Vultures, Thermals and Orographic Lift

The climate is usually related to the geomorphology, such as mountain ranges, valleys and water bodies. Thermals or rising bodies of warm air, strong winds and rain patterns are related to the morphology of the Earth's surface, with thermals commonly forming over flat plains and strong winds forming between mountains and in canyons. Leshem and Yom-Tov (1998) in assessing the relevance of Earth features to bird migration pointed to 'the combined effects of geomorphology and climate on migratory routes.' In this case, migration refers to the regular seasonal movement of birds seeking warmer climates during northern winters (in the northern hemisphere), from northern to more southerly regions.

As is evident from Chapters 1, 2 and 3, thermals are necessary for vulture soaring which is necessary for food foraging. Before we look into the issues of climate, we need to first examine the terms 'soaring' and 'wing loading'. Soaring may be defined as 'the use of air movements to sustain gliding flight' (Alexander 2003). Wing loading is defined as the ratio of the wing area to the weight of the bird 'The ability to fly slowly, and turn in small circles, is critical for exploiting thermals, and therefore the wing loading is a useful indicator of a gliding bird's capacity for this activity' (Pennycuick 2008: 266). This author also notes that the wing loading does not have any special significance for powered flights, i.e., when the bird is flapping. Slope soarers are birds that fly at high angles to the wind, such as albatrosses which fly in the sloping wind currents over the ocean waves. For these birds, high wing loading is optimal, because they need to glide faster than the wind and to glide fast they need high wing loading. For thermal soarers, such as vultures, low wing loading is needed because they can thus fly in smaller thermals. The Andean Condor is described as using both methods, thermal soaring in thermals and slope soaring over mountains (Alexander 2003).

Thermals form through convection, which is the transfer of heat through the movement of a fluid, either air or water (Fig. 6.1). Natural convection of air occurs when some parts of the Earth's surface absorb more heat than nearby areas. When this happens, the air molecules near the hotter surface begin to bounce on the surface, gaining more energy through conduction. The molecules turn into heated air that expands. When the air expands, it becomes less dense than the surrounding cooler air. The result is that the warmer air rises to form large, warm, rising bubbles of air that transfer heat upwards. Cooler air surrounding the warmer, rising air flows towards the source of the warm air, i.e., the warm ground that heated the air at the beginning. This cooler air also becomes warmer, and begins to flow upwards, following the warm air above it. This vertical exchange of air is called convection and the rising air is called a thermal. As the warm air rises it expands and cools, spreading outwards and sinking to the surface again, where is moves to the warm surface and rises, starting the cycle again. This circulation is called convective circulation or a thermal cell (Ahrens 2007).

Fig. 6.1. Thermal Formation

Bird weights and wing-loading (the ratio of wing area to body weight) are important for the use of thermals. For example, there are three weight categories of Griffon vultures. The heaviest is the Himalayan Griffon, which at 12 kg is similar to the Andean condor. It is a slope soarer, that rarely uses thermals. Houston (1983: 137) argues that 'wind speeds in the Himalayas are high, and the rising airflows over hill ridges provide the extremely good soaring conditions needed to support such a large soaring bird.' A heavy body and high wing-loading is an advantage in gliding flight as it increases flight speed in straight gliding flight, with only a small cost in the use of weak thermals.

The next weight category is the middle group, the Cape, Ruppell's Common, Indian and Slender-billed vultures (similar in weight, around 7-8 kg). 'All four species have comparable body proportions, presumably a compromise between the high wing loading that is efficient in gliding flight and the lower wing loading required for good performance when soaring in weak thermals' (Houston 1983: 138, see also Pennycuick 1972). The two lightest Griffons, the African White-backed and the Indian White-Rumped vultures (weighing about 5 kg each) live in flat savanna lands and rely more than the others on thermals, and need to use flapping flight which is very difficult for heavier birds. When the savanna climate is rainy and cloudy and the thermals are very weak or absent, flapping flight is necessary for foraging, and would be more difficult for a heavier bird (Houston 1983; see also Pennycuick 1972b; Houston 1975a).

Thermals may also determine the period that vultures remain in the roost. For example, in a study in Iowa, United States, McVey et al. (2008), found that Turkey vultures remained in communal roosts later from June to August than from in April, May, September and October. They attributed this to the fact that thermals are generated later in the morning during summer months than in spring or fall. As thermals are created by the contrast between ground and air, and in summer these contrasts are less pronounced, thermals form more slowly in summer and may require a higher angle of sun and longer exposure (Wallington 1977). The Turkey vultures (which need thermals for soaring) therefore remain at a roost when thermals are absent (Kirk and Mossman 1998). Vultures also varied their departure times, as opportunities arose (see also Mandel and Bildstein 2007). A study by Byman (2000) in the Pennsylvania winter found that Turkey vultures left their roosts earlier on warm sunny mornings than on cold mornings. The study by McVey et al. (2008) concluded that vultures left roosts later during the summer, but the lack of return time data prevents an assessment of whether the longer days in summer could be a factor for this, or it is only due to the later formation of thermals. Thiel (1976) reported a similar result for Turkey vultures. Hiraldo and Donazar (1990) also reported a similar departure behavior from the Cinereous vultures (Aegypius monachus).

A study by Xirouchakis (2007) of Griffon vultures in Crete found that Griffon departure times from roosts was determined by both thermals and wind. In one roosting colony, located near strong north-north-west winds, strong upward currents were produced; vultures used these winds for foraging for about 30-60 minutes after sunrise. In another breeding colony, where the winds were blocked by cliffs, the birds waited for 3 to 4 hours after sunrise for thermals before they could depart for foraging.

Temperature also has an effect on vulture physiology. An example is wing-spreading behavior in Turkey vultures. Ohmart and Clark (1985) found that wing-spreading during the morning pre-departure period was related to the intensity of the sun's rays, but not related to low overnight temperature. Wing-spreading was also more common when the vultures were wet than dry. Therefore, the birds would spread their wings to dry feathers and to 'ameliorate the thermal gradient between themselves and their environment' or both of these in combination.

Mahoney (1983) wrote that vultures are exposed to intense solar radiation and high day temperatures. Large birds also generate more heat from metabolism and store more heat than small birds, even when they are resting, and during exercise, e.g., flying body heat can increase ten-fold over the resting temperature (see also Kleiber 1961; Calder and King 1974). The black plumage may also retain heat (see Hamilton and Heppner 1967). This is probably a factor for the leg-wetting (urohidrosis) for evaporative cooling in Turkey and Black vultures. Turkey vultures also use panting when temperatures exceed their body temperature (Kahl 1963; Hatch 1970). Mahoney's (1983) study used four captive Black Vultures, and found that these birds had body temperatures and metabolic heat production similar to other similarly sized birds. The metabolic rates increased fivefold when the birds were running compared with resting rates, and flapping was predicted to be double this, with soaring and gliding double the resting rate. The study also found that resting Black Vultures in temperatures of 40 to 50°C were able to maintain body temperature at about 40 to 42°C for 45 minutes.

Few studies, however speculate how other vultures that do not use urohidrosis cool themselves, as Africa and Asia are at least as hot as the habitat of the Black vulture. In some studies, it has been suggested the bare heads of vultures exist as a thermoregulatory adaptation to avoid facial overheating. Many commentators suggest that bare heads also avoid the saturation of feathers with blood during feeding. This theory is however disputed (Mundy et al. 1992; Wilbur and Jackson 1983; Ward et al. 2008). Ward et al. (2008) used a mathematical model to study bare skin exposed by Griffon vultures in different postures, using heat flow through museum skins and estimates of exposed skin in different postures in hot and cold conditions as measureable examples. The skins studied had variable feather cover density, and could be used to estimate the coverage and hence the heat loss for the whole body. The results indicated that 'Postural change can cause the proportion of body surface composed of bare skin areas to change from 32% to 7%, and in cold conditions these changes are sufficient to account for a 52% saving in heat loss from the body'; the authors therefore concluded that 'the bare skin areas in Griffon vultures could be important for thermoregulation' (ibid. 168).

Rainfall affects foraging vultures. A study by Hiraldo and Donazar (1990: 130), found that 'the influence of rain on the flying of Cinereous vultures appeared to be very marked, decreasing activity and becoming practically nil for long periods of rain.' The effect of the rain was stronger during cold months, when unlike the warmer months no flights were recorded. For example, this study found that in one area of the western Sierra Morena in Spain, no vultures were seen on a strongly rainy day. On clear days in the same areas 10 or more vultures would be sighted. In another area, 130 vultures were recorded on a rainless day, while the next day with some rain only 68 were sighted. Also vultures tended to stop flying just before rain started and perched while the rain wet them. After the rain, the birds sunned themselves with spread wings before resuming flight. Some vultures also fly away from areas with rain, heading towards drier areas (Hiraldo and Donazar 1990).

Orographic lift is another weather phenomenon useful to soaring birds (Fig. 6.2). This occurs when an air mass, originating either over flat land or the ocean, moves up rising terrain, cooling as it moves up in altitude. This movement creates strong winds that blow up the slope and may also create turbulence over the uneven surface of the land. Soarers utilize winds that blow up the slopes, the turbulence created being sufficient to lift heavy birds such the condors and large vultures. Orographic lift is cited as a factor for condors and large vultures such as the Himalayan Griffon vulture favoring mountainous areas over adjacent flat lands from which strong thermals emerge (BirdLife International 2014; Rivers et al. 2014). The Bearded vulture also has a high wing loading and a long wedge shaped tail, possibly an adaptation for flight between mountain ridges using turbulent wind and orographic lift and updrafts (Ferguson-Lees and Christie 2001). Migratory birds also use such updrafts, for example over the parallel ridges of the Appalachian Mountains (Brandes and Ombalski 2004; Bildstein 2006; Mandel et al. 2011).

Fig. 6.2. Orographic Lift

The California condor is also dependent on air flow in mountainous areas (Koford 1953; Wilbur 1978). The condor historically foraged over the flat Central Valley (Belding 1879; Stillman 1967). However, it slowly changed to mountain foraging as the San Joaquin Valley changed from grassland with herds of domestic livestock and native big game, to intensive cropland, and the urban areas of Los Angeles and San Francisco (Dooley et al. 1975). Also, in midsummer about 6 h per day may be suitable for soaring, but in winter, there may be only 4 soaring hours (Wilbur 1978).

Rivers et al. (2014) conducted the 'first quantitative assessment of habitat- and meteorological-based resource selection in the endangered California condor (Gymnogyps californianus) within its California range and across the annual cycle.' Condor use of habitat was influenced by meteorological conditions and the thermal characteristics (thermal height and velocity) were the most important. In addition, the study found that condors also used orthographic lift for soaring flight. This orographic lift is common in coastal areas (e.g., Big Sur region of western North America) and mountainous areas. Here, updrafts can support the condors' weight.

The California condor needs 50 to 60 feet for takeoff on flat land with no head wind (Koford 1953). It may be helpless if it accidently alights in dense vegetation, as large plants obstruct its movement in running and flapping. For this reason it usually does not access cougar kills or remains left by human hunters, as these are usually left in dense vegetation. The ideal landscape for feeding would be flat land with some wind, or small elevations from which to launch itself into the air.

Similar results were found for the Andean condor from the high slopes of the Andean mountains (McGahan 1973; Pennycuick and Scholey 1984; Lambertucci and Ruggiero 2013). The Himalayan Griffon, as mentioned above is similar, being fairly common in the high mountains of Nepal between 900 m and 4,000 m, sometimes up to 6,100 m (Inskipp and Inskipp 1991). It also occurs in the mountains of Bhutan, usually in the alpine and temperate zones from 1,400-4,000 m, occasionally down to 400 m and up to 4,800 m. The Cinereous vulture also frequents mountainous terrain in Israel (Shirihai 1996), Armenia (Adamian and Klem 1999) and Kazakhstan (Wassink and Oreel 2007).

Climate in combination with relief, including orographic lift has an important effect on vulture foraging. For example, in a study of Cinereous vultures, Hiraldo and Donazar (1990), compared two nesting nuclei, one in a mountainous area and the other in a lowland area, and found that in the former with higher slope lift, vultures initiated flights nearer sunrise, and concluded foraging nearer sunset. By contrast, in flatland, they depended only on thermals and foraged for a shorter period each day. Over flatlands, the thermals were only strong enough for foraging about seven hours after sunrise. Therefore, vultures which nested in mountainous area, but foraged on the plains would begin flying earlier in the mountainous area, fly around in this area, and glide down to the foraging area, where they would remain perched until the thermals were strong enough for soaring flight. Those vultures that nested in the flatland area also began foraging at this later time, about seven hours after sunrise.

This behavior was not limited to Cinereous vultures; Griffon vultures in the vicinity also exhibited this behavior. There were seasonal variations; for both nesters in mountainous areas and lowlands, the vultures left the mountain nests earlier in winter (February) and returned home latest in summer (June), when the flying hours were longest (Hiraldo and Donazar 1990). The authors note that 'The arrival of vultures at the colony occurred relatively closer to sunset in summer than in winter probably because of the high temperatures in the afternoon which permit thermal lift to continue until the last hours of the day' (ibid. 131). They however noted that slope lift did not seem to greatly modify vulture foraging times, as the birds in the study area preferred plains for foraging. It was hypothesized that in other more mountainous areas, slope lift might affect foraging.