Birds in Their Habitats

by Ian Fraser

  Norell MA, Clark JM, Chiappe LM, Dashzeveg D (1995) A nesting dinosaur. Nature 378, 774–776. doi:10.1038/378774a0

  Parmesan C, Yohe G (2003) A globally coherent fingerprint of climate change impacts across natural systems. Nature 421, 37–42. doi:10.1038/nature01286

  Pizzey D, Doyle R (1980) A Field Guide to the Birds of Australia. Collins, Sydney.

  Potvin DA, Parris KM, Mulder AM (2011) Geographically pervasive effects of urban noise on frequency and syllable rate of songs and calls in Silvereyes (Zosterops lateralis). Proceedings of the Royal Society B: Biological Sciences 278(1717), 2464–2469.

  Reinberger S (2013) Birds that go wild for the city. MaxPlankResearch 1(13), 72–79.

  Russell A (1953) Murray Walkabout. Melbourne University Press, Melbourne.

  Tanaka K, Zelenitsky DK, Therrien F (2015) Eggshell porosity provides insight on evolution of nesting in dinosaurs. PLoS One 10(11), e0142829. doi:10.1371/journal.pone.0142829

  Woinarski J, Bonan A (2017) Pardalotes (Pardalotidae). In Handbook of the Birds of the World Alive. (Eds J del Hoyo, A Elliott, J Sargatal, DA Christie and E de Juana). Lynx Edicions, Barcelona, Spain, .

  Woodall PF (2017) Kingfishers (Alcedinidae). In Handbook of the Birds of the World Alive. (Eds J del Hoyo, A Elliott, J Sargatal, DA Christie and E de Juana). Lynx Edicions, Barcelona, Spain, .

  World Wide Words (2001) Stool pigeon. World Wide Words website, .

  Xu X, Norell MA (2004) A new troodontid dinosaur from China with avian-like sleeping posture. Nature 431, 838–841. doi:10.1038/nature02898

  7

  Woodlands and grasslands

  Great Western Woodlands, Western Australia: Emu family

  They materialised by the road east of Hyden in the southern inland of Western Australia when we were almost alongside them. The big male Emu watched us carefully as we pulled over, while the youngsters seemed largely heedless of any potential hazard – I guess they were used to taking their cues from their father. Stretching up to appear more intimidating, he stood most of 2 m high, his sooty brown body somewhat reminiscent of a haystack of loose shaggy feathers, and the short black fluff of head and neck framed a large patch of blue skin around his ears and down his neck. The dozen largish chicks, a couple of months old, were marked with creamy stripes running down the neck and along the body, like a flock of big bumblebees. They hovered at the edge of the Gimlet Gums that reared with oddly fluted glowing coppery trunks from the understorey of scattered shrubs and grasses.

  These Great Western Woodlands of the semi-arid Goldfields district of Western Australia represent the largest extent of Mediterranean climate woodland in the world that remain in their original state (Mediterranean implying a hot dry summer and winter rainfall). They cover 16 million hectares: an area greater than that of England and Wales combined. The apparently – and understandably – mandatory drive for Australians, 4000 km ‘across the Nullarbor’ from the east coast to Perth on the south-west coast, passes through these woodlands, but a quieter 300 km journey along the well-maintained Granite and Woodlands Discovery Track between Hyden and Norseman is a more rewarding way to see them (see Photo 27).

  The term ‘woodland’ is used loosely in day to day conversation and, even more confusingly, is employed differently in various parts of the world, but the essence of the concept in most places is a treed habitat where the trees are more sparsely scattered than in a forest; that is how I apply it in this chapter. In Australia the definition generally specifies that the total tree canopy from above covers between 10% and 30% of the ground area, meaning that the individual canopies scarcely touch, if at all; if more than that it is a forest. In Britain, however, ‘woodland’ is broadly used to describe any natural habitat dominated by trees, while ‘forest’ implies a managed plantation. There are other important implications of this scattered nature of the woodland tree cover too, the chief one being that much more sunlight hits the ground than in a shady forest, so that the understorey is quite different. Most grasses like to grow in full sun and a woodland understorey comprises more grass and fewer shrubs than a forest (bearing in mind that all these habitat concepts are better thought of as a continuum rather than discrete separate entities). As conditions change, so that tree growth becomes less and less supportable (perhaps because of decreasing rainfall or temperature, or changes in soil type, such as to cracking black soil plains whose constant expansion and contraction rips roots apart) we are left with just the woodland understorey: a grassland with few or no trees.

  I tried waving a handkerchief out the side window, which can often attract curious Emus to come close to investigate (a curiosity which in times past was exploited in various ways by Indigenous hunters), but, as they started to move cautiously towards us, another vehicle roared past in a cloud of dust, seemingly oblivious to the birds, and the Emus fled. He jumped and whirled in the air, the haystack morphing into athletic dancer, pounding down the road verge seemingly powered by little puffs of dust with each stride. The chicks followed as he eventually swerved off into the trees: it often seems to take Emus a while to work out that they can actually leave the road.

  Ratites, and the mysterious case of the flying elephant birds

  The ratites, as traditionally defined, are quintessentially ancient Gondwanans, including all the great flightless birds of the southern continents – two species of ostrich in Africa, the Emu and three cassowaries in Australia–New Guinea and three (or perhaps just two) rheas in South America. They also include the five New Zealand kiwi species, which, of course, are much smaller. In addition, there are several recently extinct species (i.e. in the last few centuries, coinciding with human arrivals) comprising perhaps four, and possibly up to seven or eight, species of mighty Madagascan elephant birds, two emus (though here, as with the elephant birds, taxonomy is uncertain) and nine New Zealand moas (see Photo 28).

  The most fundamental division of living birds is not, as one might suppose, into passerines and non-passerines (see page 47), but into the Palaeognaths and Neognaths. These reflect palate characteristics, and the division has long been recognised from anatomical studies: an analysis more recently confirmed by biochemical and genetic studies. The ‘ancient palates’ have generally been understood to comprise the ratites, plus an associated ‘sister group’, the South American tinamous (see page 42): 47 species of smaller ground-dwelling birds that fly, albeit weakly. The conventional wisdom has been that the ratites lost their flight back in the hazy mirages of Gondwanan time, and that the flightless ancestors of modern ratites were carried across the Southern Hemisphere with the fracturing of Gondwana into rafting continents. Along with the flightless rhea ancestors to South America went the cousins, the still-flying tinamous. Knowledge of the history of the component continents of Gondwana (in particular, the order in which they broke away from each other) and logic tell us that ostriches and elephant birds should be the oldest members of the group and be most closely related to each other. Likewise, moas and kiwis, stranded together as New Zealand floated away, must surely be each other’s nearest relatives. Moreover, although the tinamous are undoubtedly related to the ratites, they can’t actually be ratites because that would imply that they had somehow regained flight after being isolated in South America, and it is universally agreed that such a reversal of evolution is simply impossible (too many separate adaptations to a radically new lifestyle for all to be wound back in synch, for a start).

  But … again, that ‘but’ to send shivers down the spine of those of us fond of a neat story. The story I’ve just summarised, and which I’ve told in good faith many times over the years, seemed to explain all that we observe about the ratites, and to encapsulate the role of Gondwana in explaining what we see in the Southern Hemisphere. Except that now it seems not to do so after all … There have for some time been mutterings of unease about the narrative, not least because, although it is agreed that the various
far-flung outliers of the palaeognaths are indeed old, surely they can’t be that old? And sure enough, in the past decade a series of studies using emerging techniques have completely disassembled the story and rebuilt it from scratch, based on the apparently incontrovertible new information that seems to accumulate by the year.

  Which DNA to analyse?

  Cell mitochondria contain much less DNA than does the cell nucleus, so it is relatively easy (but still not that easy!) to obtain a complete mitochondrial analysis. Moreover, animal mitochondrial DNA evolves more rapidly than does that of the nucleus. These two observations mean that doing complete mitochondrial analyses to compare species is now a standard and powerful technique for unravelling not only relationships, but the time since their ‘most recent common ancestor’ stalked the Earth: indeed the acronym MRCA is now in widespread use in the literature without perceived need of clarification. Such analyses have rapidly become almost standard practice and have been applied to a huge array of organisms – just try entering ‘complete mitochondrial analysis’ into your search engine of choice. Moreover, as one of the key papers of this ratite evolution revolution notes, ‘ancient DNA is now a respectable and thriving industry’ (Phillips et al. 2010). DNA can be retrieved from fossil material (as indeed any aficionado of Jurassic Park already knew!) so that extinct pieces of the puzzle, such as elephant birds and moas, can now be fitted into place. And as it has turned out, they complete a most unexpected picture.

  For instance, it emerges that the tinamous, rather than being convenient ‘outliers’ to the main ratite line, are right in the middle of it – oops. The ‘oops’ is because this means that the common ancestor they all share was either flightless, which is the conventional wisdom, and the tinamous, against all conceivable probabilities, regained flight (and no-one believes that), or it flew and the others all then lost their flight independently!

  It gets worse, though, from the viewpoint of those wishing to cling to that conventional wisdom: the closest relatives of tinamous are not rheas at all, but … New Zealand moas! (To be honest, others had suggested this previously, without being able to explain the distributions.) Moreover, kiwis and Madagascan elephant birds are each other’s nearest and dearest; this was new. (I’ve given up on the exclamation marks now – it all seems a bit Alice in Wonderland really.) A series of papers has developed this theme, naturally with a lot more detail and sophistication than I am reflecting (e.g. Harshman et al. 2008; Phillips et al. 2010; Allentoft and Rawlence 2012; Baker et al. 2014; Mitchell et al. 2014).

  I confess that, when I first came across this, my reaction was that there must have been some mistake. I’m a firm believer in the parsimony principle: that the simplest evidence-based explanation is always likely to be the correct one. The more evolutionary steps that are required to explain something, the less likely it is that it happened that way. You see, the initially somewhat hallucinogenic implication of all this is that at least some of the ancestral rheas, tinamous, kiwis and elephant birds flew in to their current abodes, and independently subsequently all (except the kiwis and tinamous) grew hugely in stature – they couldn’t have flown at their current size – and lost their ability to fly. (Or it could have been the other way round: that is, they could have lost their powers of flight and then grew.)

  In fact, when the analysis was done of the time since the lines separated, and that was compared with the timing of the breakup of Gondwana, it emerges that all of them must have flown in, because each pair separated well after their current isolated continent was last connected to others. For instance, kiwis and emus/cassowaries only parted ways some 60 million years ago, but New Zealand had become isolated 20 million years before that. (Phillips and colleagues, who did this study, didn’t have the reliable elephant bird material that was available to Mitchell’s team just 4 years later, so couldn’t make the elephant bird–kiwi connection.) Moas and tinamous separated at about the same time (i.e. 60 million years ago). Emus and cassowaries diverged only ∼20 million years ago, but were already on the same continent.

  Finally, in terms of shocks, it emerges that ratites might not even be Gondwanan in origin, though that is still uncertain. Ostriches seem to have arisen as a separate line in the earliest ratite days, but whether in southern Gondwana (where the ancestor of all the rest apparently dwelled), or actually in Eurasia where they later lived, is unclear. Certainly, it appears from the fossil record that ostriches may only have turned up in Africa as recently as 23 million years ago.

  So how do the authors go about justifying the apparently impossible coincidence of all these birds independently abandoning their wings at about the same time and getting very big indeed? What could possibly have happened across the Southern Hemisphere to explain such a seemingly uber-implausible set of events? Well, something pretty cataclysmic actually did happen about then – a massive meteorite smashed into the Earth on what is now Mexico’s Yucatán Peninsula 65 million years ago and, in the ensuing deep chill as the sun was blocked out for years by the dense blanket of dust and smoke, three-quarters of all animal and plant species on Earth perished. It was a horrific time as life chilled and starved, and was burnt by widespread acid rains. Among those extinguished were all the dinosaurs, that mighty dynasty that had dominated the planet for 160 million years; well, not quite all, of course, because the bird-dinosaurs survived and thrived in the suddenly empty landscape.

  Among the niches left vacant was that formerly occupied by the birds’ immediate ancestors: the fast, erect bipedal dromaeosaurs. We know that modern birds are very prone to evolving to flightlessness in situations where there is no reason to go on spending so extravagantly to maintain an aerial lifestyle (especially on islands; see page 69). Perhaps then it is not so implausible, in such a different empty world where opportunities were many and predators were few, that a series of related birds should do so in isolation from each other, having increasingly relied on their legs, rather than their wings, which were far more expensive to fuel. Presumably too they still carried the genes of their bipedal running ancestors. Freed of the weight restriction imposed by flight, they could then grow increasingly bigger to tower over such predators as were also evolving. Phillips et al. (2010) point to the analogy of Australasian Swamphens flying (or being blown) from Australia to New Zealand in relatively recent times, and in that mammal-free Nirvana losing the use of their wings to evolve into the big flightless Takahe.

  Maybe it’s time to come back to now and the Emus, which really don’t care much about ancestry.com.

  Feathers: a bird’s best friends

  I mentioned the curiously ‘haystackish’ appearance of the Emus, whose plumage is more reminiscent of an untrimmed shaggy dog.

  Feathers are critical to a bird: they help determine where it lives and how it lives. A measure of their significance is that, despite their proverbial lightness, and the fundamentally critical importance of shedding every gram of surplus weight in the interests of flight, a flying bird’s feathers weigh two to three times as much as its skeleton.

  A feather is formed of keratin, which is unsurprising given that it is the protein that forms reptile scales, from which feathers evolved. There are two major feather types: vaned feathers and down feathers. Vaned feathers are essentially all the visible ones, flight and tail feathers, and all the body-covering contour feathers. They consist of a solid shaft, or rachis, arising from a hollow shaft, or calamus, which is embedded in the skin. From the rachis extends the vane (i.e. what we would probably think of as the feather), comprising two densely packed opposite rows of barbs. In most visible feathers, these barbs are locked to the ones alongside by lines of tiny hooked barbules, rather like velcro. Birds spend hours a day preening, meticulously running every feather through their beak to reset the barbules, making sure they’re properly zipped up so they can perform their critical functions of flight, waterproofing and insulation.

  At the base of many feathers is a short non-zippable woolly aftershaft (or hyporachis, if you’re
trying to impress someone), like another little feather growing from the base. Its function is presumably insulation, though not all birds have them. Based on the bird groups that do and don’t have them, it seems to be a primitive characteristic, with passerines (the most recent Order of birds to arise) mostly lacking aftershafts. At the other extreme, in our ancient roadside Emus and the closely related rainforest cassowaries, the aftershaft is as long as the main shaft, so each feather appears duplicated. Other ratites don’t share the extended aftershaft, but all lack barbules so the feathers don’t lock together, hence the loose shaggy or even woolly appearance.

  Other modern birds have an array of other feather types too, some of them very specialised. The other common feather type, as mentioned above, is the down feather: the soft fluffy doona that lies beneath the vaned feathers and keeps the bird warm. Chicks hatch with only down, with the vaned feathers growing through later. Down feathers have a very short rachis, or none at all, and no barbules to zip the fluffy barbs together. Of course, it is not strictly the feather itself that insulates but the layer of air it traps – like double glazing. A bird can increase insulation by means of tiny muscles just under the skin that can raise feathers – ‘fluffing up the doona’ – or can decrease it in hot weather by flattening the feathers against the body to squeeze out the air and increase conduction of heat away. We have all seen birds on a cold day looking twice their normal size, with feathers puffed out all over them.

  Bristles around the eyes of some species, especially insect-eaters, consist of just a thin shaft. They are thought to be sensory, but probably also play a role in protecting the eyes from struggling prey that are understandably less than enthusiastic about being eaten. It seems that they play no role in actual prey capture, though this has also been proposed (Lederer 1972). Filoplumes are also single shafts, but with lots of sensors at the tip. They are scattered through the plumage, but especially among the flight feathers, and apparently convey information to the brain about feather movement so that adjustments can be made. These remarkable little structures can be seen as the ‘hairs’ on the body of a plucked chook.