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Scavenging of migratory bird carcasses in the Sonoran Desert.

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 communities 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 scavengers 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 information 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 (32[degrees]47'31"N, 115[degrees]33'47"W) and Ocotillo, California (32[degrees]44'19"N, 115059'39"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 40[degrees]C.

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 undersensitivity 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 whitewinged 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 [+ or -] SE = 0757 h [+ or -] 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 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 ([chi square] = 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 nondesert 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 (Hernandez 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 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 (Hernandez 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 suggest that in some instances scavenging events might have occurred more quickly than our cameras were capable of capturing photographs. Second, during transects we also regularly encountered desert iguanas (Dipsosaurus dorsalis) which occasionally scavenge carrion (Norris, 1953). Because our cameras operated by detecting 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. Ordonez for translating our abstract into Spanish.


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Submitted 21 October 2013.

Acceptance recommended by Associate Editor, M. Clay Green, 28 April 2014.


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:

TABLE 1--Remote cameras documented scavenging of migrant bird
carcasses (n = 25) in the Sonoran Desert.

Scavenged species               Scavenger species (a)

Common name      Scientific     Common name     Scientific
                 name                           name

Ash-throated     Myiarchus      N/A             N/A
flycatcher       cinerascens

Black-           Amphispiza     --              --
throated         bilineata

Brant goose      Branta         Coyote          Canis latrans

Lazuli bunting   P. amoena      Coyote tracks   C. latrans
(2)                             (2)

Lazuli bunting   P. amoena      Coyote tracks   C. latrans

Lincoln's        Melospiza      --              --
sparrow          lincolnii

MacGillivray's   Oporornis      --              --
warbler          tolmiei

Mourning dove    Zenaida        Coyote          C. latrans

Mourning dove    Z. macroura    N/A             N/A

Nashville        Vermivora      --              --
warbler          ruficapilla

Orange-          Vermivora      --              --
crowned          celata

Red-winged       Agelaius       --              --
blackbird        phoeniceus

Townsend's       Setophaga      Common raven    Corvus corax
warbler          townsendi

Townsend's       S. townsendi   Common raven    C. corax

Unidentified     Empidonax      Kit fox         Vulpes
Empidonax        species                        macrotis

Western          Piranga        N/A             N/A
tanager          ludoviciana

White-winged     Zenaida        Kit fox         V. macrotis
dove             asiatica

Willow           Empidonax      N/A             N/A
flycatcher       traillii

Willow           E. traillii    N/A             N/A

Wilson's         Wilsonia       Coyote tracks   C. latrans
warbler (2)      pusilla

Wilson's         W. pusilla     --              --

Wilson's         W. pusilla     Coyote          C. latrans

Wilson's         W. pusilla     Greater         Geococcyx
warbler                         roadrunner      californianus

Wilson's         W. pusilla     Kit fox         V. macrotis

Yellow warbler   Setophaga      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
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Author:Rogers, Andrew M.; Gibson, Michelle R.; Pockette, Tyler; Alexander, Jessica L.; Dwyer, James F.
Publication:Southwestern Naturalist
Article Type:Report
Date:Dec 1, 2014
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