Birds in Their Habitats

by Ian Fraser

  Birds have had to develop a rather different principle of respiration from mammals, because their lungs are smaller and unable to expand very much due to the constraints of the great flight muscles and associated supporting bones. Though a bird’s lungs are relatively smaller than a mammal’s, the air capillaries are finer and more numerous, giving a greater surface area. In fact, a duck’s lung accounts for only 2% of its body volume, compared with our 5%.

  More importantly, however, birds have a unique system of thin-walled air sacs throughout the body, acting like bellows in conjunction with their lungs. The sacs are extensions of the bronchial walls (so are part of the respiratory system), but extend between the intestines, between the muscles, under the skin, even into some bones. There are usually nine of these very thin-walled sacs, though this can vary between species. Once the duck’s air sacs are factored in, the effective lung volume rises from the modest 2% to an amazing 20%.

  Largest are the two rear-most ones: the abdominal sacs. Air is drawn directly into them from the bronchi, by-passing the lungs. From there, it is pushed forwards through the lung into sacs at the front of the body, so air flows through a bird’s lung only in one direction (very different from our diaphragm-driven ‘in and out’ system) and the whole contents of the lung are changed with each breath. This ensures that the blood flow in the lung is always going against the air direction, maximising oxygen uptake and carbon dioxide expulsion. Air reaching the front of the lung has already been depleted of oxygen as it moves forwards, but the blood it meets there has just arrived in the lung and it too is low in oxygen so is capable of taking more oxygen up from the relatively low air concentrations. As the blood moves back through the lung it becomes enriched with oxygen, but is meeting more and more highly oxygen-enriched air, so oxygen uptake is possible all the way through. This is a far more efficient system than our own.

  Due to some remarkable detective work in 2005, we now know the origins of these sacs, a real key to birds’ success (O’Connor and Claessens 2005; Ward 2006). Those origins lay in very low atmospheric oxygen levels around 230 million years ago (the reasons for that are probably a mix of biological and geological, and needn’t concern us now, but such changes in atmospheric oxygen have been not unusual in Earth’s history). They were about half of today’s oxygen concentration at sea level, which equates to a modern altitude of around 4400 m. A lizard’s lung is relatively small and rigid, which is fine when oxygen levels are high, but less so otherwise. Their ancestors doubtless struggled when levels dropped, and one minor group of reptiles came up with a radical solution. We know that hitherto unsuccessful group as the dinosaurs (and especially the theropods, from which the birds arose), and analysis of early dinosaur bones has shown that they developed the same type of cavities that modern birds have: in other words, they evolved air sacs to cope with the thin air. Their descendants – the birds – inherited them and developed them to assist with flight and survival at high altitudes, on mountains and above them.

  Perhaps inevitably too, a few species have even adapted them for other uses. Male frigatebirds famously have a ludicrous great red throat pouch: an air sac just under the skin, which they inflate like a great balloon to attract females’ attention. Moreover, the male is quite shameless in flaunting it to females flying over while he sits in apparent domestic bliss with his mate on the ground! (See Photo 18.)

  Other memories of mountains

  Like fleeting glimpses of snow-slashed black peaks, which loom through gaps in the swirling clouds, then withdraw into the diaphanous curtain, here are some random mountain memories:

  • High in the Kapteinskloof Mountains north of Cape Town is a private nature reserve called Mountain Mist. I spent a memorable night there some years ago – the views alone are immense, northeast to the rugged Cederberg Mountains and west across the coastal plains 1000 m below to the Atlantic Ocean and Saint Helena Bay (reputedly the largest bay in Africa). As the sun sank over the sea, the distant strip-cultivated plains were charcoal and pale green and the sea was palest misty blue merging into the sky. The foreground tumbled away from my verandah in heath-covered rockiness (‘fynbos’ in the local usage). Proteas crowded in, and glowingly glorious Orange-breasted Sunbirds and taxonomically enigmatic Cape Sugarbirds worked the big flowers. The sugarbirds comprise just two species in a Family of utterly uncertain relationships: this one has a curved bill, brown back and white undersides streaked all over, and a hugely long tail. The sun swelled and turned that peculiarly African shade of vermilion just before disappearing. In the dusk, two sugarbirds sat up in silhouette. In the morning, the world on the mountain was in brilliant sunshine but below was a dense white blanket. I inched down the narrow rough road, sometimes through passages cut through the rocks, with mist stifling all views. Sometimes I saw a ghostly sugarbird by the road, once a silhouetted Ground Woodpecker, all in silence.

  • Another misty day, near the summit of Mount Kosciuszko in Kosciuszko National Park, the highest point of Australia: this is an ancient land that has been eroding away for a very long time and ‘Kosci’ is just 2300 m above sea level. The wind ripped the cloud into shreds but it reformed immediately. It was midsummer, but uncomfortably cold. A small flock of Little Ravens inspected the tumbled granite boulders, intently peering into crevices. They were looking for Bogong Moths: a medium-sized grey moth that emerges in spring on the black soil plains hundreds of kilometres to the north, where the larvae (known as ‘cut worms’) have spent the winter feeding on plant roots. As their ‘foremoths’ have done for millennia, the adult moths flew south at night, not feeding as they travelled, and spent the days somewhere dark and quiet, be it cave or broom cupboard. They find the same crevices that thousands of previous generations have used and spend summer in partial aestivation in the cool dark crevices. Little Ravens gorge on their fatty bodies, as do antechinuses (small marsupial carnivores). At night, inexplicably, some of the moths fly, though they still don’t feed, and bats and Southern Boobook Owls are waiting. In autumn, the survivors fly north again, lay eggs – and die. No moth makes the trip twice: the map is hard-wired into their tiny brains. And each year the Little Ravens will be waiting for them.

  • South of the ‘gringo capital of the Atacama Desert’, San Pedro de Atacama, high in the Chilean Andes at 4100 m above sea level is Laguna Miscanti: an almost impossibly beautiful lake, sky blue with the encirclement of snowy volcanoes reflected in it. The short walk to the lake is a challenge at that altitude, but eminently worth it. Andean Gulls, which live and breed only in the high Andes, drifted overhead. Scattered around the shore and on the water were massive Horned Coots, weighing over 2 kg, strangely attired with three black, tuft-tipped, fleshy wattles above the bill. In all the world they live only in these high Andean lakes more than 3000 m above sea level, in a small area around the corner where Chile, Bolivia and Argentina meet. The apparently floating nests we could see were an illusion: amazingly, the coots painstakingly carry pebbles out into the lake and drop them to make huge artificial islands on which they make their nest of vegetation. The world never ceases to astonish me (see Photo 19).

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  Photo 1. Red soil and termite mounds and polished gibbers: desert country around Warrigal Waterhole, north-west Queensland. (Page 1)

  Photo 2. Courting Budgerigar pair, Mullewa, Western Australia. A budgie can breed at just 6 weeks old. (Page 8)

  Photo 3. Ethiopian Swallows, Waza National Park, northern Cameroon, panting to stay cool in 40 plus degree heat by evaporation from the open bill. (Page 17)

  Photo 4. American Flamingo, Isabela, Galápagos, showing typical flamingo feeding strategy: pumping water in and out of the upside-down bill, filtering out tiny food items. The rich plumage colour is obtained solely from this food. (Page 23)

  Photo 5. Primary lowland tropical rainforest in Korup National Park, south-western Cameroon: the mighty granitic massif of Rengo Rock gives a vantage point. (Page 27)

  Photo 6. Golden-naped Barbet, Mount Kinabalu National Park, Sabah. The yellow and the blue are achieved by completely different methods, and the green by a combination of them. (Page 36)

  Photo 7. Ocellated Tapaculo, Paz de las Aves, north-western Ecuador: a near-mythically hard-to-see dweller of the Andean cloud forests, patiently habituated here to come to offered worms. (Page 39)

  Photo 8. Wire-crested Thorntail hovering: one of the most dramatic of hummingbirds (a big call indeed!). This one was attending the feeders at Aguas Verdes, eastern Andean slopes, northern Peru, which is another ecotourism success story. (Page 41)

  Photo 9. Rufous-bellied Euphonia calling 45 m above the ground, seen from a canopy tower in a huge Kapok Tree, Sacha Lodge, Ecuadorian Amazonia. (Page 53)

  Photo 10. Medium and Large Ground Finches scrounging, Baltra Airport, Galápagos: their different-sized bills evolved to allow them to co-exist by eating different-sized seeds – though not from plates! (Page 55)

  Photo 11. Common Cactus Finch, Puerto Ayora, Galápagos. The cactus finches have evolved as complete cactus specialists, eating flowers, fruit, stems and the insects on them, using their long sharp bills. (Page 60)

  Photo 12. Tasmanian Nativehen, Copping, Tasmania: the ‘turbo chooks’ have lost the use of their wings and rely on their long strong legs for safety. (Page 64)

  Photo 13. The Titicaca Grebe is flightless and restricted to huge Lake Titicaca in the high Andes of southern Peru. In this case, the lake is effectively an island in the mountains; retaining flight would have made the bird susceptible to being blown out to die in the hostile montane landscape. (Page 69)

  Photo 14. Flightless Cormorant pair at seaweed nest, Fernandina, Galápagos. The wings are clearly far too small to allow flight. One bird has just returned from fishing and they are performing a greeting ceremony. (Page 70)

  Photo 15. Red-tailed Tropicbird, soaring in courtship display off the wild Malabar Cliffs on the northern edge of Lord Howe Island. (Page 75)

  Photo 16. Andean Condors soaring up out of Colca Canyon, southern Peru, where they roosted for the night; they are riding rising air currents as the morning warms. (Page 89)

  Photo 17. Red-capped Parrot, Albany, Western Australia, where it is endemic. The elongated upper mandible, used for extracting tiny seeds from big Marri eucalypt capsules, is evident. (Page 97)

  Photo 18. Magnificent Frigatebird pair at nest, North Seymour, Galápagos. The male has inflated his preposterous red throat sac to impress both his mate and other females flying over; it evolved from one of the air sacs that make bird respiration so efficient. (Page 101)

  Photo 19. Laguna Miscanti, at 4100 m above sea level in the Atacama Desert of northern Chile, supports populations of birds including the huge, an
d very restricted, Horned Grebe. (Page 103)

  Photo 20. A pair of Waved Albatrosses in a breeding colony on Española, Galápagos, perform an elaborate bill-clashing bond reinforcement ceremony as one returns from a fishing expedition. Their sole chick is the object of intensive care. (Page 133)

  Photo 21. Cocha Salvador, deep in the primary lowland rainforest of the Manu Reserved Zone in southern Peruvian Amazonia, is an oxbow lake, formed from a cut-off bend in the Manu River. In a morning there, we saw four birds that are the only members of their Families. (Page 134)

  Photo 22. The Limpkin, here at Cocha Salvador, is one such bird, a single-species Family. It is not at all related to the ibises, which it superficially resembles. Its calls have been used in movies from Tarzan to Harry Potter! (Page 137)

  Photo 23. Hoatzin, Tambopata National Reserve, southern Peruvian Amazonia. This remarkable bird is the only member of an entire Order, one of only two such birds in the world. It is also the only bird that has evolved colonies of gut bacteria to digest cellulose. (Page 138)