New World Vultures and Other Avian Scavengers

This section looks at the relationship between New World vulture species and each other, and with Golden Aquila chrysaetos (Linnaeus, 1758) and Bald eagles Haliaeetus leucocephalus (Linnaeus, 1766), ravens, crows, and buzzard species in northern North America, and with tropical eagles and hawks in Central and South America.

Concerning the relationships between the vulture species, as summarized in Chapter 3, research has been conducted on the dominance hierarchies between Andean Condors and King vultures (where the condor is usually described as dominant); King vultures and the Cathartes and Black vultures, where the King vulture is dominant (except where crowds of Black vulture swamp the other species); and between Cathartes vultures and Black vultures (where there are different opinions on which group is dominant, but the Black vulture is usually numerically dominant). Size is important, but in the case of the Black vulture, so is numerical superiority.

Numbers at a food source, arrival time, size and aggressive behavior influence the relationships between the smaller vultures. The Cathartes vultures, with their sense of smell, are described as the first arrivals at carcasses, especially in forests where vegetation may obscure food sources (Stager 1964; Olrog 1985; Houston 1986, 1988; Graves 1992; Snyder and Snyder 2007). Black vultures lack a strong sense of smell, and tend to follow the Cathartes vultures to food, and have been described as dominant over them, while other studies find they are rather subordinate except when in large numbers (Wallace and Temple 1987). Turkey vultures prevail over Black vultures in slightly more than half of the one to one confrontations, but Turkey vultures can be dominated by large numbers of Black vultures (Stewart 1978; Wallace and Temple 1987; Buckley 1996). For the Greater Yellow-headed vulture, a forest scavenger with a strong sense of smell, the main relation is to lead other vultures to carcasses. In encounters with the Turkey vulture at carcasses, the Greater Yellow-headed vulture may be dominant (Schulenberg 2010), or the Turkey vulture may be dominant (Hilty 1977).

The King vulture is the common larger vulture of Central and South America, although it is out-numbered by the Greater Yellow-headed vulture in the Amazon rainforest, and by the Lesser Yellow-headed, Turkey and American Black vultures in more open habitats (Houston 1988, 1994; Graves 1992; Restall et al. 2006). It may find carrion in dense forest (Lemon 1991), but it may also follow the Cathartes vultures with strong olfactory senses, to carcasses (Houston 1984, 1994; Beason 2003). It is dominant over the smaller, weaker billed Cathartes and Black vultures, but may help them by tearing open carcasses, and is itself dominated the Andean Condor (Wallace and Temple 1987; Lemon 1991; Houston 1994; Gomez et al. 1994; Muller-Schwarze 2006; Snyder and Snyder 2006). Similar to the King vulture, the California condor does not have a sense of smell and often follows Turkey vultures to carcasses (Nielsen 2006). Condors are usually dominant over Turkey and Black Vultures (Koford 1953).

The Andean condor is usually dominant over other avian scavengers, such as Turkey, Yellow-headed and Black vultures (Wallace and Temple 1987). However, in some cases, smaller vultures may cause problems for the condor (Carrete et al. 2010). As described in Chapter 3, flocks of Black vultures were able to dominate the Andean condor even when Andean condors arrived first to carcasses (especially in mountains) and also when the Black vultures arrived first (mostly in the open plains). Therefore, the numbers of condors were negatively related to the abundance of black vultures (both in plains and mountains, but more so in the plains). 'Although black vultures do not completely prevent the arrival of Andean condors to carcasses, they represent serious obstacles for feeding. Thus, while dominance hierarchy at carcasses could be related to body size, carcass consumption was determined by species abundance' (Carrete et al. 2010: 390).

Although some studies recorded some relation between the two species (the condor being dominant, but helping smaller vultures by opening carcasses), it is also mentioned that they did not coexist until the recent expansion of the range of the Black vulture. The hypothesis was that both species are in competition for food, with the degree of competition and dominance dependent on human presence and the abundance of either species. The results showed that the superior size of the condor could be a factor in dominance, but the Black Vulture's superior numbers where enough for dominance over the condor. The Black vulture therefore was a serious hindrance for condor access to food supplies (Carrete et al. 2010).

A study by Wallace and Temple (1987) examined the relations between Andean condors, King vultures, Turkey vultures, Black vultures and a non-vulture, the Caracara Polyborus plancus in competition at carcasses. The results showed that Turkey vultures arrived first at the carcasses (92% of the carcasses), Black vultures second (72% of the carcasses) and Andean Condors third (70% of the carcasses). Aggressive inter-and intra-species interactions were ranked from most to least dominant, considering size, gender, age, for each species.

These were: (1) adult male Andean condors; (2) older juvenile male Andean condors; (3) adult female Andean Condors; (4) older juvenile female Andean condors; (5) younger juvenile male Andean Condors; (6) younger juvenile female Andean condors; (7) adult King vultures; (8) older juvenile King vultures; (9) younger juvenile King vultures; (10) adult Caracaras; (11) juvenile Caracaras; (12) adult Turkey vultures; (13) juvenile Turkey vultures; (14) adult Black Vultures; and (15) juvenile Black Vultures. In conflicts, adult Turkey Vultures dominated juvenile Turkey vultures, but less so than adult Black vultures dominated their juveniles. Adult and juvenile Caracaras dominated Turkey vultures (91, 90% respectively) and Black Vultures (85, 72% respectively, but not when the Black vultures were numerous). Turkey vultures were slightly dominant over Black vultures one to one, but not when the Black Vultures were numerous (commonly over 50 birds). King vultures were always dominant over the smaller vultures, but lost all encounters with Condors. Large numbers of Black vultures could snatch food in Condor presence with frequent aggressive contact (Wallace and Temple 1987).

The evidence hints that numbers are at least as important as size, and among the New World vultures at least, the smaller birds are much more numerous. An important relation among the vultures therefore concerns the inability of larger vultures to obtain food. Adaptability is also an issue: Black vultures (and to a lesser extent Turkey vultures) are expanding their foraging habitats into urban and other modified environments, while still occupying natural environments. This competition between numerous, small generalists and large, rarer solitary foragers is an important problem and falls within the opinion that the larger vultures are past relics from a time of megafauna carrion.

Roosting is another important activity that influences inter-species relations. All Cathartids, with the exception of the King vulture are known to form communal roosts, as do nine Old World vulture species, including the seven Gyps vulture species. Communal roosts are hypothesized to serve as information centers were birds share foraging information (Ward and Zahavi 1973; Rabenold 1983). This hypothesis was tested for the relationship between communally roosting Turkey and Black vultures. The evidence of shared information would be revealed when their departures from communal roosts at the commencement of foraging would 'deviate from random both in direction taken and time' (Rabenold 1983: 303). A common characteristic of both Black vultures and Turkey vultures is to commence morning foraging with flapping, directional flight, where they are aware of the food location. When the food source is unknown, then the behavior is different; foraging commences later in mid-morning, synchronized with the thermals, with soaring until later directional flight towards a sighted food source.

Evidence shows that Turkey vultures leave roosts earlier than Black vultures, and most return within two hours of nightfall, sometimes to different roosting spots for each night (Rabenold 1983). There was evidence that departures were grouped in time, the departing birds were directionally related (not correlated with weather conditions), with these directions changing daily and the departing groups were more numerous than the returning groups. Possibly, these behaviors reveal a common purpose for foraging; departure is more important than later individual foraging, and group directional change indicates adaptation to different, daily food locations (Rabenold 1983). Such conclusions for vultures are supported by studies of other species (e.g., Emlen 1971; Krebs 1974; Erwin 1978; Jones 1978 on bank swallows Riparia riparia, great blue herons Ardea herodias, six species of terns and reed cormorants Phalacrocorax africanus), despite the fact that the latter species were in breeding mode, and possibly found safety from predators in group foraging. Turkey and Black vultures, despite being communal roosters, tend to be solitary breeders, with pairs siting nests in remote tree or cliff locations, with roosting in the old location continuing for the bird not on the nest with the young (Rabenold 1983).

Roosts as information centers is not conclusively proved (Ward and Zahavi 1973; Rabenold 1983). The group behavior could be to ensure more effective foraging and not necessarily include transfer of information at a roost. Despite the visual evidence, why would a bird lead others to a food source (e.g., Alexander 1974)? Possibly, due to their keen eyesight, this is unavoidable. The need for help in breaking into a carcass is one possible reason (Jackson 1975; McHargue 1977; Rabenold 1983). Information sharing might be demonstrated by observing roosting birds that sighted food on the first day and determining if other birds followed them to food sources on the second day, before these other birds sighted their own food sources. It is important to determine if the primary bird is followed before it sights food, as after it sights food it would be followed anyway, whether or not information had been shared at the roost. As the author (Rabenold 1983) acknowledges, more research must be done to establish the actual meanings behind such possibilities. To this we may add, there must be more research on both New and Old World vultures, as if communal roosting confers advantages in food access, there are important links with habitat changes that affect roosting places.

Concerning the relations between New World vultures and other avian facultative scavengers, the main species groups are eagles, corvids and smaller raptors such as caracaras. Some authors have written that Black and Turkey vultures have no 'major predators in North America away from their nests' (Stolen 1996: 43; see also Townsend 1937; Jackson 1988a, 1988b). These observations suggest that they have reason to be wary of large predatory birds. Stolen (1996: 43) acknowledges that there are 'few published accounts of interactions of either the Black Vulture or the Turkey vulture with large predatory birds' however, 'competitive interactions due to overlap in use of carrion might be expected.' Glazener (1964) gives examples of Turkey vulture interactions with the Crested Caracara Caracara plancus (the caracaras harassed the vultures at the carcasses until the latter disgorged morsels, which the caracaras then ate.). Regarding eagles, Oberholser (in Bent 1937) stated that 'Bald Eagles Haliaeetus leucocephalus do not . . . hesitate even to pursue the vultures and compel them to disgorge, when if it fails to catch the coveted morsels before they reach the ground it alights and devours them.' Audubon (in Bent 1937) noted that the arrival of eagles at a carcass caused vultures to '. . . retire and patiently wait until their betters are satisfied'. Coleman and Fraser (1986) recorded an incident where an immature Golden eagle (Aquila chrysaetos, Linnaeus 1758) disturbed a large, feeding flock of Turkey and Black vultures and attacked one Black vulture, causing it to regurgitate before it flew away. In one incident a Bald Eagle actually killed a vulture that 'for some reason was unable completely to disgorge' (Audubon 1937).

Stolen (1996) gives an interesting study in central Florida, where small mammal carcasses were placed to attract vultures, contributing to interactions between both Black and Turkey vultures and Bald eagles. In both incidents several vultures (seven Black and three Turkey vultures in the first incident, and nine Black vultures and one Turkey vulture in the second incident) were feeding or sitting near a raccoon carcass, and flew away at the approach of one Bald Eagle, which merely flew over the carcass. Another study by the same author concerns the impact of Bald Eagles on a communal roost of mostly Black vultures with a few Turkey vultures. In one incident, Black vultures 'appeared agitated' and '. . . flew between perches and looked around more' when Bald Eagles were harassing Ospreys within 100 meters (ibid. 43). In another incident, a Bald Eagle was sitting in the vultures' roost tree, and all the vultures were sitting in low trees about 200 m distant (possibly to avoid the eagle). However, in two other incidents, the presence of Bald Eagles at the roost site appeared to have no effect on the behavior of the vultures. Stolen (1996: 43) notes that 'These observations suggest that interactions with large predatory birds may constitute a selective pressure on Black and Turkey Vulture behavior.' This author argues that the eagle-vulture interactions seem to be more important at carcasses; the relevant evidence concerns how vultures regurgitate food in response to harassment or threats from predators which may distract potential attackers (for different opinions, see also Maynard 1881; Brown and Amadon 1968; Jackson 1988a, 1988b; Ritter 1983; Townsend in Bent 1937).

Concerning the Golden Eagle, there is evidence that this bird is dominant over all the New World vultures in North America, including the California condor. Snyder and Synder (2000) found that California condors avoided nesting where Golden Eagles were common. Another study concerned the relationship between California condors, Golden Eagles, Turkey vultures, Ravens and Coyotes (Koford 1953). Other animals discussed in this case study were Cougars (Puma concolor Linnaeus, 1771) and Black Bears (Ursus americanus Pallas, 1780). Species not discussed but possibly sharing the condor's foraging areas were American Crows, Red tailed Hawks, feral dogs and wolves. Golden Eagles, Turkey vultures and ravens usually arrived to start feeding before condors, and left later than them. Turkey vultures roosted much closer to carcasses than condors. Cougars only provided food for the scavengers by killing animals and were not competitors at kills. Black bears by contrast were described as possible competitors for carcasses, but were not recorded in the study.

Condors were dominant over ravens and Turkey vultures. Although ravens occasionally fed within one meter of condors, they jumped from feeding at the approach of condors, flew away when condors descended, and flew from alarm faster than condors. Ravens crowded at a carcass appeared to attract condors. Turkey vultures did not approach condors as closely as ravens did. They would fly from a carcass when a condor approached, and rarely tolerated condors within 1.5 meters. Normally, Turkey vultures waited several meters away, for the condor to finish feeding before approaching the carcass.

Two animals dominated condors: the golden eagle and the coyote. When one eagle was feeding and condors approached, the eagle drove the condors away and then resumed feeding, a feat repeated several times. One eagle chased 16, 14 and 12 condors in separate, successive incidents.

When the eagles left, the condors rushed the carcass. A common tendency was for condors to become bolder after being chased several times. In one case a condor was feeding on a sheep, on the opposite side from an eagle. Then a group of condors approached and crowded the eagle off the food. In another incident, an eagle and a condor stood seven meters apart, then the eagle approached and pecked the condor, which retreated and stood distant. The results of the study showed the dominance ranking to be golden eagles at the top, followed by condors, then Turkey vultures and finally ravens, with the strongest difference between eagles and condors, and the smallest between Turkey vultures and ravens.

Rodriguez-Estella and Rivera-Rodriguez (1992) also recorded Crested Caracaras being dominant over Turkey vultures at carcasses in the Cape Region, Baja California Sur, Mexico. The Crested Caracara (Polyborus plancus, now named Caracara cheriway Jacquin 1784) is described as an opportunistic but mostly carrion feeding raptor (see also Sherrod 1978; Glazener 1964) and is smaller than the Turkey vulture; length is 49-58 cm (19-23 in), wingspan of 107-130 cm (42-51 in), and weight 800-1,300 g (1.8-2.9 lb). Caracaras and some other raptors are generally dominant over Turkey and Black vultures (e.g., Striated Caracara Phalcoboenus australis Gmelin 1788; see Strange 1996; Catry et al. 2008).

In another study by Dwyer and Cockwell (2011) in the Falkland Islands (Malvinas), interactions were studied between the Variable Hawk (Buteo polyosoma, now classified as Geranoaetus polyosoma Quoy and Gaimard 1824), Striated Caracara, Southern Caracara (Caracara plancus, Miller 1777), and Turkey vulture (Cathartes aura jota). Variable hawks were aggressors in 98% (96-100%) of interactions with Striated Caracaras, 82% (69-95%) of interactions with Turkey vultures, and 80% (72-88%) of interactions with Southern Caracaras. Southern Caracaras were aggressors in 100% of interactions with Striated Caracaras, and 90% (80-100%) of interactions with Turkey vultures. Turkey vultures were aggressors in 71% (61-82%) of interactions with Striated Caracaras. These results put the Turkey vulture as more passive towards the Variable hawk and the Southern Caracara, and dominant over the Striated Caracara.

In North America, there are other scavengers that take the place of vultures during the winter. Studies of these scavengers, in the absence of vultures are important, because they illustrate the potential for competitive conflict for periods when vultures are present, the role of facultative scavengers as substitutes for obligate scavengers, and the importance of vultures if scavenging is incomplete or has different result from vulture presence.

For example, Wilmer et al. (2003) give the example of the Greater Yellowstone Ecosystem in the northern Rocky Mountains in winter. In this study, neither the Black vulture nor the Turkey vulture is mentioned. The primary winter scavengers were, in order of their dominance at carcasses, coyotes, Golden eagles, Bald eagles, ravens and magpies. Five species of mammal carnivores existed in the area: coyote, wolf (Canis lupus), cougar (Puma concolor), grizzly bear (Ursus arctos), and black bear (U. americanus). Elk were the main prey of wolves and human hunters (the main carcass providers in the area) and consequently were also the main food of the scavengers (see also Murie 1940; Gese et al. 1996; Lemke et al. 1998; Crabtree and Sheldon 1999a,b; Mech et al. 2001).

Bald eagles and ravens had the largest feeding radii, due to long distance flying capabilities (see also Heinrich 1988; Buehler 2000). As communal rosters, they might transfer resource location information among themselves (see also Marzluff et al. 1996; Buehler 2000; Dall 2002). Magpies had small feeding radii due to their lower mobility (see also Bekoff and Andrews 1978; Trost 1999). The Golden Eagle, although similar in flying ability to the Bald Eagle, was relatively sedentary and solitary, with smaller feeding radii, as 'they lack the degree of social interaction that enhances information transfer concerning resource locations' (Wilmer et al. 2003: 997, see also LeFrank Jr. and Clark 1983).

Where humans provided carcasses in localized areas, these were dominated by local scavengers with lower feeding radii (such as magpies). Where wolves provided the carcasses, these were widely dispersed and dominated by scavengers with wider feeding radii (such as Golden Eagles). Although vultures were absent, the other sources cited in this book allow speculation on their presence. Both Black vultures and Turkey vultures have wide feeding radii, but as noted in the study by Koford (1953) Golden Eagles are dominant over Turkey vultures at food sources. Turkey vultures and Black vultures are approximately even, but the Black vulture is numerically dominant. Turkey vultures have been found to be dominant over ravens, and presumably Black vultures, at least in large numbers would also be dominant.