Some bird species have an incredibly small distribution. Consider, for instance, the Helmeted Honeyeater. The state bird of Victoria, Australia, very small numbers of this bird can be found in the Yellingbo Nature Conservation Area, and almost nowhere else.
At the other end of the distribution scale are bird species with ranges that almost defy belief. One such bird is the Eurasian Tree Sparrow which can be found from Afghanistan to Brunei; from Christmas Island to Iraq; from Japan to Ireland. Beyond its native distribution, Eurasian Tree Sparrows have been successfully introduced to far-flung lands including Canada, southern Australia and the Philippines, and are sometimes seen in Israel, Gibraltar, and Timor Leste. These sparrows are clearly incredibly flexible and tolerant. However, the flexibility and tolerance of no creature is infinite.
Sixteen kilometres off the north tip of Western Australia, Troughton Island has a permanent human population of just two. It is less than one km2 in area, and although the island has an airport and an automated weather station, it has no standing drinkable water. Is it the sort of place where a population of Eurasian Tree Sparrows might persist?
Between 6 and 8 August, 2016, a survey of sea turtle nesting on Troughton Island was conducted by Uunguu Rangers from the Wunambal Gaambera Aboriginal Corporation and staff of the Western Australia Department of Parks and Wildlife. At that time the island’s resident caretakers, Brad and Heather Newman, pointed out four small sparrows foraging in the grass near the airstrip. These were later identified as Eurasian Tree Sparrows. Another expedition later that month found these four sparrows drinking from a dripping air conditioner. Three of the four sparrows were killed. The two carcasses that could be retrieved were tested for avian influenza, and found to be free of the disease. The fourth sparrow was not seen after September of 2016, and had presumably died.
An earlier caretaker on the island, Peter King, had reported as many as seventeen Eurasian Tree Sparrows on the island in 2011 following a storm. King noted that some of the birds had bred.
Reporting on these events, Anton Tucker of the WA Department of Parks and Wildlife and his colleagues speculated on the origin of the Eurasian Tree Sparrows on Troughton. The closest population of this sparrow to Troughton Island is four hundred kilometres away in Indonesia. This bird species is thought to periodically take up residence in new lands after hitching rides on large ships, and virtually all earlier records of this species in Western Australian have been associated with ports. However, this form of dispersal seems unlikely in the case of the sparrows on Troughton Island. The island cannot accommodate large cargo ships, and is visited infrequently by supply ships. Instead, the seventeen Eurasian Tree Sparrows first seen on Troughton Island in 2011 were likely blown there by a substantial storm.
According to the Australian Bureau of Meterology, Cyclone Carlos blew across the north and west coasts of Australia between 13 and 26 February, 2011. Very heavy rains caused widespread flooding and damage to roads and properties in and around Darwin, and forced the evacuation of the community of Nauiyu.
Beyond heavy rains, the cyclone may have also brought sparrows from the north to Troughton, if only for a time. The lack of available drinking water, combined with the presence of predatory Children’s pythons almost guaranteed that the plucky Eurasian Tree Sparrow would not survive on the island indefinitely.
Tucker, A. D. 2017. Invasive Eurasian Tree Sparrows Passer montanus on Trougton Island in the North Kimberly of Western Australia: a cyclone-induced colonization attempt? Australian Field Ornithology 34:67-70.
Photo credits: Eurasian Tree Sparrow Macau photograph – www.pinterest.com; stamp – www.bird-stamps.org