Spotted Bowerbird's bowers in successive years
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The male Spotted Bowerbird that has its territory on our property at Eulah Creek, NSW, has displayed some ingenious skills through the years. By visiting the bower regularly we have been able to monitor the changes that occurred and have tried to relate them to their causes.
Note upfront: The bowers shown below were all found under Wilga trees. With their low, almost horizontal branches and dense foliage they appear to provide optimal cover for a bower.
Overview of the location of the bower of a Spotted Bowerbird on our property, under a Wilga tree, which in turn stands under a taller ironbark eucalypt (July 2013); the bower is visible at the end of the bright ray of light extending under the outer branch of the Wilga tree
Bower "no. 1(a)"
It is possible that the bower spotted by us in 2007 was in its first year of use. It was made primarily out of grass halms, without a stable base of twigs (see first photo below). Note also the scarcity of trinkets to adorn the alley.
View of the bower of a Spotted Bowerbird with its trinkets, such as e.g. unripe fruit from our garden; the alley is aligned roughly East-West
A year later the bower was much improved. It had received a solid base made of strong twigs, into which the grass halms were then stuck. This makes the alley look much tidier and the walls have become a lot higher. In addition, the number of trinkets has increased, but it is still quite small.
Bower "no. 2"
After a small wattle tree fell onto the Wilga under which the bower was located, the bird moved to a new spot, under another Wilga, and brought all the trinkets to its new bower about 20 metres on (see photo below). This bower, although lacking the massive base of its predecessor, is very tidy again. By now the bird had found a broken green bottle and a similarly coloured marble.
NB: The two bowers are much too close to each other to belong to different birds. There is confirmation of this statement in the photos below.
Relocated bower; note the symmetry of the arrangement - bones at the back of the bower, grey and black at the front, red trinkets outside around it, green glass shards and metal at the centre, with a marble as the "masterpiece", perched precariously on the shards
Bower "no. 1(b)"
A year later the foliage had fallen off the wattle tree that had fallen onto the Wilga covering the first bower. This seems to have changed conditions in a way that made use of the first bower possible again.
In autumn, after the end of the year's breeding season, the male bird refurbished it. Note in the photo below how the new grass halms are firmly rooted in the strong base made of twigs. Once the bower was re-established, the bird started moving trinkets back to it. This is confirmed by the fact that the marble, previously in use as the masterpiece of bower no. 2 (above), has now come back to the refurbished old one.
At the time when the last photo above was taken (May 2011) both bowers were in a well-maintained, usable condition contemporaneously. Both had similar alignments. The first bower is aligned with early-morning sunlight, in the second bower the mid-morning sun illuminates the alley with the glittering shards most conspicuously. The bird was busy moving shards from bower no. 2 back to the original one, no. 1(b). All this happened at a "quiet" time of year for the bird.
Less than two months later (early July) bower no. 2 had been stripped of all its adornments. Instead, bower no. 1 was completely refurbished (see photos below).
There were no major changes for two years, but at the end of the 2013 breeding season we noticed a peculiarity in the shape of the bower. Note how the two side walls of the bower in the photos above are nicely symmetric, with equal heights and angles.
All views shown above were taken looking from east to west. The two shots below were taken from west to east.
By April 2013 one of its sides was much lower than the other, making the whole construction looking clearly asymmetric, as shown in the photo below.
The photo above was taken with a flash. The resulting homogeneous illumination almost completely camouflages the most important effect of the bower's design. This becomes apparent only when looking at the bower in natural light (below).
It is very likely that the asymmetric design has a purpose, namely to get good illumination onto the central heap of glass shards by the early morning sunlight. We have previously observed birds entering the bower from the western side, an approach that will ensure that the sparkling light is always visible and does not get obscured by the body of the bird entering the alley. If correct, this interpretation suggests that bowers can undergo seasonal adjustments.
Bower "no. 3"
Despite the adjustments made to bower no. 1(b), something must have been wrong with it, because only three months after the photo above had been taken, the male bowerbird decided to move again. This time a new bower was built just two meters from no. 1(b), under the same tree, but closer to the edge of the protective tree crown (see photo below).
New bower at the edge of the protective tree cover, with remnants of the old bower, no. 1b, still visible in the background in July 2013 - at the time the male had a harem of at least four females with him
Note that, contrary to previous convention, this new bower is now North-South aligned. The morning sun does not illuminate the shards and marble at the centre of its (symmetrical) alley.
At the time when the photos above were taken (July 2013), the male Spotted Bowerbird had a harem of at least 5 females around him. We observed up to 6 birds around our house simultaneously.