Birds in Their Habitats
Page 24
Then, in 1901, the Duke of York (later to become Britain’s King George V) visited New Zealand and at Rotorua the local Maori presented him with a Huia feather as a sign of respect: he put it in his hatband and at that moment the fate of the spectacular bird was sealed. ‘Huia plumes were reduced from sacred treasure to fashion accessory’ (Szabo 1993). The price of an individual feather was £1, so a single bird was worth £12 – at a time when a labourer could expect to earn between £2–3 a week. Organised shooting parties went out and shot every Huia they could find. Around road and rail construction camps hundreds of birds were killed. As the birds got ever scarcer, prices for a single feather rose to £5. Moreover, rather than spur the desperate rescue efforts that we might hope to see today, the attitude seemed increasingly to be one of ‘it’s doomed anyway, so we might as well make what we can out of it’. The last confirmed sighting of the bird was in 1907, just 6 years after the duke’s fatal gesture. This was effectively a planned extermination, abetted by government: islands were set aside as reserves, but no birds reached them. Indeed, a pair destined to be the first were diverted as a gift (dead of course) to Lord Rothschild in England (Szabo 1993; Fuller 2002; Higgins et al. 2006).
Most extinctions don’t happen like that, though in past years a lack of concern was usually a hallmark. The cause (or causes, there’s rarely just one) was clear, but that isn’t always the case either. I alluded earlier to two emu species that have become extinct since European settlement. One was from Kangaroo Island, off the coast of South Australia, the other was from King Island in Bass Strait between the mainland and Tasmania. Both island emu forms were regarded as ‘dwarf’, with the Bass Strait birds being smaller (about half the size of modern mainland Emus). Both were reported to be very common, but barely survived the initial contact with Europeans – in both cases represented initially by whalers and sealers keen to supplement their meat supply. The great French scientific expedition led by Nicolas Baudin visited both islands in 1802, and captured live birds from each. Unfortunately, the science was a bit rubbery on those occasions and proper records were not kept of which came from where. He took on board an unspecified number (apparently four or five) at King Island, and two from Kangaroo Island, but they were allowed to mingle; only two reached France, where they lived for another 19 years to be perhaps the last survivors of their kind (West-Sooby 2013).
In a sense it is immaterial to the story, but the confusion thus engendered is worth mentioning. It was originally assumed that all the birds that arrived alive in France came from Kangaroo Island. When someone finally got round to studying the skeletons of the two birds, it transpired that they were different from each other, and the specimen on which the Kangaroo Island Emu description was based (a skeleton in the Paris Musée d’Histoire Naturelle), was most likely from King Island, so things got tricky. It fell to Shane Parker of the South Australian Museum to write a new description of the species (as he asserted that it was) based on subfossil leg bones from the island: he called it Dromaius baudinianus to honour Nicolas Baudin (Parker 1984). Today there is far from unanimity on the species status of the King and Kangaroo Island Emus: the Australian Government regards them as separate species (Department of the Environment 2017a, 2017b), while a mitochondrial DNA study of King Island Emu subfossil remains concluded that they fell well within the variation expected from mainland birds (Heupink et al. 2011). Perhaps it doesn’t matter nearly as much as the fact that these magnificent animals – the products of at least tens of thousands of years of adapting and evolving in isolation – were crushed from existence and wiped from the record virtually overnight, and we can’t even say why.
Astonishingly the King Island Emus were gone from the planet by 1805, just 3 years after Baudin reported them in abundance. Certainly the whalers were hunting them with dogs, but King Island is 60 km long and over 20 km wide and it seems unlikely that this alone could have resulted in extinction in such a short time. Even more surprising is the total loss of the Kangaroo Island Emus: it took a little longer, but they had gone by 1827. My surprise here is based on the size of Kangaroo Island, which is 145 km long and 45 km wide, and heavily vegetated. It simply seems too hard to accept that hunting alone could have achieved their complete destruction in that time, especially bearing in mind that kangaroos were also being slaughtered there by the thousands and are still common to this day. Perhaps the most likely explanation lies in a greatly increased burning regime, which could well have been fatal to a ground-nesting bird. We will simply never know: all we can do is wonder and mourn.
Perhaps it’s time to think of things less grim (though we can’t, and shouldn’t, ever fully forget what has been and is being done to the world by our cupidity, arrogance and ignorance – always a toxic brew).
High Veld, Wakkerstroom, South Africa: Widowbirds
It was a remarkable performance, and one that required some suspension of disbelief to absorb. A black streamer fluttered slowly above the grasslands: it took a bit of effort to see it as a bird. My diary at the time described them as resembling ‘overdecorated runaway toy kites’. The Long-tailed Widowbird (yes, a male widow) flapped slowly, showing off his glowing red and white shoulder ‘epaulets’, but the real show-stopper was the tail: the streamer that first caught our attention. He is not much bigger than a House Sparrow, with a wing span of some 15 cm – but the tail is close to half a metre long! The long feathers hang loosely, forming vertically flattened pennants dangling down behind him.
Wakkerstroom, a busy little village some 230 km south-east of Johannesburg, lies in the vast flat expanse of the high veld: that great grassy plateau in east-central South Africa. It is nearly 1800 m above sea level, which means winter can be tough (freezing at night and still cold during the day; frost and snow are common). Wakkerstroom started out as a 19th-century service centre for the local agricultural communities. (Actually it started out as Uysenburg and went through a couple of name changes before settling on Wakkerstroom in 1904; it was an attempt to acknowledge the Zulu name of the local river – but translated into Afrikaans!) Today it makes a great deal of its living by catering to the birdwatchers who flock to it, appropriately, to enjoy the rich and special birdlife of the grasslands and wetlands that surround the town, including several threatened species. Birdlife International (a worldwide association of bird conservation organisations) has designated the Wakkerstroom area as an Important Bird and Biodiversity Area, meaning that it has significance above that of surrounding areas, as defined by specific criteria. Many houses in town have been bought and renovated by birding folk, often to be rented out as accommodation for visiting birders. There are at least two birding shops in town, plus a Birding South Africa place just out of it, which also has a shop, as well as its primary purpose of running a training school, whose graduates now work as birding guides in Wakkerstroom.
Tall tails and true
Widowbirds form a pretty dramatic group within the large African and southern Asian Family of weavers, named for their superb woven grass nests, usually domes with side openings, often built in colonies. In most species, the male is the weaver, and his skill in the art is judged by the females: only if his nest meets her exacting requirements will he get to mate with her. Male Long-tailed Widowbirds take a quite different approach, however. He holds a territory, within which he perfunctorily twists a few blades of grass together, all that is left of his ancestors’ nest-weaving prowess. Instead of providing a nest, he competes with the neighbouring males for the females’ attention by means of his amazing display flights, though he will also display from a perch, where his tail is equally visible (Craig 2017). One might think that a skilfully constructed nest might be of more value to a female widowbird than a spectacular tail, but apparently she doesn’t see it that way. Having fallen for ‘the male with the tail’, she builds her own nest in his territory, along with up to four others similarly infatuated.
Thanks to some clever experiments (albeit a bit tough on some of the males!) in the early 1980
s, we know that females make their choice as to whose territory to nest in simply on the length of his tail! In that experiment, territory-holding males were caught, and some had a length of their tail removed and taped onto that of another male. The birds were tested in groups of four – one with a short tail, one with an even more extended one, and two with the original length tail, though one of those had his tail snipped and then reattached, just in case that process had an effect. The test was to compare how many nests were added to each territory after the procedure. Unsurprisingly, the short-tailed males did worst, but less expected was the finding that the males with ridiculously long tails were a great hit with the females, even though they’d never encountered such a phenomenon before! (Andersson 1982). (You’ll be pleased to know that at the end of breeding season he moults into a drabber short-tailed version of his breeding glory, so that next season the tail-cropped males were back to their normal gorgeous and popular selves.)
There has been intense debate on the subject of ‘sexual selection’ since Charles Darwin pondered it, observing that some males carry such extreme ornamentation that it must be detrimental to their very survival. It wasn’t until Malte Andersson’s experiments on the widowbirds, however, that someone actually sought experimental evidence that females did in fact prefer such over-the-top expressions of machismo. In the case of the widowbirds, the large tail could have been to intimidate rival males, but though he will chase angrily an intruder, the tail isn’t spread during that pursuit.
It seems that in showing off his ridiculously exaggerated nether appendage he is saying something along the lines of ‘see how I can survive despite carrying this burden around behind me – there’s your proof that I have great survival genes that would be perfect for your chicks’.
We can hardly discuss interesting tails and sexual selection without mentioning peacocks – technically either the Indian (Blue) Peafowl or the Green Peafowl of South-East Asia, but we can afford to follow familiar usage here and continue to refer generally to peacocks. (There is also the little-known Congo Peafowl, but it doesn’t have a particularly impressive tail and the following material refers to the Asian species.) The peacock’s ‘tail’ is actually not really a tail at all, but comprises enormously extended tail coverts – the usually short feathers that cover the base of a bird’s tail above and below – for which the term ‘train’ is generally used. In a twist on the normal state of affairs, the actual tail feathers merely support a peacock’s coverts. Although Malte Andersson and colleagues pioneered the experimental study of sexual selection with his Long-tailed Widowbird work, others have carried it on with studies on peacocks (mostly on captive or feral birds).
Marion Petrie and colleagues studied a feral population in southern England. They found that females inspected a series of males, and invariably mated with the one with the most eye-spots in the train (Petrie et al. 1991). Just to make sure it was the eye-spots that the females were basing their judgement on (as opposed to some other factor correlated with elaborate trains), they snipped some spots from the train of successful males during winter, and next spring they were significantly less successful (Petrie and Halliday 1994). The researchers were also able to do what Andersson could not with wild birds: they caged eight males whose mating success over the past two seasons was known, and randomly allocated four females to each. Even the females that drew the short straw in males mated rather than produce no chicks at all, but they weren’t going to give their all for a bloke they knew was below par – after all, they only had to check out his tail. Those finding themselves with proven successful males laid more eggs than their less fortunate sisters (Petrie and Williams 1993).
But were all these picky females right in their judgement? It would seem that they were. One winter, two foxes managed to penetrate the perimeter fence and kill five peacock males; while only a third of the males had failed to achieve any matings the previous year, four of the five deaths were in this group. The numbers are small, but statistically significant, and highly suggestive (Petrie 1992). Those that fail to impress females are either less fit than others, or lack some survival skills – either way, they are not the best choice as father of your chicks, and females know that. They are right about that too: the eggs from the experiment above were taken and hatched under identical conditions in an incubator, and the chicks released into the wild situation at an appropriate time. Offspring of the successful males (the ones the peahens preferred) survived better than those of the also-ran males (Petrie 1994).
A longer tail is an impediment in the wild because it is more liable to be seized by a predator, and is heavier to drag around. As with the widowbirds, the male peacock is proclaiming his superior fitness and survival skills with his ability to overcome the handicap.
Is he telling the truth though in so proclaiming his superiority? Again, experimental evidence comes down on his side. A French team led by Adeline Loyau went straight to the internal health of peacocks by checking heterophil levels in the blood (these are cells which are produced in response to infections). The study found that peacocks with the most eyespots in the tail and the most displays per hour had the lowest heterophil levels. Conversely, if they injected a successful bird with lipopolysaccharides, or endotoxins, which in the blood produce a strong immune response called endotoxemia, his display rate fell. But, males with more eyespots were better able to resist such attacks and to continue to display at a relatively high rate (Loyau et al. 2005)!
(I should acknowledge that, more recently, a Japanese study has sought to dismiss all previous studies, including the ones cited above, because their own study, reported in a provocatively titled paper that directly challenges the title of Petrie et al. (1991), didn’t show such results (Takahashi et al. 2008). However, as some of the previous authors have noted in a reply (Loyau et al. 2008), all this shows is that the birds in that particular study seem to have responded differently for reasons unknown, not that all the previous studies must be mistaken. Scientific disagreement is rarely dull!)
Another excellent example of a tail hugely modified into a display tool is that of the Superb Lyrebird: a large, very primitive songbird of south-eastern Australian wet forests. (The closely related Albert’s Lyrebird of the temperate rainforests of the New South Wales– Queensland border ranges has a similar, but less extreme, rear adornment.) The male’s 16 tail feathers are like those of no other bird’s tail. The outer two are long and broad, curving outward into S-shapes with broad black tips and semi-transparent chestnut bars along their length. Twelve inner feathers are filmy and lack barbs, so they are loose and open, grey above but silvery below. Two central ones are long and stiffly quill-like. Normally they are all carried in a long sheath-shape behind him, but, when he is displaying, the whole structure is thrown forwards upside down over his head, displaying the shimmering silver underside. At no point is the tail carried so that the big outer two feathers form a lyre-shape (which gives the birds their name): that was the artefact of a British taxidermist, which been perpetuated by artists.
The evolution of the tail
Archaeopteryx and other early dinobirds unsurprisingly had long, floppy feathered reptilian tails with up to 23 articulated bones. These were good for steering when running across the ground, but not so good in the air. Moreover, they would have been heavy. It is then also unsurprising that, in a relatively short period of time, the tail shortened dramatically, both by a reduction in the number of bones, and by the shortening of individual bones. A chook, for instance, has just five short tail bones and a pygostyle: a short rod comprised of a few bones fused together. Along with associated fat and muscle, it forms the uropygium, which on the (invaluable, for anatomy) roast chook appears as the ‘parson’s nose’. This supports a stiff fan-shaped tail of strong feathers (the remiges) all arising from the same point, with huge aerodynamic advantages over the floppy ancestral tail. The oldest known ‘modern’ bird tail belonged to Hongshanornis longicresta: a small bird from 125 million year old Chinese d
eposits (Chiappe et al. 2014). The tail has a key function in efficient flight, but, as long as this is not compromised, it can be used for other purposes or, perhaps more accurately, some compromise is permissible, as with the widowbirds and peacocks, if the purpose is sufficiently important. Lyrebirds can scarcely fly at all, being content to glide downhill and walk up again afterwards.
Some nightjars are also given to extreme caudal exhibitionism. The extraordinary Lyre-tailed Nightjar of the tropical Andes has a pair of outer tail feathers up to 80 cm long. Groups of males gather to flaunt their tails to prospective mates by flicking them provocatively while hovering above the ground (Cleere and Kirwan 2017).
Tails have been put to other purposes too. Groups as disparate as woodpeckers, Northern Hemisphere creepers, South American woodcreepers and some swifts have evolved stiff pointed shafts that are used as props while clinging to a vertical surface – extracting prey from bark crevices for most, or clinging to a cliff while nesting in the case of the spine-tailed swifts. Here, the vane (the ‘feathery bit’) simply ends before the tip of the shaft.
Australian Brushturkeys (big forest-dwelling mound-dwellers of eastern Australia) have their tail feathers arranged, or least held, to form a vertical fan, presumably for steering when running through dense vegetation. Willie Wagtails (a common and familiar member of the fantail Family throughout the whole of mainland Australia) hunt mostly on the ground and constantly flick their tail from side to side to scare up insects from the grass.