by Ian Fraser
The swallows and bee-eater also held their wings partly extended to expose the large blood vessels under them to the air, further enabling heat loss. In part, the swallows looked so glossy because they had pressed their plumage close to the body, squeezing out as much of the insulating air as possible. I do wonder why they didn’t use the shade though. I guess you need to be tough to live in the Sahel.
I have an abiding memory of a dramatic sunset that night: a huge sun setting white, rather than red, through a bare thorn tree, with a silent flight of Black Crowned Cranes drifting over in silhouette.
Waza: guineafowl and sandgrouse
Next morning we drove into the park, past herds of antelope, big Roans and Korrigum (formerly regarded as a subspecies of Topi) in a landscape seemingly too sparsely grassed to support such a large biomass of animals. A pair of Common Ostriches and a huge Arabian Bustard stalked away from the vehicle.
We stopped near a muddy waterhole and watched as mixed flocks of doves streamed into the nearby thorn bushes, dropped to the ground and walked cautiously to the water’s edge. A mob of Helmeted Guineafowl came en masse, while keeping a wary eye on us across the water. Guineafowl have always appealed to me: I would be tempted to keep some if their extraordinarily raucous cackling wasn’t guaranteed to test the patience of neighbours beyond reasonable limits. Plump round tailless bodies, dark grey spangled all over with white stars, are carried on strong grey legs. Their white face (at least in this west African race) is set off with red bill and wattles, and a funny little cassowary-like casque on the head. This flock isn’t just a casual gathering: Helmeted Guineafowl are known to stay with the same flock, which can contain up to 200 birds, for years. Unlike other guineafowl species (all African), Helmeted flocks will happily mingle at watering holes or food sources. On the other hand, they will unite furiously to repel a predator. This group would have ambled to water, feeding as they went, as soon as they left their roost site to which they return every night (e.g. Martínez and Bonan 2017).
Staying cool in a hot land: a tricky balance
As with most problems, there is no perfect solution to the crucial dilemma of staying cool when the surrounds are very hot. Panting works quite well, but, as temperatures rise, so do the spectres of two problems involved with it. If it’s dry – as in Waza – the loss of body water is increasingly dangerous. Albright et al. (2017) looked at the impact of a 4°C temperature rise on five species of south-western US desert passerines, with regard to evaporative water loss (i.e. through panting). They showed that small birds lose water at a relatively higher rate than larger ones, so are at higher risk. The basis to this lies in physics. A smaller bird has a relatively larger surface area than a large one – more on this on page 160. But, given that they don’t lose water through sweating it seems counter-intuitive that this could be relevant. However, it is not just the body surface that is relatively larger: the area of surfaces such as the upper digestive tract and interior of the mouth from which water is being evaporated is also larger.
On the other hand, if humidity is high, panting simply doesn’t work – you can’t evaporate water into a saturated atmosphere. However, we’re becoming aware of the complexities of another use for the bill in this situation. Bills can dump excess body heat by radiation and convection without losing water, and this works just as well at conditions of high humidity (it seems that this is the major purpose of the ridiculously large bills of the tropical toucans – Tatersall et al. 2009). Allen’s Rule predicts that such organs used in heat exchange will be disproportionately larger in warmer climates, but only up to a point … As the surrounding temperature approaches the bird’s body temperature, radiation and convection cease to work. Even worse, if the surrounds are hotter than the bird, a large bill actually becomes a hazard by absorbing heat.
Janet Gardner of the Australian National University and colleagues set out to test Allen’s Rule for a wide range of Australian passerines, while also exploring the role of humidity (Gardner et al. 2016). They measured 2864 museum specimens of 36 Australian passerine species, collected across their range from 1970 to 2012. In fact, they found no simple correlation of bill size and summer temperatures. In addition to the problem of a large bill becoming a heat trap at very high temperatures, it is very probable that it is also a problem in a desert winter when heat conservation is significant.
However, they did demonstrate a strong correlation between summer humidity and bill size, especially at somewhat lower maximum temperatures (at higher temperatures than the bird’s body temperature, the issue of heat gain via the bill kicked in again, so the large bill was no longer advantageous whatever the humidity). Once temperatures get to the dangerously high levels above body temperature, evaporative cooling again becomes the only option, despite its inefficiency in humid situations and danger of dehydration in dry ones. This means that birds in a warming world living in already stressful hot lands may have ever-decreasing windows of thermal regulation opportunity open to them.
Smit et al. (2016) observed the responses to heat of a large number of desert bird species in the Kalahari. They confirmed that panting is effective at losing heat in dry conditions, but at a considerable cost in water loss, so it pays to put it off as long as possible (i.e. until it gets hot enough to be essential). They found that birds were better able to defer panting until higher temperatures if they were smaller, so could better dissipate heat via their proportionately larger surface area, if they could avoid excess activity (i.e. foraging in particular) when it was hot. If they didn’t drink much, so were obtaining most of their water from their food, it was even more essential to use panting as a last resort.
However, you’ll not be surprised to hear that there are even more complications to a bill’s thermoregulatory function than surface area. Inside is a system of chambers called conchae, whose size and complexity determine both heat exchange and moisture retention during breathing. Danner et al. (2017) carried out detailed analyses of these conchae in two subspecies of Song Sparrow in the United States: one from arid coastal dunes, the other from a more humid situation. They took CT scans and radiographs of the bills, and showed that birds living in the arid situation had larger and more complex conchae, especially towards the bill tip, for better moisture retention.
Soon after their departure, their place was taken, to my very great pleasure, by a flock of Chestnut-bellied Sandgrouse. This Family of mostly desert specialists is found throughout much of Africa, plus western, central and southern Asia. Superficially pigeon-like with streamlined bodies, unlike the guineafowl they fly hard and far. Again unlike the guineafowl, which sometimes seem determined to be the centre of attention, sandgrouse specialise in being unobtrusive, from their soft brown cryptic colouring to their habit of lying still when nervous and avoiding any conflict with their fellows, which might attract predators. They rely on small hard seeds, and must fly daily to water, like Zebra Finches and Budgerigars in Australia. Here they may gather in huge numbers – at Lake Kabara in Mali up to 50 000 Chestnut-bellied Sandgrouse have been reported (de Juana and Boesman 2016). But my favourite story about sandgrouse concerns their remarkable provision of water for their chicks, which lie in a simple scrape nest on the baking ground.
In 1896, prominent English ornithologist and conservationist – the splendidly named Edmund Gustavus Bloomfield Meade-Waldo – reported that he’d seen a male Pin-tailed Sandgrouse walk into water, soak his belly feathers with water, and carry it to nearby chicks, which immediately sucked it up. Sadly, and surprisingly given his excellent track record, people refused to believe him and then dismissed others who were reporting the same phenomenon. It was not until 1960, long after his death, that the world became reluctantly convinced by the mounting evidence. The male sandgrouse’s belly feathers are superbly adapted to the purpose. A gram of them can hold up to 20 mL of water: four times what a kitchen sponge can manage. The feather barbules are soft and coiled, forming a felt-like material that absorbs water by capillary action and e
nables him to carry up to 40 mL of water for tens of kilometres. When he arrives, he stands over the chicks, which suck the water from his belly. He might have to make up to three trips a day for them (de Juana 2017). I could only hope that some of these Waza birds were loading up to feed thirsty babies.
Shortly after that, a most unpleasant stomach bug brought me very low indeed and I have limited memories of the rest of my time in the Sahel, but that’s how birding can go sometimes and I would prefer my memories of Waza to be of guineafowl and sandgrouse than what followed. Unlike the now fearful people of Waza, I have a choice.
The Atacama: the driest desert
The mighty Atacama in northern Chile and southern Peru is a desert like no other. Parts of it are the driest places on Earth, with some locations recording no rainfall at all in the 450 years of European records. Overall, the average rainfall is 15 mm per annum, but the coastal cities of Iquique and Arica average less than 3 mm a year. It has been subject to ‘extreme hyperaridity’ for at least 3 million years, making it the oldest consistently arid area on the planet. One spin-off of the aridity is that in some places there are mountain peaks over 6000 m above sea level with no snow cover, although much lower Andean peaks right on the equator do have permanent snow caps. The basis of this aridity is the presence of the cold Humboldt Current against the Pacific coast: winds blowing across the chilly sea pick up very little water. Moreover, the Andes form a barrier to moisture-bearing winds blowing west across the Amazon basin from the Atlantic, creating a rain shadow. The west coasts of Australia and southern Africa also have deserts right to the sea due to similar cold currents, but even the forbidding Namib Desert in Namibia can scarcely compare to the Atacama. Near the sea, the camanchaca – the pall-like morning sea mist – can bring enough moisture for some tenacious plants to survive. Higher in the Andes, just enough moisture leaks over the ridges from the Amazon for some growth, mostly in the form of tough hard cushion plants. In between, from 1000 to 2000 m above sea level, is the ‘absolute desert’ where the zones of ‘has never rained’ lie. Here you can stare out of the vehicle window at a landscape of sand and pebbles that stretches to the horizon, uninterrupted by green.
Flamingos
Yet, counter-intuitively, there are still birds in the Atacama. Flamingos for instance … Now, although I may well be losing my remaining marbles, the evidence is not in that observation. Yes, there are flamingos – three species in fact – that live and breed in the mighty Salar de Atacama, comprising 3000 km2 of salt flats in a basin with no outflow in the mid-level Andes of Chile, near the desert town of San Pedro de Atacama. Water runs down from the soaring, snow-capped peaks that loom jaggedly into a sharp blue sky to the east. There is not much water, but enough to form shallow trapped lagoons super-saturated with salt. In these live the remarkable brine shrimps, which flamingos can’t resist.
Laguna Chaxa is a surreal world. Fabian’s Lizards peep out from little salt caves and splash through water that our feet would find caustic in pursuit of the myriad flies that swarm across the surface. Chilean, Andean and Puna (or James’) Flamingos, Andean Avocets and Puna Plovers are among the mouth-watering birds (from a birding point of view!) that work the shallow waters. All around, where the water has evaporated, is a plain of churned-up mud sparkling with salt crystals. And beyond, the Andes, snow-splashed above completely arid brown slopes.
The taxonomy of flamingos
Flamingos belong to an ancient lineage, so old that their relationships to other bird groups are unclear, though some smart money is currently on a connection with grebes. Once there were many species scattered throughout the world, including Australia, until just a couple of million years ago when the vast inland lakes dried out, but now there are just six species (or perhaps only five, depending on who you ask). The Greater Flamingo (of Africa, southern Europe and southern Asia), the American Flamingo (from the Caribbean and the Galápagos) and the Chilean Flamingo (from central Peru south along the Andes to Tierra del Fuego and east to the Atlantic) form a close species group. The Lesser Flamingo of Africa is given its own genus, and the Puna and Andean Flamingos, both Andean specialists, form a third genus.
The Laguna Chaxa flamingos are doing what is normal to them, but extraordinary to us. The head is upside down between the bird’s feet, the horizontal inverted bill pumping water in and out. The large, fatty, highly sensitive tongue with numerous fleshy spines is the pump, working at up to 20 beats a minute, sucking in particulate-laden water and expelling unwanted items via a complex set of movements. This diet makes their feathers pink too, from the β-carotenes contained by the shrimps (see Photo 4). Chicks begin life grey and ‘pink up’ as they eat their shrimps. ‘Pink’ doesn’t really do it justice, however, because flamingos flaunt a rich range of red-pink-orange – the ancient Egyptians used a flamingo-shaped hieroglyph to indicate red. (To keep them coloured, zoo flamingos are fed canthaxanthin, a naturally occurring β-carotene that is also used in tanning pills, mostly illegally.) All three of the Salar de Atacama species breed, often together, in colonies of cone-shaped mud nests topped with a bowl that holds one egg. Feeding the chicks involves another very singular flamingo adaptation – they produce milk! Well, they don’t really, of course – that is definitively a mammalian characteristic – but, unlike almost all other birds, they synthesise a protein to give their chicks a boost. The protein is derived from glands throughout the upper digestive tract. Also unlike mammals, both flamingo sexes produce it and, among other birds, only pigeons and male Emperor Penguins have duplicated the trick.
After a few hours among the flamingos in this driest of deserts, driving back in the evening into the bustling oasis town of San Pedro de Atacama, with its green trees and rows of gringo-oriented souvenir stalls, seems stranger still.
Other memories of deserts
Like sand grains picked up by sudden breezes passing over desert dunes, some other desert memories drift into my mind:
• Waking in the Botswanan Kalahari Desert to a pair of Southern Yellow-billed Hornbills catching the earliest light, then giving a brief wing-stretching, bowing display, bubbling quietly the while.
• Okaukeujo Camp in Etosha National Park, based on a vast dry salt pan in north-western Namibia. The camp ground is dominated by huge Sociable Weaver nests, which can contain up to a tonne of grass and be metres long and high. The tiny neat black and white spotted architects are everywhere. From the top of the nest peer a couple of tenants: an exquisite chestnut, grey and white pair of Pygmy Falcons, just 20 cm long. Although they mostly eat out, they have been known to snack on the landlords’ chicks.
• A magnificent Black-breasted Buzzard, chestnut and dark chocolate, drifting along the face of mighty red Uluru (for a while known as Ayer’s Rock) in central Australia.
• Sitting on a brick-red sand dune near Cameron’s Corner where New South Wales, Queensland and South Australia meet, watching, for the only time in my life, a family of white-streaked rusty little Eyrean Grasswrens bouncing about like ping pong balls between tussocks of cane grass. In all the world, they live only on dune crests in the Lake Eyre basin in the central Australian deserts.
References
Albright TP, Mutiibwa D, Gersond AR, Krabbe Smith E, Talbot WA, O’Neill JJ, et al. (2017) Mapping evaporative water loss in desert passerines reveals an expanding threat of lethal dehydration. Proceedings of the National Academy of Sciences of the United States of America 114(9), 2283–2288. doi:10.1073/pnas.1613625114
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