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On a wing and a thermal, the flight of vultures

Text: Tim Jackson. Article from the November 2012 issue of Africa Geographic Magazine.

Vultures are reputed to be among the avian world’s best flyers, but it’s a reputation that merits a little closer scrutiny. They don’t, for example, get far by flapping their wings. What they’re good at is taking advantage of wind power to get around. But even this has its drawbacks.
Vultures are reputed to be among the avian world's best flyers, but it's a reputation that merits a little closer scrutiny.
Getting up

A cliff-nesting Cape vulture launches itself into the air. Once airborne, it will use the updraughts created over the cliffs to gain altitude before moving away in search of food.Vultures are, literally, not early risers. You don’t see them out and about until the sun is well above the horizon and conditions are favourable to get them – and keep them – airborne.

They sit and wait for the first air currents of the day before launching themselves into the sky, but on still days they may not take off at all – they’re just too heavy.

Some vulture species (Rüppell’s, Cape and bearded) nest on cliffs.

Perfectly poised, a Cape vulture soars on outstretched wings that, with a special arrangement of muscles, can hold that position for hours on end.They make use of updraughts produced when the wind blows over hills and mountain ridges (called slope lift) to get airborne and then gain height by flying backwards and forwards at right angles to the direction of flow. They use a similar pattern of flight to gain height in rising air currents that are generated over leeward slopes (wave lift).

Facts of flight
  • Vultures using slope (below) lift tack back and forth across the direction of the wind in order to gain height. The same pattern of flight exploits wave lieft (bottom) on the leeward side of a slope.Vultures have a special arrangement of muscles that enables them to keep their wings extended in the soaring position for hours at a time.
  • Aside from being hugely energy-expensive for vultures, powered (flapping) flight would give the birds an estimated foraging range of less than 40 kilometres per day. Soaring extends this to about 150 kilometres.
  • The avian high-flying record is held by Rüppell’s vulture. One was sucked into an aircraft engine off West Africa at an altitude of more than 11 000 metres.
  • From an altitude of 300 metres a vulture can reach any point on the ground in a 4.5-kilometre radius within six minutes.
  • A young Cape vulture fitted with a tracking device in Namibia flew across six countries in southern Africa and covered at least 64 000 kilometres.

For these species – especially Rüppell’s vulture, which specialises in feeding on large migratory antelope, like wildebeest in the Serengeti – the distance between their home cliffs and potential food sources can be a problem. Much depends on the whereabouts of carcasses; if they’re close by, or if there are large antelope herds (with an increased likelihood of individual animals that have fallen to a predator or have died of natural causes) in the vicinity, then the vultures will expend comparatively little energy on finding food. Sometimes, though, they travel 100 kilometres or more from their cliffs in search of a carcass, and in doing so test their energy reserves to the limit.

Without the aid of a cliff-generated updraught, tree-nesting vultures like the white-headed have to flap their way into the air before finding a thermal to ride.And what of the vultures that don’t breed on cliffs? Lappet-faced, white-backed and white-headed vultures build their nests in the tops of trees, and even cliff-nesting species roost in trees outside the breeding season. These vultures rely on flapping to get airborne, and must then find a thermal to help them gain height. To make use of a thermal successfully requires quite a bit of skill. As the air in it rises faster in the centre, the massive bird flies in as tight a circle around the centre as it can. But to do so it must bank over at an angle, which can cause it to sink. If it banks at too tight an angle, it will end up sinking rather than rising in the air column.

In general, thermals are strong enough to carry vultures aloft from about two hours after sunrise, though much depends on the prevailing weather conditions. A thermal-riding vulture can reach an altitude of 1500 metres or more.

Vultures using slope lift (below) tack back and forth across the direction of the wind in order to gain height. The same pattern of flight exploits wave lift (bottom) on the leeward side of a slope.

What is a thermal?

Thermals works as 'carriers' for vultures because the warm air moves upward faster than a bird sinks - typically at speeds up to 36 kilometers per hour.Thermals occur when dark patches of ground (including urban areas and roads) are heated by solar radiation and in turn warm up the air above them more quickly than surrounding areas, causing the air to rise in columns – or thermals.

A good indicator of a thermal is a cumulus cloud at its top, which forms when the warm air cools as it ascends and any moisture that it contains condenses into visible droplets.

Thermals work as ‘carriers’ for vultures because the warm air moves upward faster than a bird sinks – typically at speeds of up to 36 kilometres per hour.

Getting around

Vultures cover distances in search of food by repeatedly gaining height in one thermal and gliding downward in the desired direction to the next. A thermal can often be identified by the cumulus cloud above it.Once carried into the air by slope or wave lift, cliff-nesting vultures, like their tree-nesting counterparts, have to find thermals to ride. On overcast days they don’t move around much, but when the sun is out to warm up the ground, they can travel over considerable distances by ‘hopping’ from one thermal to another, rising with the ascending air as high as they can, then exiting the thermal and gliding steadily downward towards another in their chosen direction of travel. Their aim is to find the next thermal before they descend too far. They enter it at a low point and repeat the performance, able to travel in this way at an average ground speed of 45 kilometres per hour.  They can easily cover 140 kilometres or more in their commute from nest to food, a journey that would take roughly three hours one way. Huge wingspans – up to 2.8 metres in lappet-faced vultures – and broad wings facilitate this style of low-energy, flapless flight, while a low wing loading (ratio of bird weight to wing area) enables the bird to manoeuvre at slow speeds.

Vultures cover distances in search of food by repeatedly gaining height in one thermal and gliding downward in the desired direction to the next. A thermal can often be identified by the cumulus cloud above it.

While in the air the vultures look for food on the ground, and in this respect the slow pace of their soaring flight is ideal. Some species, like lappet-faced and white-headed, are thought to be territorial. They defend their airspace from interlopers by means of aerial altercations and when scavenging they appear at a carcass only a few at a time. Others, notably white-backed and Rüppell’s, seem not to be territorial and commute over large distances in search of food. When they do find a dead animal they descend upon it in large numbers, with the result that more than 100 birds are sometimes seen squabbling over the remains.

Wings over Africa

Vultures have some of the largest wingspans of the avian world, relative to their size.Their rectangular wings, with a large surface area, are ideally suited to making the most of ascending air currents and enable the birds to soar effortlessly. The wingspans of the most frequently seen African vultures are:

Hooded vulture 170-182 cm.

White-backed vulture 218 cm.

White-headed vulture 230 cm.

Riippell’s vulture 241 cm.

Cape vulture 255 cm.

Lappet-faced vulture 280 cm.

Getting down…

The altitude at which vultures fly gives them a good field of view of the ground below – and of the movements of fellow vultures in the sky. One of the most amazing sights in Africa is the sudden descent of masses of vultures onto a carcass. Drawn from far and wide, they home in on a dead animal – but how do they do it?

Taking off from flat ground is a costly business for a vulture such as this lappet-faced, and it expends a lot of energy in getting airborne after feeding at a carcass.The lappet-faced vulture is often the first species to descend on a kill. In Tanzania’s Serengeti National Park biologist Hans Kruuk observed that the number of these huge birds that arrived first at a carcass was disproportionate to the size of the local population. They, it is believed, scan the ground most assiduously for signs of death. For most other species it’s a case of follow the leader. Colin Pennycuick, an expert on vulture flight, reckons that most vultures – and white-backed and Rüppell’s in particular – are more intent on watching the antics of other vultures than they are on sighting a kill themselves.

Vultures readily cue into the movements of other individuals to find a thermal, so it's not unusual to find a large number of them soaring higher and higher as they gain altitude on the rising current of air.The first vulture to descend starts a chain reaction: those closest to it are the first to make a move, dropping out of the sky with amazing speed, and others further away follow suit. Several hundred of the scavengers have been recorded descending onto a carcass in less than an hour and from distances estimated as far as 35 kilometres away. Most will feed well on a large carcass, like that of a buffalo, but only the early birds will get a meal from a small antelope like a duiker.

Latecomers will depart hungry.

…and up again

Getting to a carcass is one proposition, but taking off with a full crop is quite another. Ungainly on land, the heavy birds run along the ground and flap their wings energetically in their efforts to lift off; if they don’t succeed at once they quickly become exhausted. Sometimes they flap laboriously to a nearby tree to await the assistance of a passing thermal.

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