Karl Marx once claimed; “A spider conducts operations that resemble those of a weaver, and a bee puts to shame many an architect in the construction of her cells. But what distinguishes the worst architect from the best of bees is this, that the architect raises his structure in imagination before he erects it in reality” (1).
Whether this statement is applicable to bowerbirds, however, is a matter open to debate.
For these passerines, native to Australia and Papa New Guinea, are capable of building incredible retreats known as ‘bowers’, ground based lodgings woven around young trees.
Taking years to perfect, these works of architecture are produced by males from 17 of the 20 known species in the bowerbird family (2).
Construction of a bower alone is a considerable feat. Armed with nothing but their beaks and dry foliage at hand in their environment, some species such as the Vogelkop can erect dwellings that dwarf their creator (figure 1), whilst other species such as the great bowerbird substitute size for intricacy to boast equally wondrous galleries (figure 2).
However the bower itself is often insufficient for the purpose for which it was constructed – to become a personal bachelor pad – and thus additional, careful decoration is required to win over choosy females.
As with the construction of the actual bower structure, subsequent embellishment gives the observer an impression of pre-determination on the bird’s part. Collections, which can include flowers, fungi and beetles, are segregated into geographical locations as if their creator were practicing feng shui (figure 3). Even more interestingly the categorization, often based on colour schemes, varies between species in a manner akin to culture, a phenomenon not yet recognized in birds (2).
But what has lead to these creatures constructing these small architectural wonders? Simple. Sexual selection. The process by which one sex – in this case the female – chooses their sexual partner based on ‘measurable’ phenotypic traits, these in turn being a signal for good quality heritable characteristics.
This form of selection has become dominant in bowerbird species due to their polygynous nature. One male may mate with numerous females whilst they in turn will likely (75%) visit just one bower before copulating (3). A result of this behavior is that females are left to raise their young alone, an expensive task, which as such intensifies selection pressure on females to identify the best mating partner.
It is the bowers, a key part of bowerbird courtship, which likely allows females to identify which males carry the best bouquet of genes. Acting as honest signals – the cost being the tradeoff between feeding/resting time and time spent building/maintaining the bower – these constructions are likely evidence of selection via the handicap principle proposed by Hamilton and Zuk (4). This hypothesis states that individuals with superior gene combinations can afford to devote more time, energy or resources to attracting a partner relative to less fit individuals. This may be via growth of a cumbersome tail as in peacocks, or as is the case in bowerbirds the building of bowers. Essentially, the presence of a trait which represents a fitness tradeoff, and which can be scrutinized by members of the opposing sex in order for the hosts overall fitness to be assessed (4).
As with many other birds, colour is of incredible importance in this family. Some species such as the satin bowerbird show particular fondness for specific colours – blue in the case of the aforementioned species. Conversely my own hypothesis is that this species fondness for decorating its bower with blue ornaments (figure 4) arises from sensory bias. The satin bowerbird species itself has bright blue eyes and perhaps it is the case that females of the species are disproportionately attracted to the colour for this reason. Resultantly, it would be selectively advantageous for males of the species to choose decorations that best match the interest of their female counterpart.
In other species a full repertoire of colourful ornaments may be employed, and in these species it is easy to understand why decoration of a bower evolved above beautification of the male body. Flexibility. Whilst birds of paradise for example are indeed mesmerizing, their plumage is limited by their genetic makeup. Bowerbirds however are free of this constraint, enabling them to woo potential mates with an array of colours and textures that far exceeds those at the disposal of any species I can think of, save perhaps the chameleon (5).
So the aptly named ‘seduction parlours’ of the bowerbirds, emblazoned with bright flowers and shiny beetles are a result of 40 million years of unique evolution (3). Sexual selection has allowed for a grand diversity in the design of each species gallery, the exquisite decoration that has followed and the courtship proceedings that accompany it. With the finest bowers owners attracting a disproportionate proportion of sexually active females, selection is intense. It may take years of building before the male’s abode is sufficient to attract a mate and in the meantime all he can do is perfect his woven masterpiece and lace his grand walkway with his growing collection of trinkets.
As to whether these birds can imagine beforehand the structure they are creating, I am unsure. Certainly many bird species have developed tool use and problem solving skills associated with high cognitive performance and perhaps this family is yet another example of birds boasting intelligence at a level previously considered unique to higher primates such as ourselves.
With just one species of this family threatened, my reason for writing this blog was not to encourage conservation, simply to share what I have learnt and hypothesized about these amazing animal architects. It was the photos I viewed on the national geographic website –some of which are referenced in this article- that encouraged me to begin this particular blog and indeed to develop this website, a small space where I hope to continue writing about a subject that has inspired me throughout my time in university.
- National Geograhic, July 2010, Birds That Decorate, pages 68-81
- Hamilton and Zuk 1982: http://www.ncbi.nlm.nih.gov/pubmed/7123238
All photos courtesy of National Geographic.