Birds in Their Habitats

by Ian Fraser

  The Lammergeier doesn’t perform the task by keeping the bone in the gut for a long time – the canal is not significantly longer than that of meat-eating raptors – and it can digest a rib bone in just 24 h. Rather, it has a dense layer of acid-secreting cells in the oesophagus and stomach, producing a highly acidic environment, akin to battery acid. As the bone softens (and remember the Lammergeier can’t chew it up like a hyaena, so must swallow it whole, lengthwise), muscular contractions break it up. In this way, they can deal with bones up to 250 mm long and 35 mm in diameter (Elphick 2014). Lammergeiers have reportedly even been seen flying with the end of a bone protruding from the mouth, waiting for the other end to digest so they can swallow it all!

  It is also well documented that, when faced with bones too big to get down whole, it will carry them into the air and drop them from heights of up to 150 m onto favoured and repeatedly used rock platforms. Bones of 4 kg (half to two-thirds of the bird’s weight) have been recorded as being lifted and shattered in this way, and multiple attempts may be required.

  Amazing, but why bother? Vultures, despite a common prejudice, actually don’t like very putrefied meat, though their harsh internal environment destroys many of the harmful bacteria ingested with the meal, and they can tolerate others to a surprising degree. There comes a point, though, when the carcase is no longer an acceptable food source. Moreover, they have serious competition in the form of other bird and mammal scavengers, bacteria, fungi and insect larvae, so after a while all that is left is … bones. These last for a very long time, so a Lammergeier living in the high mountains, where the density of large mammals and thus their carcases is low, can greatly extend its food supply by being able to use a single carcase for weeks or months until all the skeleton is gone. It’s a bit like having a deep freeze to keep fresh your single bulk purchase until you’ve eaten it all.

  Torres del Paine again: New World vultures

  Unexpected origins

  Inevitably we’ve wandered far indeed from the condors demolishing the chulengo carcase in the icy, windy mountainscapes of Torres del Paine, so let’s go back there to pick up another story they have for us. The question to introduce this story is a pretty basic one – ‘what are they?’. Well, as we’ve noted, they are the largest of the New World vultures, but that’s just begging the question, which is ‘what are the New World vultures?’. It’s now well established that they are not just Old World vultures that found their way to the Americas. In fact it seems they are not very closely related at all, despite appearances and behaviour. The vultures of Africa and Asia belong to the same Family of birds as eagles and hawks, the Accipitridae; the American vultures belong not only to a different Family (Cathartidae) but, according to many taxonomists, to an entirely different Order of birds. Their resemblances are due to parallel lifestyles, and animals (including birds) tend to look the way they do because that makes them good at what they do.

  We can readily imagine Old World vultures deriving from a hunting bird such as a Wedge-tailed Eagle or Whistling Kite (to use Australian examples, though there are equivalents elsewhere too), which heavily supplement their diet of prey with carrion. But what was the ancestor of the condors and other American vultures? There is still not consensus (not an unusual situation with taxonomists, who deal with vast stretches of time, often incomplete evidence and evolving investigative tools), but the numbers seem to be increasingly behind an old idea: that their closest relatives are storks (they aren’t storks, but share a common ancestor). It’s not hard to envisage a bird with the carrion-eating lifestyle of a modern Marabou Stork giving rise to a dynasty of carrion specialists (Elliott 2016).

  Other almost impossible foods

  The Lammergeier may be the most extreme example of a bird specialising in a resource that is unavailable to others (no other vertebrate, even the Spotted Hyaena, relies so heavily on bones), but it is not unique. Others have cracked the secrets of using potential foods that are physiologically unavailable to most animals. Ants and eucalypt leaves, for instance, are both appalling foods, both chemically and physically, but their saving grace is in being an almost infinite resource, so, if you can solve the problems inherent in digesting them, you are guaranteed a food supply. Woodpeckers and Australian treecreepers are two groups that prey extensively on adult ants but, although many insect-eaters snap up the odd ant, very few make an effort to do so consistently. There are many South American passerines, including antbirds, antpittas, antthrushes and antwrens, that follow the army ant columns (in some cases exclusively) but they’re after the insects terrified into motion by the ants, not the ants themselves. On the other hand, no bird eats eucalypt leaves; indeed very few birds eat leaves at all, but that’s a topic for elsewhere (see page 139).

  The 8000 species of sawflies (wasp relatives) are found across much of the world. In Australia, there are fewer than 180, of which many, not unexpectedly, have learnt to eat eucalypt leaves. They deal with the toxins by separating them from the edible components and, rather than wasting them, add insult to the plant’s injury by storing them as caustic yellow oils in the foregut to regurgitate in times of their own need (hence the name ‘spitfire’). Not many birds will risk this chemical warfare, but one seems unfazed by it. The Gang-gang is Australia’s smallest cockatoo: ashy grey with pale mottles and bars and a wispy crest comprising separated spiky feathers like starfish arms. Moreover, the males have a clownishly bright red face, head and crest, and their voices creak like corks coming out of bottles as they fly overhead or sit quietly feeding. They are an eternal delight and I often have the pleasure of their company at home, because Canberra is the only city where they can be found throughout the urban area, including the city centre.

  A particularly fond memory of them, however, is of a male sitting in a leathery-leafed Snow Gum high in the Snowy Mountains of Kosciuszko National Park in the Australian Alps of south-eastern Australia. Alongside him was a cluster of steel-grey tubular spitfires, frantically tapping their heads on the leaf in warning to their colleagues and dribbling yellow fire. He was quite unfazed as he perched on one foot, picking up the larvae one at a time in the claws of his other, delicately extracting the gut sac with the tip of his beak and dropping it to the ground before thoughtfully eating the rest like a connoisseur of eucalypt lollies.

  Spitfires closely resemble caterpillars: the unrelated larvae of moths and butterflies. Most of these have limited defences beyond camouflage, but quite a few have evolved irritating bristly hairs or long thin ones whose tips can pierce skin and be shed, or even contain toxins. These are pretty effective against most predators, but not all. Many cuckoos and cuckoo-shrikes (this combination is a curious coincidence, since they are entirely unrelated, in separate Orders) specialise in hairy caterpillars. Cuckoo-shrikes have learnt to rub the hairs off on branches before eating them, though cuckoos are also able to eat large quantities of the hairs without ill effect, regurgitating them in pellets. As with the spitfires and the Gang-gang, these birds aren’t so much eating these awkward unpleasant meals despite the fact that others don’t, but because they don’t. Like the ants, hairy caterpillars and spitfires represent a major untapped food supply for which there is little competition; if you can stomach them, you’ll be a lot less likely to go hungry.

  A few other birds have also opted for such a diet that is downright hazardous and quite unavailable to almost anything else, giving them a free run at it. There are four ‘honey buzzards’ in the genus Pernis (one breeding in Europe and three in both temperate and tropical Asia). They are not true buzzards and don’t eat honey, so best not to enquire into ‘what’s in a name?’ in this case. Their chosen dietary niche is one of the most forbidding imaginable – the nests of wasps, hornets and bees, which they raid as a major part of their diet for larvae, pupae and the waxy comb itself. They dig into the ground for buried nests and take nests from trees. To answer the obvious question, it seems that small stiff dense feathers, which are especially scaly on the face, protect the skin
from stings (Birkhead 1974). Moreover, Sievwright and Higuchi (2016) have suggested that a ‘unique filamentous substance’ on feathers, especially around the eyes, could be evidence of chemical defence, but this has not yet been established.

  It is regularly reported that the honey buzzards eat the waxy comb, but it is not clear whether this is simply in order to extract the contents, or if they do digest the wax itself. If so, this would be a most unusual situation: wax certainly contains nutrient value, but it is hard to extract. In fact, only the honeyguides (a small Family of 16 species in the same Order as barbets and toucans, from Africa and Asia) are known to have mastered the trick as a group. Although earlier studies suggested that bacterial colonies were employed for the purpose, more recently unusual enzymes including lipases have been found in some species (e.g. the Lesser Honeyguide; Downs and van Dyk 2002) and there now seems to be doubt about the bacterial theory. It has been suggested that some seabirds can also do so (to cope with waxes in crustaceans), as well as some passerines that eat wax-coated berries.

  Incidentally, the honeyguides don’t generally adopt the full-frontal assault tactics of the honey buzzards: although their skin is unusually thick, it is not impervious to stings, and birds have been found dead with numerous bee stings. They tend to attack the hive early in the morning before the bees are active, or find abandoned hives, which apparently are more common in hot climates than temperate ones. They will also take ‘leftovers’ from hives that have been attacked by other animals, especially humans – the behaviour of Greater Honeyguides in southern Africa in leading people to a hive is well documented.

  More specialists: fitting the bill

  So far, these examples haven’t involved the development of a specialised bill for eating unconventional food, which is a bit surprising given that the bill is the primary food-gathering organ of a bird. Such bills certainly exist, however. One unusual bill form, which has evolved on several occasions for different functions, involves a long slender upper mandible that significantly overlaps the lower one.

  The Red-capped Parrot is endemic to the south-west of Western Australia, and is the only member of its genus (perhaps closest to the rosellas). Although it also eats a range of seeds and fruits, it specialises in the tiny seeds of Marri (Eucalyptus calophylla), which are held in a large, hefty cup-shaped woody seed case up to 50 mm long. The parrot nips off the entire fruit and holds it in one foot, anchors it further with the broad-tipped lower jaw, and delicately extracts the seeds from the cup with the thin upper mandible (Forshaw 2002) (see Photo 17).

  The Long-billed Corella is a medium-sized white cockatoo with a red-orange face and breast band, and a massive bill with an elongated upper mandible. It lives in open woodlands and grasslands (mostly now dramatically altered by farming and grazing) of inland south-eastern Australia. It uses its bill not for delicate extraction, but for serious excavation into the soil: when a flock has been working a paddock it has very obviously been cultivated! The Murnong, or Yam Daisy (Microseris lanceolate), once covered vast areas of Australian temperate grasslands and was a key food item for the Long-bills. Accounts from southern New South Wales and northern Victoria (i.e. Long-billed Corella heartland) tell of swathes of Murnong flowers turning the plains golden to the horizon. Their small sweet tubers were harvested by Aboriginal people, eaten raw or roasted to a delicious treacly consistency. European settlers learnt the trick from them. There are stories of wagon wheels turning up thousands of Murnong tubers from the soft soil, leaving them to rot on the surface. Then the sheep came, eating the plants and learning to push into the soil to eat the tubers as well. The plough finished the job. Initially, Long-billed Corella numbers crashed, until they learnt to eat the wheat that replaced the Murnong. This was, of course, a capital offence and populations declined again as large numbers were shot. Now they survive largely on exotic lily-related tubers, for which their specialised bill is pre-adapted.

  In South America, and to a minor extent in central America, the Caribbean and Florida, two raptor species also have similar bills – and, almost without exception, raptors are carnivores, with no interest in seeds or tubers. In this case, the food for which their bill evolved presents a similar problem to the Marri fruit – large, smooth and hard with a small opening. The big apple snails (Pomacea spp.) are found widely in South America and provide an excellent food source to anything that can breach their defences: a challenge that is beyond the capacities of most birds. Snail and Slender-billed Kites, however, use their long sharp upper mandible to cut the columellar muscle that holds the snail’s body to the shell, after which the meaty delicacy just falls out.

  Among the very few other birds that have learnt to breach the defences of apple snails are the two species of openbill storks (one African, one Asian): in their cases the snails are Pila spp. These storks have a most peculiar bill indeed, in which the lower mandible bows outwards towards the tip, and is twisted slightly sideways, creating a distinct gap of several millimetres when the beak is closed. There has been a lot of debate as to how they use this structure to extract the snails, not least because the process is both rapid and mostly occurs under water. It is now agreed, however, that they do not break the shell, or use the gap to carry snails away. Rather, they use a concentration of stalk-like pads near the tip of the straight upper mandible to hold the snail in place against the ground or mud, then use the angled lower mandible to push past the operculum (the hard plate that protects the shell opening) and, like the American kites, to cut the columellar muscle. Perhaps even more astonishingly, the Asian Openbill produces a narcotic chemical in its saliva, which flows down the bill and relaxes the operculum muscle, facilitating access (Elliott 2016).

  It’s important to remember that all these specialists can, and do, also eat ‘normal’ food (i.e. as defined by their less specialised close relatives) if circumstances warrant it, but use their ‘super powers’ to escape competition from those lacking them.

  Even the apparently logical notion that the New World vultures evolved to fill an empty niche caused by the absence of ‘real’ vultures is at odds with reality. Until just 10 000 years ago, there were Old World vultures in America, and there were New World vultures in the Old World, though not apparently for the last 20 million years.

  However, the concept of birds evolving into something quite different to fill an empty niche is not at all fanciful. Take the Southern Crested Caracaras waiting patiently for their turn beyond the scrum of condors on the carcase. This is quite a substantial bird, with a wingspan of up to 1.3 m and weighing over 1.5 kg, but it is dwarfed by the condors. It has short feathered trousers, its back and flight feathers are brown-black, while most of the rest of the body is barred with fine white markings on a dark background. A white throat and cheeks are topped by a black cap and blue bill with a broad red base. The bill alone leaves no doubt that it is a bird of prey: either vulture or mainstream raptor. BLAA! (That was the quiz show buzzer announcing our humiliation and loss of points for this incorrect assumption.)

  This solid predator, which spends a lot of time walking across the ground in search of food as well as scrounging carrion, is a falcon. Not even ‘evolved from a falcon ancestor’, but a fully paid-up member of the Family Falconidae. Nonetheless, it eats almost anything, from mammals to birds to reptiles to frogs to crabs to worms to insects to carrion – not at all a falcon’s diet. So what niche does it fill that was empty enough to tempt a falcon from the skies to the ground? The answer seems to be that of crows. When I first encountered a flock of the little brown Chimango Caracaras working over the ground in the Patagonian heaths, I was irresistibly reminded of Little Ravens in the high country of south-eastern Australia foraging in just the same way. When South America collided with North America just 3 million or so years ago there were no corvids (i.e. members of the Family Corvidae, such as crows, ravens, jays, magpies and nutcrackers) in the southern continent. While other vertebrates, such as North American blackbirds and camels, spread throughout South Amer
ica, it seems that the crows foundered in the vast rainforests of the north and, even today, the only corvids in South America are a few species of rainforest jays. This left the vast plains of the south with an opportunity apparently too good to refuse, and falcons filled it. There are still mainstream falcons in South America too: it’s just that some of their ancestors chose another path.

  Flying high: how do they do it?

  Andean Condors are found to at least 5000 m above sea level (about the level of Patapampa Pass, where I struggled to walk a few steps), but that’s nowhere near the limits of altitude for birds. There are reports from planes of migrating birds and big raptors at double that altitude, but at 10 km above the ground each breath contains less than a third of the oxygen that is available at sea level. Most of us could not survive just being there, let alone undertake prolonged strenuous exercise. In an experiment some 40 years ago, mice and sparrows from sea level were put into a chamber with an atmosphere equivalent to 6100 m (i.e. 20 000 feet, hence the arbitrary metric figure). The mice were prostrate, as would we be, but the sparrows appeared unaffected (Tucker 1968). How do they do it?