=25) in the Sonoran Desert.
Scavenged species Scavenger species
a
Common name Scientific name Common name Scientific name
Ash-throated flycatcher Myiarchus cinerascens N/A N/A
Black-throated sparrow Amphispiza bilineata ——
Brant goose Branta bernicla Coyote Canis latrans
Lazuli bunting
2
P. amoena Coyote tracks
2
C. latrans
Lazuli bunting
2
P. amoena Coyote tracks C. latrans
Lincoln’s sparrow Melospiza lincolnii ——
MacGillivray’s warbler Oporornis tolmiei ——
Mourning dove Zenaida macroura Coyote C. latrans
Mourning dove Z. macroura N/A N/A
Nashville warbler Vermivora ruficapilla ——
Orange-crowned warbler Vermivora celata ——
Red-winged blackbird Agelaius phoeniceus ——
Townsend’s warbler Setophaga townsendi Common raven Corvus corax
Townsend’s warbler S. townsendi Common raven C. corax
Unidentified Empidonax flycatcher Empidonax species Kit fox Vulpes macrotis
Western tanager Piranga ludoviciana N/A N/A
White-winged dove Zenaida asiatica Kit fox V. macrotis
Willow flycatcher Empidonax traillii N/A N/A
Willow flycatcher E. traillii N/A N/A
Wilson’s warbler
2
Wilsonia pusilla Coyote tracks C. latrans
Wilson’s warbler W. pusilla ——
Wilson’s warbler W. pusilla Coyote C. latrans
Wilson’s warbler W. pusilla Greater roadrunner Geococcyx californianus
Wilson’s warbler W. pusilla Kit fox V. macrotis
Yellow warbler Setophaga petechia Kit fox V. macrotis
a
N/A indicates the carcass was not scavenged during the monitoring period; — indicates the carcass was scavenged but the scavenger species was
not identified.
b
Three carcasses were within the frame of the same camera setup.
544 vol. 59, no. 4The Southwestern Naturalist

Chihuahuan Desert of New Mexico, where birds occurred
in only 1.6% of scats collected in spring and 1.5% of scats
collected year-round (Hern´andez et al., 2002). Our study
suggests that, though the proportion of birds in the scats
of mammalian scavengers might be consistently low in
general, those studies might not well-represent individual
scavengers occupying areas where migrant passerine
carcasses occur in disproportionately high numbers.
We observed kit fox dens, a pack of coyotes including
juveniles, and four common raven nests in our study area
(TP, unpubl. data). Each of these species was breeding
during our study; coyotes March–May (Webb et al., 2004),
kit foxes March–September (Zoellick et al., 1989), and
common ravens May–July (Smith et al., 1981). Thus, we
speculate that scavenged migratory bird carcasses were
likely provided to offspring in each of these species. We
also observed an emaciated kit fox, a coyote with only
three legs, and a common raven with a broken mandible.
It might be that injured scavengers in our study area
depended on the carcasses of migrating birds. If so, then
cascading effects from the mortality of avian migrants
within an ecological barrier could have influenced the
ecological community of our study area. Future research
investigating coyote scats in our study area, particularly
with emphasis on seasonal difference in scat composition,
would help resolve the potential differences between our
findings and those of previous researchers and would
clarify the ecological role of the carcasses of migratory
birds in migration corridors.
Though remote cameras enabled us to document some
scavenging events we would not have seen otherwise, the
cameras failed to detect all scavengers. In these cases we
speculate carcasses were scavenged in one of two ways.
First, based on our observations of common ravens in
flight dropping to the ground and then immediately
flying up again with a scavenged migratory bird carcass, we
FIG. 1—Migrant passerine carcasses scavenged in the Sonoran Desert. a) Kit fox scavenging Wilson’s warbler. b) Coyote scavenging
mourning dove. c) Common raven scavenging Townsend’s warbler. d) Greater roadrunner scavenging Wilson’s warbler (see Table 1
for scientific names).
December 2014 Rogers et al.—Scavenging of migratory bird carcasses 545

suggest that in some instances scavenging events might
have occurred more quickly than our cameras were
capable of capturing photographs. Second, during tran-
sects we also regularly encountered desert iguanas
(Dipsosaurus dorsalis) which occasionally scavenge carrion
(Norris, 1953). Because our cameras operated by detect-
ing differences in temperature, and because iguanas are
ectothermic species, we suggest that if reptiles like desert
iguanas scavenged carcasses (DeVault and Krochmal,
2002) our cameras might not have detected them. We
do not know if both, either, or neither of these hypotheses
are correct. Nevertheless, because scavenged carcasses
were consumed diurnally and nocturnally, and human
surveys for carcasses were largely conducted diurnally, our
study supports prior assertions that failure to account for
removal of carcasses by scavengers might bias studies
toward lower estimates of mortality (Dwyer and Mannan,
2007; Ponce et al., 2010), particularly when a diverse suite
of predators are active throughout the day and night.
Scavenging events tended to happen relatively soon after
sunrise as ravens flew along the length of the power line
right-of-way. Because most scavenging events occurred
nocturnally or relatively soon after sunrise, our data also
suggest that survey time period strongly influences
detection probability. The later in the day that surveys
occur, the greater the likelihood that carcasses are
scavenged before surveys begin.
Our study was relatively limited in scope, including
only a single spring migration in an area where spring
and fall avian migrations occur annually. Future research
comparing multiple years of surveys, and comparing
surveys during spring and fall migration, would likely
reveal additional species of migratory birds consumed by
scavengers as well as potential differences in the species
composition of scavengers and migratory bird carcasses by
season.
We thank R. Abe and E. Gilbreath for assistance with data
collection, A.M. Dwyer, D. Eccleston, and R. E. Harness for
comments on an early draft of this work, and D. P. Ordo˜
nez for
translating our abstract into Spanish.
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