Bringing birdsong back to Guam
Spending time in Guam’s forests can be an eerie experience. One can be hiking through lush ravine forest in the volcanic south of the island, or beautiful and diverse karst forest in the coralline north, and experience near total silence. Where one would expect to hear a cacophony of birdsong, the only sounds are the occasional buzz of insect wings, the rustling of a skink in the leaf litter and the sough of wind through the trees.
The collapse of Guam’s avifauna has been well documented. Dr. Julie Savidge’s doctoral work, published in 1987, isolated the culprit - the invasive brown tree snake - among a host of other potential factors in a textbook example of ecological problem solving. There are few bright spots to this story. The health of Guam’s forest is declining and an entire generation of islanders has grown up without the chance to interact with their native avian community. In one of the last lines of her thesis, Dr. Savidge did offer a glimmer of hope in an otherwise bleak prognosis: “Starlings have recently been observed nesting on artificial structures, and populations might be augmented using nest-boxes placed on concrete telephone poles.”
Fast-forward 30 years and those prophetic words are becoming reality. Our project, led by the very same Dr. Savidge and her husband, Dr. Tom Seibert, aims to investigate the potential for using predator-resistant nest boxes to promote the recovery of Micronesian Starling (Aplonis opaca) populations on Guam. The ultimate goal is to apply similar techniques in reintroducing another cavity nester, the extinct-in-the-wild Guam Micronesian Kingfisher. The range of the once-ubiquitous starling, locally known as sali, has contracted dramatically and is now restricted to Andersen Air Force Base on the very northern tip of the island (where we are focusing our efforts) and Cocos Island, a snake-free sandbar and seabird colony, off the southern village of Malesso’.
Preliminary surveys found this normally forest-dwelling species displaying an impressive level of flexibility in its nesting habits: besides the more natural nesting sites in coconut palm crowns and hollow trunks, we found sali nests in window shutters, streetlamp poles, roof gables and even exhaust pipes. While it was promising to find a relatively robust breeding population, many of the nesting sites appeared to be vulnerable to inclement weather, predators and potentially lethal internal temperatures.
Our first challenge was to design a suitable nest box. Given that we would need to install the boxes in exposed locations in order to keep them safe from potential snake predation, we needed to build a structure that would maintain relatively cool and even internal temperatures despite Guam’s sweltering tropical heat. We built a number of prototypes based on established designs, mounted them in full sun and monitored their performance using data loggers.
One design stood out: a modified version of the British Trust for Ornithology’s European Starling nest box constructed out of white PVC decking consistently approximated ambient temperatures. We added an overhanging roof and ventilation gaps, modifications that helped further buffer the internal temperatures. In the fall of 2015, we built 50 boxes based on that design, half with cedar walls and half fully made of PVC. We installed the boxes on concrete streetlamp poles and then we waited.
For six long months, the birds showed no interest whatsoever in the nest boxes. Although sali are curious birds, we did not see a single bird investigate a box over that period. We opted to modify the boxes in a somewhat desperate attempt to make them more appealing. We enlarged the entrance hole, roughened the roof and front panels, and added a perch. Incredibly, we observed birds building nests in a number of boxes barely a week after making the modifications. Our tinkering was rewarded! In the year and a half since that development, more and more sali have adopted our nest boxes. More than half of the 70 nest boxes currently installed have birds regularly breeding in them. We have recently surpassed the mark of 300 chicks fledged from our boxes!
Besides monitoring the nest boxes, our team has been radio-tracking fledglings in order to monitor their survival and dispersal. We have also been spending long hours in the field netting and tracking juvenile and adult birds in order to document their movement patterns and home range size. Analysing the sali’s diet is helping us understand its crucial role as a seed disperser within its environment. The picture of what is required to recover this species, and in turn to restore some level of function and connectivity - as well as birdsong and beauty - to this ecosystem is rapidly becoming clearer.
Posted 9 November 2017