Meet the Spotted Whistling-Duck, Australia’s newest resident breeding species.
This piece first appeared in the December edition of Australian Birdlife magazine.
It may seem unimaginable today, but seeing a Cattle Egret on your patch would once have been an immense thrill to an Australian birdwatcher. Initially unknown in Australia, from the 1870s onwards the Cattle Egret began a rapid expansion from its original range across south-western Europe, tropical and sub-tropical Africa, and southern Asia. The species crossed the Atlantic into South and Central America, reaching as far north as Canada, where it first bred in 1962. In Africa it expanded southward, arriving in South Africa by 1908. The species had spread as far as Australia by the 1940s and New Zealand by the 1960s.
Today, Cattle Egrets are well established through the better-watered regions of Australia’s north and east and even occur, less frequently, across the driest parts of the continent. The 1940s-era birdwatcher would have been there during the first wave of this self-introduction and been able to chart its progress across the Australian continent. Until now, the only other species of bird to have self-introduced into Australia in historic times is the Kelp Gull which arrived, presumably, from New Zealand in the 1940s and first bred here in 1958. Is it possible, however, that we are currently witnessing a self-introduction of another species, albeit on a less spectacular scale?
It came as quite a surprise to Australian birders when Spotted Whistling-Ducks were first sighted on the Australian mainland, at Weipa on the western coast of Cape York, in March 1995. It’s worth noting that this record followed closely in the wake of Tropical Cyclone Warren. Even more remarkable was that this turned out to be no one-off vagrant record – in the years that followed, the species would continue to be reported at Weipa in flocks up to 14 birds. Later it would be recorded at Chilli Beach and Lockhart River, with records as far south as Mission Beach in 2014.
The first confirmation of the species breeding in Australia came in 2000 on Cape York Peninsula, where it has now established a small but widespread breeding population. It was eventually recorded across the Gulf of Carpentaria in the Northern Territory in late December, 2011, when a lone bird was seen at Leanyer Sewage Ponds near Darwin. The similarities with the initial Weipa record went beyond its appearance at a sewage works to include its timing – immediately after a cyclone (this time, Tropical Cyclone Grant) had passed through. Spotted Whistling-Ducks may since have established a small breeding population on Melville Island, north of Darwin, with flocks of up to 130 birds recently reported by fishing charter vessels in the vicinity of Goose Creek.
The global spread of Cattle Egrets can be explained by its commensal relationship with large herbivores, cattle in particular, and the expansion of pastoralism in many parts of the world throughout the twentieth century. But what, if anything, is behind this self-introduction of Spotted Whistling-Duck to Australia’s north?
The Spotted Whistling-Ducks traditional range extends across the southern Philippines and through the Moluccas and Lesser Sundas to New Guinea (where it is the most common duck) and the Bismarck Archipelago. Throughout this range its status is listed as Least Concern, but it is rarely found in concentrations larger than 100 birds. The population is yet to be comprehensively surveyed as much of its home range is in remote areas, but estimates have placed the world population at between 10,000 and 25,000 individuals. It’s a decidedly lowlands resident over most of New Guinea, rarely venturing above 200 metres, and inhabiting freshwater swamps, marshes, pools, and occasionally mangroves and watercourses where there are grasslands with scattered tree cover.
When dealing with a species so little-known even in its heartland, any answer to such a question of self-introduction will involve a lot of conjecture, but it is an interesting exercise all the same. Adding further intrigue, other species, like the Collared Imperial-Pigeon and Red-capped Flowerpecker, are regularly recorded on the Torres Strait Islands between the tip of Cape York and Papua New Guinea. Neither of these species has yet to make the short flight south to be recorded on the mainland of Australia.
Considering the proximity of the first record to Tropical Cyclone Warren, the role of severe weather systems can’t be ruled out. Many northern areas of Australia share similarities of habitat type, climate and even faunal assemblages with the lowland areas of New Guinea. Perhaps ducks are being picked up by these storm systems and pushed the short distance over to comparable Australian habitats. Being a mostly sedentary species, once relocated over Torres Strait into suitable habitat, birds may make no attempt to return north. Most Australian records occur in November, December and January (the early wet season), which may support the idea of birds being caught up in tropical storms during the build-up.
Another suggested cause of southward movement is the destruction or degradation of habitat in New Guinea. The changes that have been wrought in New Guinea’s ecosystems by decades of mining are difficult to overstate. Mining operations in highland areas have drastically changed all aspects of the drainage systems below them. Some river supply routes once plied by all manner of vessels are now all but unnavigable due to changes in sedimentation which has resulted in ever-shallower rivers. How these changes to river levels and water quality might affect the habitability of connected wetlands and waterways along their length may have something to do with the displacement of waterfowl and other animals that rely on them.
In the last two years there has also been a worsening drought across the highlands of New Guinea related to the burgeoning El Niño. Drought in the highlands of New Guinea almost certainly has a different meaning in relation to rainfall than it has across most of Australia, but it’s a relative term, and the resulting drop in rainfall has had disastrous effects on crops in many areas. As a lowland species this drought may not be enough to directly affect the movements of Spotted Whistling-Duck but it is certainly connected to lower river levels across the region, which could be one of the many variables relevant to our question.
Finally, we are now undoubtedly seeing the impact of global climate change on a number of habitats and the animal communities that live within them. Many cuckoo species, for instance, are arriving earlier and travelling further south than ever before. It would be extremely difficult to definitively tie this to the comparatively recent southern excursion by Spotted Whistling-Ducks but it is certainly going to be part of a complex set of factors involved.
One of the most difficult variables to eliminate in researching this sort of phenomenon is ‘reporting bias’. Reporting bias is the effect that might lead you to draw erroneous conclusions based on the absence of records when the reason for the absence of records is not be the absence of birds, but the absence of observers. We see this every time we look at a bird distribution map of Australia. Point records form recognisable clusters around remote population centres and highways, with large blank areas in the deserts which aren’t traversed by accessible roads or tracks. Anyone who has visited these areas can attest that they certainly aren’t bird-less; the birds are there but the paucity and sporadic nature of reports from observers in these areas makes species occurrence appear negligible in comparison to more well-travelled and well-watched areas.
So we may say that 1995 saw the first record of Spotted Whistling-Duck in Australia, but we must acknowledge the limitations of that phrase. Perhaps they have previously existed in small numbers across northern Australia, before records (or notice) was taken, making their self-introduction more of a re-colonisation. Perhaps they did occur before 1995 but in areas so remote that they were never noticed before this date. It is doubtful that the birds could have been mistaken for one of Australia’s other whistling-ducks, as the Spotted Whistling-Duck is distinctly different in appearance to both of those species, being dark brown along the flanks and undertail with bold white spotting. It could be possible to an immature bird for Wandering Whistling-Duck, with the flanks presenting as streaked rather than spotted as in the adult, but this is an error unlikely to have been repeated on a regular basis by the many experienced birders who visited the Cape York region in the decades preceding the initial sighting.
To fully understand the nature of bird movements and the driving forces behind them, it is necessary to have comprehensive, long-term data. For many species this simply doesn’t exist - and Spotted Whistling-Duck is one of them. Happily, this is one area where we all play a role in setting the record straight and filling in the blanks. Projects like the Atlas of Australian Birds and, more recently, online databases such as Eremaea Birds and eBird, are showing bird watchers to be right at the coal face of citizen science.
While we may just be guessing at the motivations of Spotted Whistling Ducks in their progressive southern peregrinations at present, when we do finally come to understand the movements of this mercurial species, it’s likely to be the observations of bird watchers like you and me that we rely on.