Old World Vultures and Other Avian Scavengers

The particular avian relationships for the Old World vultures, apart those between each other, concern storks, corvids, Larus gulls, eagles, kites and other predatory raptors. In Northern Europe, facultative scavengers dominate as vultures are largely extinct. Apart from the Larus gulls, in northern Europe other scavengers are the White Tailed Eagle (Haliaeetus albicilla Linnaeus 1758), Red Kite (Milvus milvus Linnaeus 1758) and Raven (Corvus corax Linnaeus 1758) (Gu and Krawczynski 2012). In Asia and Africa, there are more avian competitors for vultures including the huge storks and far more eagle species than in Europe of North America.

Before the relationship between vultures and other scavengers can be understood, it is first necessary to examine the relationship between the vultures themselves, as this may shape the outlook towards similar species. Old World vultures may be classified, as described in Chapters 1 and 2, into into three groups, the large tearers, the large and medium-sized pullers and the small pickers (Kruuk 1967; Konig 1974). The tearing species, include the huge Cinereous and Lappet-faced vultures and the smaller Redheaded and White-headed vultures. The pullers are the Griffon vultures, with long necks. The peckers are the small Hooded and Egyptian vultures (Konig 1983).

In competition between vultures, size is a factor for dominance (Kwuk 1967; Lack 1971; Konig 1974; Houston 1975). There are various reports concerning aggressive interactions between these feeding groups or species. Studies by Kruuk (1967), Brown (1971) and Anderson and Horwitz (1979) ranked six species of African vultures according to size and dominance: (1) Lappet-faced vulture 7.5 kg; (2) White-headed vulture 5.9 kg; (3) Ruppell's vulture 6.4 kg; (4) African White-backed vulture 5.7 kg; (5) Hooded vulture 1.9 kg; and (6) Egyptian vulture 1.9 kg. In this list, the White-headed vulture (a tearer) is dominant over the Ruppell's Griffon (a puller), despite the latter's larger size.

All larger vultures dominate the Hooded and Egyptian vultures, while the slightly larger and more aggressive Hooded vulture dominates the Egyptian vulture. On most occasions, the larger Cinereous and Lappetfaced vultures dominate the large griffons, but there are reports of griffons dominating the larger birds (Konig 1973, 1974). There are cases where the larger species do not dominate the smaller species, when they do not assume a threatening posture. Konig (1983) gives the example of the Lappet-faced vulture, dominating smaller griffons only when a strong threatening posture is adopted, by displaying white feathers and red skin. Of 30 observed conflicts between the Lappet-faced vultures and griffons, in 24 cases the larger bird won by assuming a dominant posture, while in six cases when it was not in the posture, it was driven away. The assessment was that the motivating force of hunger was at least as important as a fixed dominance based on size.

Konig (1983) notes that there is little competition between two tearers; the larger Lappet-faced vulture and the smaller White-headed vulture in Africa. The former is aggressively dominant, while the latter is more solitary, observational and quicker in movement (due to its smaller size). It forages in the early, pre-thermal morning, earning access to kills. Konig further notes that there is little competition between the griffons and the Lappet-faced vultures, because the griffons eat softer internal organs and the Lappet-faced vultures eat the tougher hide, sinew and muscles. The Cinereous and Indian King vultures are described as similar, but they do not share ranges with species with similar feeding methods. A similar relationship exists between the Himalayan griffon, the smaller Griffon vulture and the yet smaller Indian White-rumped vulture, Indian vulture and Slender-billed vulture in Asia. The first two, larger birds both nest on cliffs, the Indian vulture mostly on cliffs, and the Slender-billed and White- rumped vultures mostly in trees (Konig 1983).

Studies have also focused on dominance and exclusion between the African White-backed vultures and the larger Ruppell's griffons in central Africa, and the African White-backed vulture and the larger Cape griffon vultures in southern Africa. The White-backed vulture weighs a little above half of the griffons (5.3 kg, compared with about 8 kg). The larger birds are near the weight for a bird 'at which prolonged flapping flight becomes impossible' (Konig 1983; see also Pennycuick 1969), while the lighter White- backed vulture easily uses flapping flight, an important ability for savanna country, due to the seasonally cloudy and rainy climate that may restrict the formation of thermals for soaring flight (Houston 1975). An important hypothesis is that the griffons have developed a larger body to cope with the scarcer food supply of the mountains, and to dominate other vultures.

There is also 'temporal segregation' between vulture species (Kendall 2014: 12). Carrion may be more available in the morning than in the afternoon, as there is less chance that diurnal scavengers have acquired it. Morning activity differs among vultures, as they rely on thermals to fly. These warm air zones develop slowly after sunrise heats the ground and heavier birds with higher wing-loading require stronger thermals than lighter birds. In a study of the largely solitary Lappet-faced vultures and their competitors, the commoner and social White-backed vultures, the former are heavier than the latter (mean, range: 6.78, 6.10-7.95 as against 5.46, 4.15-7.20 kg) and have wider wings, which are also longer (wing span: 2.80 vs. 2.18 m) (Spiegel et al. 2013). This means that the larger birds have lower wing loading (6.4 vs. 7.8 kg m^-2) (see also Pennycuick 1971; Mundy et al. 1992). Therefore the Lappet-faced vulture can utilize weaker thermals than the White-backed vulture and can search smaller more permanent foraging areas, while the latter requires wider foraging using fast direct flight (Pennycuick 1972). The Lappet-faced vulture was more efficient in searching, in terms of 'first-to-find, first-to-land, and per-individual-finding rate measures' (Spiegel et al. 2013: 102).

In a related study (Kendall 2014) tested the hypothesis that the larger Lappet-faced vulture would be more abundant at carcasses in the morning than White-backed vultures and Ruppell's vultures. The results showed the opposite, as the social White-backed and Ruppell's vultures were more abundant at carcasses in the morning, and Lappet-faced vultures were more abundant in the afternoon. Gyps vultures when full, fed less in the afternoon allowing the larger bird to feed. The Lappet-faced vultures had lower foraging success than the Gyps vultures. The results hinted that the factors for dominance of one species over the other must be critically assessed.

Size, wing-loading, foraging efficiency, numbers at a carcass and feeding specializations are important social factors. These are relevant to the relations between vultures and other species. For the large Old World vultures, the main competitors are Marabou storks (Leptoptilos crumenifer, Lesson 1831) and large eagles, kites and corvids. In the study above, Spiegel et al. (2013) noted that apart from the Lappet-faced and White-backed vultures, the other avian competitors were the Tawny Eagle (Aquila rapax, Temminck 1828), Bateleur Eagle Terathopius ecaudatus Daudin 1800 and Yellow-billed Kite (Milvus aegyptius, Gmelin 1788).

In studies of interspecific relations of assorted vultures and non-vulture scavengers, body size and social foraging may determine competitive ability at carcasses (Kendall 2013). This case study of the Masai Mara National Reserve included seven species of raptors: two eagles—Bateleur and Tawny eagles—and five species of vultures—Lappet-faced, White-headed, African White-backed, Ruppell's vulture and Hooded vultures. In the dominance relations, size appeared to be a factor, even between the vultures and the eagles. Weights of the listed species were, in order of smallest to largest: Hooded vulture 1.9 kg, Bateleur 2.2 kg, Tawny eagle 2.3 kg, White-headed vulture 4.3 kg, White-backed vulture 5.6 kg, Lappet-faced vulture 6.8 kg, Ruppell's vulture 7.6 kg. The Hooded vultures had low beak strength, the White-backed and Ruppell's vultures had medium beak strength, and the other species had high beak strength (the White-headed and Lappet-faced vultures and the eagles) (the source is adapted from Mundy et al. 1992, by Kendall 2014).

The Lappet-faced vulture affected the feeding of the smaller species, including the Ruppell's, White-backed and Hooded vultures, and also the Tawny eagle and the Bateleur eagle. The Bateleur eagle was also affected by the Ruppell's vulture and the Tawny eagle. It is noted that the Lappet-faced and African White-backed vultures were able to force leaving behavior in Bateleur eagles. The White-backed vulture negatively affected the Tawny eagle. Both the White-backed and Ruppell's Griffon vultures were social. White-backed vulture groups could dominate the larger, more solitary Lappet-faced vulture. The social behavior of the Ruppell's vulture did not have the same effect. Lappet-faced vultures were also affected by the presence of the much smaller Bateleur Eagles and Hooded vultures. Tawny eagle presence also increased the likelihood that White-backed vultures would feed.

Smaller species located more carcasses than larger species. Bateleur eagles discovered the largest number of carcasses and did not arrive at any carcass after another species. Superior search efficiency could be due to flight height in foraging or visual acuity, from variations in wing-loading among the different species (see also Pennycuick 1972). Other studies have found that Bateleurs tend to fly at lower heights than other raptors, while Ruppell's vultures tend to fly higher than average, possibly making these species less and more reliant on other birds' behavior respectively (Mundy et al. 1992; Watson 2000). The White-backed vulture had similar foraging capacity to the Tawny eagles and Hooded vultures, possibly due to the social foraging of the former species (they outnumbered Tawny eagles almost eight to one and Hooded vultures 35 to one).

Birds that were either good searchers (e.g., the Bateleur) or socially dominant (e.g., Lappet-faced, Ruppell's and White-backed vultures) generally were found in high quality landcover (where wildlife density was high, human settlements were low), while other species with low social dominance and lower search efficiency (e.g., Hooded vultures and Tawny eagles) were commoner in low quality land cover with low wildlife density. Some of the higher quality searchers, such as the Bateleur, White-backed, and Lappet-faced vultures were also found to be more common morning foragers, possibly increasing food access. Of these more dominant species, only the Ruppell's Griffon vulture was a late forager despite problems with food access when it arrived to well attended carcasses. The late arrival of this species has been attributed to its nesting on cliffs distant from the feeding grounds (Pennycuick 1983; Kendall 2013).

Eagles were also found to be aggressive towards Lappet-faced vultures in Saudi Arabia (Shobrak 1996). Six species of eagle were recorded in this study of the Lappet-faced vulture: Bonelli's eagle (Aquila fasciata, Vieillot 1822), Booted eagle (Hieraaetus pennatus, Gmelin 1788), Golden eagle, Imperial eagle (Aquila heliaca, Savigny 1809), Spotted eagle (Clanga clanga, Pallas 1811), and Steppe eagle (Aquila nipalensis, Hodgson 1833). The other common scavenger was the Brown-necked raven (Corvus ruficollis Lesson 1830), a slightly smaller bird than the Common raven (Corvus corax Linnaeus 1758), with a length of about 52-56 cm. In this study, there was pronounced conflict at carcasses. Lappet-faced vultures had 71.5% of conflicts with each other, and 29.5% with other species. For Brown-necked ravens intraspecific conflicts were 39.7% and interspecific conflicts were 60.3%. For the eagles collectively, intraspecifc conflicts were 63.3% and interspecific conflicts were 36.7%. Lappet-faced vultures won more interspecifc conflicts (70.9%, losing 29.1%) than the eagles (64.8%, losing 35.2%) or the ravens (25.8%, losing 74.2%). Only the eagles attacked these vultures (Shobrak 1996).

In a similar study looking at eagle-vulture relations, Mundy et al. (1986) found that the Black eagle (the African Verreaux's eagle) (Aquila verreauxii Lesson 1830) attacks the nests and young of the Cape Griffon vulture, which shares its principal, mountainous foraging and nesting areas. In some cases, the eagles which nest in pairs would fly into the vulture colony and attack the vultures. Five out of 13 attacks resulted in the removal of a vulture nestling by the attacking eagle. Six attacks were unsuccessful. There were also four attacks by eagles on both Cape vultures and Bearded vultures: these resulted in the death of the former species. The eagle attacks on vulture nestlings and eggs may be seen as predation (Mundy et al. 1986). However, these authors dispute the opinion of Pitman (1960) that the attack on the adult vulture that resulted in its death was predation, because the eagle did not alight on the ground and feed on the vulture. An eagle was once seen eating the nestling of the White-headed vulture, in the nest of the latter, despite the fact that the White-headed vulture (unlike the Cape Griffon Vulture) does not share breeding areas with the eagle.

The Marabou Stork also has important relations with African vultures. This is a huge bird with a height of 152 cm (60 in) and weighing up to 9 kg (20 lbs) (Likoff 1986; Stevenson and Fanshawe 2001). The wingspan has been disputed, with measurements up to 3.7 m (12 ft) and 4.06 m (13.3 ft) published, but there is no verified measurement over 3.19 m (10.5 ft) with most measurements averaging 225-287 cm (7-9 ft). The large bill measures from 26.4 to 35 cm. Females are smaller than males (Hancock et al. 1992).

Marabou storks may wait until the large vultures have opened a kill before they start feeding, but they may also be dominant over vultures, ranking with the Lappet-faced vulture as the most dominant bird at the carcass (Hancock et al. 1992; Cheney 2010).

In Africa, kites and crows are common facultative scavengers that interact with vultures. One study compared the Hooded vulture's foraging and competition with those of the Pied Crow (Corvus albus Statius Muller 1776) and the Black Kite (Milvus migrans Boddaert 1783), classified by some as the Yellow-billed Kite (Milvus aegyptius Gmelin 1788), subspecies Milvus aegyptius parasitus) (Campbell 2009). The findings of this study indicated strong competition between vultures, kites and crows in urban settings, centred around meat and waste production, and human presence.

Vultures competed with crows for meat from butchers' tables in abattoirs. In meat production areas, the vultures were more numerous than the crows or kites, but were less mobile, and tended to fly overhead or perch and stand more often while the crows entered windows and doors. Crows and vultures had similar flight distances of about 2 m (the distance between an approaching human and a bird before the bird takes flight). Vultures, crows and to a lesser extent kites competed in observational perching spots, space for hovering and close soaring and ground forays for snatching of human discards or refuse. Conflicts between birds erupted when people discarded meal remnants, garbage or unwanted trade items.

Both vultures and crows, but not kites, gathered in areas of human communal eating, but unlike the meat production areas, crows outnumbered vultures and were faster when snatching deliberately thrown food. Unlike vultures, crows were more adept at foraging in gutters, on sidewalks, public toilets, under vehicles, house windows and old clothing. In these locations, flight distances were longer than in the meat and waste production areas.

In rural areas, all three species were correlated with human presence, especially farm clearance work, bush fires and tractor ploughing. Vultures, crows and kites perched in branches, the ground or circled overhead. The flight distances were much longer than in urban areas, on average about 9 m, but where there was food crows and to a lesser extent vultures approached to within 1-2 of people. For the vultures and the rarer kites, feeding was largely on animals killed by farm clearers (mostly monitor lizards, rats and snakes), while crows took such food and also human discards, crop residues and insects on the ground and in flight (Campbell 2009).