5
Some species, such as kookaburras and many parrots, do not build nests at all but rely on natural structures such as tree holes (Heinsohn et al. 2003), or on other cavities as some finches do (Brazill-Boast et al. 2010). Pardalotes and rainbow bee-eaters tend to dig long horizontal tunnels into the side of an embankment rather than construct a nest. Finally, there are some species, such as lapwings, that do not build nests at all (HANZAB 1993), but nest on the ground making minimal adjustments. Yet others prefer not to build nests themselves but seek abandoned nest sites (Liker and Székely 1999).
However, birds handle materials all the time. Whether they use their beaks to make nests, stab or hold a potential meal, manipulate food in the beak or even ‘mouth’ each other as part of courtship or appeasement, the beak and feet are used in various tasks to achieve certain outcomes. Most birds build some form of nest and do so efficiently, often producing amazingly complex structures. Orioles have suspended nests that are secured at the top. The tiny Queensland yellow-bellied sunbird also builds a suspended nest complete with roof and front shade over the entrance. Its shape is interesting (see Fig. 5.1). Pittas, pheasant coucals and lyrebirds tend to build bulky nests near or on the ground that are partly domed and may even have a side entrance or a forecourt. One can generally recognise a nest because it has a typical species-specific design, shape and structure (Hansell and Overhill 2000).
Fig. 5.1. Nest adaptation or incidental design similarity? (Left) The typical nest of a sunbird; (right) the larval casing of a bagworm moth (Lepidoptera), which is almost exactly the same size, colour and texture as the nest of the sunbird. It is possible that this incidental or actual camouflage may give sunbird nests some protection by being mistaken for the unattractive bagworm moth casing.
Apostlebirds, white-winged choughs, swallows and martins, as well as willie wagtails and magpie-larks, build nests that are partially or entirely pasted into a firm structure by using mud and, in the case of swifts also saliva or by adding unusual materials such as spider webs (Fig. 5.2).
Some species have never become good nest builders: for instance, the tawny frogmouth and the crested pigeon as well as some other doves and pigeons (HANZAB 1996). Both, entirely unrelated species, use a few sticks and put these on a fork, resulting in very flimsy constructions to the point that quite often eggs tend to crash to the ground or nestlings fall out (Kaplan 2007a) and are doomed unless some human passer-by happens to see them before predators do. Among the largest number of orphaned nestlings and juveniles are tawny frogmouths rescued every year with great predictability. The largest ‘nests’, reused as often as possible and often shared by several females, are the mounds of the brush turkey. These incubating structures may be 10 m across and occasionally even as high as 5 m, consisting of loose leaf-litter and other vegetation. Hence, there are gradations in skill and complexity in nest building.
It is not common to place nest building or building of other structures under tool use, although there are a few exceptions (e.g. the bowers of bowerbirds, discussed below). Nest building rarely rates a mention in cognitive science because it is widespread, usually successful and it tends to be stereotyped (Harrison 1975): hence it is not usually considered a complex cognitive task. However, this view is being revised and now falls under the description of ‘physical cognition’ (Bailey et al. 2014). An example of this might be the nest building skill of a willie wagtail. Its nest is firmly wrapped in spider webs (Fig. 5.2). To obtain the spider web and use it effectively for nest building requires some skill. I have watched willie wagtails take part of a spider web off its fixture during hovering flight. When it has a firm hold of the web in its beak, it flies backwards so that the sticky web does not get entangled in its feathers. It then flies backwards further and specifically against the direction of the wind so that the web flutters by the bird’s side away from its body and it then collects the loose end with one foot (it seems to prefer the left foot) and carries its cargo safely to the nest sight. It is easy to see that, without considerable skill, this fragile and sticky material, although strong, could easily end up in tatters if not manoeuvred expertly.
Indeed, there is plenty of evidence that learning is required to master the skills necessary for nest construction, a point driven home when one watches the clumsy attempts of some juveniles in building nests.
Some very specific examples of nest building may in fact be well explained under the rubric of tool use, innovation and even creativity. Birds are adept at using a variety of materials (sticks, spider webs, mud, down feathers). There are many other challenges that face a species, demanding problem solving or abandoning old practices in order to survive if the environment changes. For instance, birds often do not find a suitable site for nesting. As constant decline of flora needed for bird survival continues almost unabated, there is the question whether birds can be innovative and try to find alternatives. We know that many birds of prey, within Australia and elsewhere in the world, have started to colonise urban spaces: nesting on top of bridges, on high-rise buildings and in eaves, and other public places. Making use of such alternatives is innovative.
Fig. 5.2. Willie wagtails use spider webs to make their nests firm and protect them from disintegration due to wind and weather. The web holds the nest together as well as offering some water resistance. (Photo by John Lang, CSIRO Science Image Archive.)
Innovations in nesting locations
Nesting innovations are not as rare as might have been supposed. Nicolakakis and Lefebvre (2000) surveyed just the north-western part of Europe, including publications of a 30-year period, and found 176 nesting innovations. Unfortunately, no similar review is available in Australia, but, given the substantially larger number of species found in Australia compared with north-western Europe, one could speculate that nesting construction and innovations in nesting locations and design might be more widespread than is currently documented.
There is an extensive literature on nesting sites concerning logging and other disturbances such as fire and agriculture, and these are important studies to monitor effects on bird numbers, but they are not designed to tell us anything about innovation. It is not clear whether nesting innovations have increased over time as a function of human encroachments of various kinds – such as removing trees, expanding agriculture, housing, transport or manufacture – and whether innovation therefore has become crucial.
Nicolakakis and Lefebvre (2000) found that feeding innovations and forebrain size were correlated, while nesting innovation was not. Perhaps this is no surprise at all, since the large group of parrots with their big brains was not part of the sample. Parrots and cockatoos rely on nest holes and thus, like the kookaburras, do not build their own nests. Yet almost all these species, especially cockatoos, have developed astounding beak and foot coordination and can manipulate food and even the smallest seeds with surprising agility and accuracy. Glossy black-cockatoos although endowed with massive beaks, for example, manage to husk the tiny Allocasuarina (she-oak) cones. They may have fed on these for millions of years because some related species on which they feed today have been found to date back to Gondwana (Pepper et al. 2000). Many of the parrots, but chiefly cockatoos, are now out on a limb, literally, because their reliance on large available nest holes makes them vulnerable to changes in the landscape, especially to loss of old native trees. Indeed, availability of nest holes has been declining and competition for remaining nest hollows has become a theme of great importance (Heinsohn et al. 2003). Sulphur-crested cockatoos tend to displace eclectus parrots from nesting hollows and pink cockatoos have such specific requirements that they prefer to go back to the same nest (called nest faithfulness), which exposes them to poachers or they miss out on breeding altogether (Rowley and Chapman 1991). The most extreme form of nest competition is perhaps found in the eclectus parrot. In their 4-year study of this species in the Iron Range National Park on Cape York Peninsula, Heinsohn et al. (2003) found that females had to begin occupying a nest hole many months before breeding commenced to secure a site. Indeed, females had to spend up to 9 months in semi-breeding confinement, just to ensure that they had a chance to lay eggs and rear a clutch and, when they do, monsoonal rains may waterlog their nest hole and spoil the eggs (Heinsohn et al. 2003).
The palm cockatoo is an exception, although competition for a suitable nest hole may not be less fierce. Palm cockatoos place sticks and bark within their tree hollow – these are not just whimsical bits thrown about, but are fashioned very precisely to suit their purposes and the width of the hollow. They collect sticks, then defoliate them and remove any bark or small side branches and then break each stick to the exact size to fit the nest. In order to do so, they hold a stick in one foot and use the beak like a saw, rolling the stick backwards and forward in their beak and, with substantial force applied to the stick (up to 2 cm in diameter), they eventually succeed in obtaining the exact size needed. Once sticks are the correct size, the male hands them over to the female, which splits them and thus provides a flat edge on top. The bottom of the nest is then fashioned into a several-layered platform and lined with some bark. It is thought that the structure provides a sieve through which excrement can fall and also protects eggs and hatchlings from water gathered at the bottom of the hollow should rain enter (HANZAB 1999) before the wet season begins in earnest. In the context of palm cockatoo’s elaborate tool use described in the previous chapter, these abilities are clearly extended to nest building. Apparently, some of the sticks that have been discarded for the purpose of drumming may then be used for nesting. There is even evidence that good sticks get carried from one place to the next.
Tool use, already described, and nest building may well be skills that can inform each other. In tool-using birds, the use of sticks or objects is in principle no different in skill than building a nest, but in terms of its brain activity it may be very different. In cases of new challenges, such as dealing with new surfaces or contexts, prior repertoire may not be sufficient preparation. Problem solving, or at least trial-and-error learning, will then almost certainly be involved. This challenge may hold both for location of building as well as for materials selected. These speculative conclusions would be very worthwhile testing. It was surprising how many examples of innovations in nest building among native Australian birds have been documented and how surprisingly abundant these instances are, as Nicolakakis and Lefebvre (2000) had also found worldwide.
For example, kookaburras are known to create or use existing tree holes for nesting. The trees they require, however, either have to be quite old (usually at least 100 years old) or dead to have room for suitably sized nesting holes. Finding both categories together is increasingly in short supply. Because kookaburras are long-lived, territorial and usually permanently pair bonded, one could hear them reliably in the same localities year after year on the Northern Tableland of New South Wales. However, it was clear that none of the pairs had raised any offspring for some years. An ornithology club was greatly concerned by this. The next breeding season arrived and some members of the local club started investigating whether there were any nesting kookaburras. In the wider Tamworth area they found one nesting pair and did so in the most unlikely place: in a barn filled with bales of compressed hay stacked right up to the ceiling of a shed. The birds had hollowed out a nest section from one of the upper bales and they successfully raised one young in that unusual and innovative context. There has been one previous report of kookaburras nesting in a haystack in Bathurst, New South Wales in 1941 (Eastman 1970). More commonly, kookaburras have been noted to hollow out nest holes on ledges of a cliff face, as was reported at a deserted quarry in Springvale Melbourne (Mattingley 1926).
Many birds of prey do not build their own nests but either use cliff edges or search for deserted stick nests built by other species. Falcons are particular candidates for taking over already existing nests. More unusual examples of such reuse are of black-faced cuckoo-shrikes reusing an old nest of magpie-larks and of tawny frogmouths brooding in a discarded mud nest of white-winged choughs (Dickinson 1930).
That birds may find unusual locations for their nests is not all that uncommon. Chestnut-eared (zebra) finches were found to nest on fence posts (Geary 1926), and after a severe hailstorm we rescued live nestlings found under the dead body of a female eastern rosella in an old fence post. Grey shrike-thrushes had made their nest in a bucket that had been placed sideways in a Banksia tree, a nest of scarlet-breasted robins was seen in the blind end of a length of spouting running along a house, a nest of superb fairy-wrens was found in a vegetable garden bed and a ground nest of brown thornbills in a depression on the ground. A nest of dusky robins was positioned on a pole under a house, while a nest of a pair of scrubwrens was placed in a hanging basket with planted ferns on someone’s verandah in Tasmania (Fletcher 1934). In some instances, building nests close to humans may at times be safer than further away, if and when people are prepared to give their feathered guests some protection. An extreme example of this was reported from Pelsart Island off Geraldton Western Australia. Here, western silvereyes had built their nest suspended between two spiral electric light wires inside a house (Tarr 1949). Nests of magpie-larks and willie wagtails were observed at Port Kembla, just south of Wollongong New South Wales, built on angle-iron framework of a high-voltage outdoor sub-station (Sefton 1956). Falcons have a reputation worldwide for using unusual nesting sites. Occasionally, these have been found on telegraph poles, electric power poles and transmission towers (Bunnell et al. 1997), a practice that has also been observed in black falcons in Australia (HANZAB 1993). There is no doubt that poles and steel structures protect nestlings, especially from cats and rats, and even snakes may find it too difficult to get up steel structures (Fig. 5.3).
Fig. 5.3. Unusual nest location. Magpies have chosen this unusual nesting location – a hanging sculpture, suspended by wires high above the houses of Canberra. The hollow steel structure offered some distinct advantages in terms of safety from predators but Canberra authorities would not permit it and called in a cherry picker to remove the squatters early in the nest-building stage.
Some of these sites may seem puzzling but quite often they represent choices. These choices are sometimes made when more conventional sites are also available and it is telling that the authors who observed these nests followed the wellbeing of nestlings and found that they were successfully raised in these contexts. One can surmise that in many cases birds choose sites that may be perceived as providing more safety than others. Experience is also important and one might expect that more experienced birds may make better and, at times, more unusual choices, but this has not been tested.
Books on bird nesting and general field guides will inform the reader that pardalotes tend to build domed nests in tree hollows or dig burrows into the ground or tunnel sideways. However, the pardalote nests that I found over 2 consecutive years in the New England region of the Northern Tableland of New South Wales were located at the bottom of, and built into the lower part of, a magpie nest. The pardalotes went in and out of their nest and were easily visible. Their activities were open and unimpeded. This was particularly surprising because magpies normally guard their nest with utmost care and trespassers are typically not welcome. However, the magpies did not object. It is not clear whether there was a kind of symbiotic relationship in this – in nesting terms, as far as we know, this is a nesting innovation. It is certainly of benefit to the pardalotes in the sense that it provides them with an inbuilt protection and alarm system. It may be of advantage to magpies because pardalotes are efficient consumers of unwanted pests in magpie nests. Whether or not these practices remain isolated events or whether increased pressure caused by many introduced predators remains to be seen.
Nesting innovations – type of nest
Nesting innovations are rarely recorded and, if so, they are treated at best as a curiosity. Using different materials than is common for a species is worth noting here because it may involve challenges for construction, require different forms of tie-down or binding, and thus fall well outside the norm of nest building for the species.
Once a nest of silvereyes was found on Heron Island nesting in a small flowering Cassia bush a mere 4 feet (1.2 m) off the ground (D’Ombrain 1964). It was not only the location but also its unusual appearance that made it remarkable. No reeds and grasses were visible in this fluff ball, rendering its appearance lifeless and looking like unspecified discarded material. The reason for the appearance was that the roofed nest was covered entirely by the hair of grey wallaby (D’Ombrain 1964).
John Murray sent me photos of a wire nest (Fig. 5.4) he had found and that, prior to a storm in the previous year, had been occupied by a currawong. The central structure of this nest was entirely made up of wire and pliable coated electric wiring. The stiffer metal wire would have required substantial strength and adjustment to make it work for the purpose of constructing a nest. The inner part of the nest apparently showed traces of the traditional building material of twigs, leaves and downy feathers. Similar reports have been made in Japan of jungle crow nesting material.
Fig. 5.4. Unusual nesting material. (Left) The outside of a currawong nest is usually an untidy tangle of vines and larger twigs that have been jammed or woven in or around existing tree branches as a kind of tie-down. Unlike the rough exterior, the inside of a nest is usually neatly rounded and padded with small leaves, down and, in many localities, also lined with threads of sheep wool found on barbed wire or in the field. (Right) A currawong nest whose entire outside structure consists of strong metal wire that has been bent, except for an intact metal clothes-hanger. The imaginative use of wire is as astounding as the physical strength it would have taken to bend the wires into the required shape. (Photos: John Murray.)
Another example was sent to me from Canberra – it contained the beginnings of a nest that magpies had started to build in one of Canberra’s lofty design features. It was promptly destroyed by Council workers (see Fig. 5.3).
In 2013, the Coffs Harbour newspaper, The Advocate published an image of a magpie nest in a palm tree (28 September 2013, p. 36). It is a difficult feat to secure a nest in such a way that the ageing of a frond (i.e. detaching naturally) will not happen within the 4–5 weeks in which nestlings are raised. Moreover, the weight of nestlings could increasingly bend the palm fronds and make the entire nest slide down. This has not happened. It could well be an important innovation because the sprawl of suburbia has so far resulted in tall and strongly branched native trees disappearing and being replaced by shrubs and, at the coast in particular, by palm trees. Even the palm trees tend not to be local or native to Australia, posing a range of problems for the birds. Any adaptations they can make now may guarantee their future in these drastically altered environments.
In a final example, I would like to relate the nesting habit of an osprey pair. The osprey, one of Australia’s two fish-eating birds of prey and at home in most coastal regions and some inland waterways, has never featured much in accounts of nest building. Among Australian birds of prey, they are one of the few that build rough, but substantial, nests in trees close to waterways. Real estate for osprey may be difficult to obtain because waterfronts are also favoured spots for swimmers, beachcombers and anglers and finding secure and private spots may at times be difficult, especially when a tree of reasonable branching size is needed to enable nest construction. On the Kalang River, where it converges with the Bellinger River on the mid-north coast of New South Wales, in the small town of Urunga, a pair of ospreys had found an ingenious, if somewhat uncomfortable, position for nesting on a steel railway bridge crossing the river. In its upper arches, the top layer had a criss-cross design that had enabled the birds to place large twigs and branches above it and, over many years of nesting there, they had built a nest substantial in height and width (Fig. 5.5). People dining at a nearby restaurant at water level always gazed and marvelled at the brave osprey pair that endured the incredible noise and vibrations of several freight trains and one passenger train (Sydney–Brisbane line) per day passing through directly underneath the nest – and one might also marvel at the soundness of nest construction – the vibrations could easily loosen and dislodge vital nest structures. Every year the osprey pair successfully raised one offspring, with the Kalang River and the nearby surf beach providing the necessary meals of fish.
In 2012, the bridge was completely renovated and the nest was removed. The new structure was unsuitable because the old net of steel mesh between the joists and arches had also been removed, but a large wire basket or dish was constructed in its stead and welded onto the top of one of the bridge arches of that same bridge and more or less in the same position where the nest had been before. Cleverly, a team of people had carefully conserved the old nest and slid it back onto the new wire platform. In 2013, the osprey pair had returned. The ospreys that had not been seen nesting anywhere else in the intervening 2 years, now went about reusing the nest on top of the wire basket (The Advocate, 21 Sept 2013). The human construction of the base of the nest was fundamentally different from the arrangement that had existed before, yet the birds (or rather the female) recognised its potential immediately. This would have required a different method of laying and anchoring the first layer of twigs and branches than it had done before and a different way of securing the surface, but this was mastered and achieved. I cannot imagine that this feat would have been possible without some insight or problem-solving ability. Clearly, the human construction looked nothing like a tree fork and was different from the structure they had known before. In all cited cases, the birds had found innovative solutions.
Fig. 5.5. The Osprey and the railway bridge. The railway bridge near Urunga between Coffs Harbour and Kempsey on the Sydney–Brisbane rail line. Some years ago a pair of ospreys constructed their huge platform nest and returned to it year after year and successfully raised one offspring each year. Photos taken in 2009.
A further example of ingenuity was the initial failure of willie wagtails to build a nest around a high wire cable. Once completed and when a bird was standing on the side, the entire nest dropped and tilted by 180 degrees, now suspended. Instead of vacating the site, the birds rebuilt a nest on top of the one that was hanging down and tied the two together, giving it stability despite the slippery cable. The nest now had more the shape of an hourglass but in it offspring were successfully raised (Sefton 1956).
In the obverse, details of a nest may well be subject to correction when details do not measure up to the best specifications. For instance, Stanback and colleagues tested nests of house wrens, at home in North America, with either small (28 mm) or large (38 mm) entrance holes. They found that the birds altered their nest architecture to compensate for vulnerability to intruders by adding new sticks to render the entrances smaller and therefore less accessible (Stanback et al. 2013). Their study showed clearly that the birds had very definite ideas about design and safety. They could not be tricked into accepting the structures with large cavity holes as the experimenters had designed.
Also, for the first time it has been tested experimentally whether birds would actively choose to camouflage their nests (Bailey et al. 2014). Captive zebra finches were provided with differently coloured nesting materials against different wall colours. The researchers found that the zebra finches chose the material that blended most closely with the background. This is the first evidence that birds may actively select for materials that best camouflage their nest.
It is now clear that the more complex the structure of the nest, or the more challenging the location, the more juveniles may have to learn the art of construction, be this in finding a suitable location, suitable height and direction (in terms of such concrete factors as wind directions, sun exposure), method of tie down, best materials to use and perhaps even the colour and type of material and how to make the nest less obvious to predators. The question is whether nest building itself as one of the challenges of raising offspring may not be a cognitive task. Is its capability alone not a sign of ‘intelligence’ or of creativity when building innovations are involved (Gould and Gould 2007), or when the best solutions are found or longstanding habits adapted for a given environment? Most educators of human children would certainly think so and perhaps it is time to apply the same thinking to other species. Equally, to devise ways of finding a nesting place that is entirely unconventional and has none of the characteristics of a natural choice of nest sites one would imagine requires some flexibility, as any new solution might, and it is certainly adaptive.
In a recent paper, Owens and Bennett (1995) wanted to test four variables that represented competing ecological explanations for avian life history in evolution. It is worth raising this here because the four variables they chose were food type, foraging range, developmental mode and nesting habit. They justified this by saying that all of them are crucial variables in explaining mortality, fecundity and population growth. Their results were interesting. Of all four variables only one was found to be significant and this was their fourth variable: changes and diversification in nesting habits. Apparently nesting habits developed some 40 million years ago and significantly influenced the fortunes of birds. Nesting habits seems to have some explanatory power for life-history variations among living birds (Owens and Bennett 1995).
Finding safe nest sites and being innovative in procuring such sites has thus apparently had a major influence on successes and failures in lineages of birds. Interestingly, in their statistical analysis, nesting habits were even more important than variations in food resources. In a recent study, male zebra finches were tested in how they use nesting material and how experience influenced their activity of nest building. Bailey and colleagues found that learning about nesting materials was considerably more important to nest construction than had been assumed and males made different choices based on experimentally offered choices (Bailey et al. 2014). The longstanding belief that choice of structurally appropriate nest material is genetically determined may thus not hold under scrutiny. Such questions are now usually discussed under physical cognition.
There is a further dimension to nest building that relates nest building to partner bonds, their quality and the quality of the partners individually. Soler et al. (1998) hypothesised that species in which both sexes contribute to nest building have larger nests than those in which the nest is built only by one sex, because both sexes are using the nest-building process as a signal of their quality. Comparing 76 passerine species (and allowing for size differences), they found indeed that there was a size difference and the larger nests of joint nest builders indicate greater investment in reproduction. The investment in reproduction both in quality and time is thus pronounced in pairs with strong bonds and especially in cooperative species. Apostlebirds, for instance, build their mudnest as a group. As I observed last year south of Inverell, New South Wales, one individual stayed at the nest site while several others (I observed as many as five) were commissioned to supply the mud and did so continuously as a relay-team from dawn onwards and for several hours while the individual bird at the nest site kept shaping the mud ensuring it would not run down the side and be lost. Then the group stopped supplying mud abruptly by about 9 am – presumably to allow the mud to dry off a little while the shaping of the mud at the site continued by two birds. A new shift was recruited for the afternoon. The activities were accompanied by much raucous vocalisation. They also called each other over when a new site of mud that was deemed appropriate had been discovered. It was clear that this was a highly coordinated activity in which every single bird knew exactly what to do.
Today, there are still very few attempts to ask whether nest building or any construction, at least by some species, is merely part of the Darwinian concept of extended phenotype (genetic predisposition expressed externally) or the result of a mental plan. We certainly have clear evidence that in some nest-building species learning is required to perfect the ability. However, as Hansell (2005) rightly argued in his book Animal Architecture, ‘constructed objects themselves can be a poor guide to the complexity of underlying neuronal processes’ (p. 125) but they are usually pivotal in an individual’s or species’ life history.
At least this much is undisputed: nesting strategy and innovation is anything but marginal to the survival of a species. Good builders get rewarded by raising clutches; bad builders might not even get to find a mating partner.
Building a bower
While males in many species do not contribute to nest building, there is a group of birds in which the males construct bowers (but do not participate in nest building). These are structures on the ground that may take different shapes, depending on species, for the sole purpose of attracting as many females as possible to this construction so the male can mate with them. The western New Guinean Vogelkop bowerbird builds the most complex bower of any species in this family: it has 1-m high huts and domes, together with struts and entrance hall, and lavish decorations, a colourful blend of fruit and flowers. Diamond (1987) proclaimed the structure built by the Vogelkop bowerbird as ‘the most elaborately decorated structure erected by any animal other than humans’.
Satin bowerbirds and regent bowerbirds build comparatively simple bowers: just two walls of grasses that are raised and form a natural alley through which a single bird can just fit. They also decorate, and sometimes ‘paint’, the bower as well. It seems a difficult task to learn because juvenile satin bowerbird males often group together and jointly build one bower as a trial. Some juvenile males visit adult males to watch while they are constructing a bower and some may even try to steal especially attractive features of the adult male’s bower (Maxwell et al. 2004).
It has long since been recognised that some aspects of bower building go beyond mere skill development. Researchers found not only great individual differences and preferences in design but also individual insistence on maintaining their design. If some decoration pieces were removed, males compensated for such loss by increasing bower construction behaviour and hastening bower painting (both less time-consuming tasks than finding new decorations). In other words, males were able to assess the quality of their own displays and make decisions about how to augment them (Bravery et al. 2006; Bravery and Goldizen 2007). It also seems that females choose bowers not just for their decorations but also for the male’s display at the bower. It seems that the artistic forms of the construction inspire both male and female choice. Patricelli et al. (2003) found that attractive bower decorations facilitate attractive behavioural displays. It seems stating the obvious that there is an innovative, creative dimension to the entire process.
The great bowerbird of northern Australia is certainly among the most impressive bower builders and can make the point of creativity, innovation and individuality perhaps even more strongly than others. In the great bowerbird, the bower walls are smaller (although higher than those of the satin bowerbird’s bower) and decorations rather dull. These tend to consist rather uniformly of pebbles, stones, perhaps some bones and some glass – most pebbles and stones are bleached and a dull grey (Fig. 5.6). At first, this seems far less inviting and spectacular than the offerings of other bowerbirds. Regent bowerbirds in particular are known to mix a muddy greyish blue or pea green ‘saliva paint’ in their beak and then use it to decorate or ‘paint’ their bowers. Indeed, as already mentioned in the previous chapter, regent bowerbirds are known to use wads of greenish leaves as ‘paintbrushes’ to help spread the substance, representing a well-known example of tool using by birds.
The display of the great bowerbird is not without colour and drama though. When a female enters the bower, the male stands on the court and displays his purple and turquoise nuchal crest (the feathers that fan out from the back of his neck). The simplicity of the great bowerbird’s court is, in fact, deceptive, but this has been discovered only recently. It is not so much colour that matters but creating a visual illusion. The female is asked to judge the male’s achievement by the visual perspective of the court. As researchers (Kelley and Endler 2012) discovered recently: arranging the pebbles from large to small creates a visual illusion known as a forced perspective. The female viewing the male’s display over the court apparently judges the perspective and the quality of the illusion as the most important variable and it is the quality of this illusion that is apparently associated with mating success.
In a rather clever experiment, Laura Kelley and John Endler of Deakin University (2012) heightened the quality of the forced perspective to determine whether males maintained it at the new higher level, decreased it back to its original, or allowed it to decay at random over time. They found that the males actively changed it back to their own original values. Then they compared this aspect of bower court presentation among various male bowers of the same species and over two breeding seasons and found that differences in the quality of visual illusion among their bower courts were consistent within and between two breeding seasons. This suggests that the level of forced perspective is actively maintained by each individual male as an intentional design.
Fig. 5.6. Bower of a great bowerbird. The front as well as the back of the bower contains river pebbles of a uniform grey plus bone fragments and a few pieces of broken glass, uniformly of dark green colour. The bower is almost twice the length of the bower of the satin bowerbird and more than twice the length of that of regent bowerbirds that build a very basic bower. Photograph taken by the author in 2013 at Adel’s Grove, outback north-western Queensland.
Whether it is appropriate to discuss this skill and preference of style in terms of cognitive dimensions is another matter entirely. Most scientists would probably say that this is too far-fetched and there is too little evidence to support this notion. However, Kelley and Endler (2012) provide compelling evidence that bower design is anything but random, especially in key features, and individually different in quality and effect. This behaviour cannot be explained as an inability to learn because, in that case, one might have expected relative indifference to the interference of the researchers. Replacing the pebbles is a good deal of additional work for the bird and would not be done if indifference or lack of comprehension of the forced perspective were the issue.
Individuality is an essential feature in species that compete for female attention, and in the case of great bowerbirds the male asks a female to judge his bower and court. The intention is that she will come to the conclusion that his bower offers an element of surprise and interest. The latter is a mark of creativity and in this case cannot be explained by different levels of mechanistic skill development because, as the experiments showed so clearly, individual birds were adamant about their design and insisted on ‘righting’ the disturbance back to their original design.
Innovations, as Kummer and Goodall (1985) had defined them, were characterised by having found ‘a solution to a novel problem or a novel solution to an old one’. Although their definition was based on research on primates, the definition is wide enough to apply to vertebrate behaviour in general. In nest building, there are plenty of examples of a wide range of birds creating new nesting opportunities in new locations demanding some skilful new solutions. Bowers have long intrigued scientists and naturalists alike. The tool use, design, colouring and now even visual illusions are clever, and even artistic. In primates, the latter is referred to as cultural transmission.