Spotted Bowerbird's bowers in successive years
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The male Spotted Bowerbird that has its territory on our property has displayed some ingenious skills through the years. By visiting the bower regularly we have been able to monitor the changes that occurred and to relate them to their causes.
Note upfront: Both bowers shown below were found under Wilga trees. With their low, almost horizontal branches they appear to provide optimal cover for a bower.
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.
A year later the bower was much improved. It had received a solid base from 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.
After a small wattletree fell onto the Wilga tree under which the bower was located, the bird moved to a new spot and brought everything belonging 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
A year later the foliage had fallen off the wattletree that had fallen onto the tree 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 the second bower (see 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 simultaneously. 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 the second, newer bower back to the original one. All this happened at a "quiet" time of year for the bird.
Less than two months later (early July) the second, newer bower had been stripped of all its adornments. Instead, the original bower 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 undergo seasonal adjustments.