New Zealand’s banded dotterels are arriving in Australia later each year as the climate warms

On the open beaches, shingle riverbeds and bare volcanic plateaus of New Zealand, the banded dotterel is one of the country’s most familiar and best-loved shorebirds. Known in Australia as the double-banded plover, this small, charismatic bird breeds nowhere else on Earth but New Zealand. While a portion of the population stays put throughout the year, an estimated seventy per cent undertake one of the more unusual migrations in the avian world: rather than flying north towards the tropics as most migratory birds in the region do, they turn west and cross the Tasman Sea, a journey of almost 3,000 kilometres, to spend the southern winter in Australia. It is a remarkable feat of endurance for a bird that weighs only a few dozen grams, and one that has presumably played out for thousands of generations. Now, new research suggests that this age-old ocean crossing may be quietly running out of time, and that climate change is the likely culprit.

A study conducted by the Department of Ornithology at the Max Planck Institute for Biological Intelligence, published in the international bird science journal Ibis, has documented a striking shift in the timing of this migration. By analysing more than a decade of observations submitted by birdwatchers to the citizen science platform eBird, the researchers found that banded dotterels are arriving on their Australian non-breeding grounds approximately four days later each year on average between 2014 and 2025. Crucially, the team detected no comparable change in the date at which the birds depart Australia to return to New Zealand for the breeding season. The consequence of this asymmetry is dramatic: the total estimated time the birds spend on their Australian wintering grounds fell by around a quarter in just over a decade, shrinking from a substantial residency to a markedly shorter stay.

The numbers behind this finding are sobering. The researchers divided the birds’ known Australian non-breeding range into grid areas of roughly 23,000 square kilometres each. For every one of these areas, they tracked how sightings reported through eBird built up at the start of the non-breeding season and tailed off at its end, using these patterns to estimate when the birds arrived in and departed from each part of the range. This continent-scale approach put the average stay at 173 days in 2014. By 2025, that figure had fallen to 129 days, a reduction of about twenty-five per cent. For a migratory bird, the length of time spent on the wintering grounds matters enormously, shaping how much fuel can be accumulated, how much stress the demanding sea crossing imposes relative to its rewards, and ultimately how likely an individual is to survive from one breeding season to the next.

What could be pushing the birds to delay their departure from New Zealand year after year? The study points to a large-scale atmospheric phenomenon known as the Southern Annular Mode, a broad climate index that describes the north-south position of the strong westerly winds that circle Antarctica. Human-caused global warming has driven a southward shift in these winds, and since the 1950s this shift has made calm, mild fall and winter weather increasingly common in New Zealand. When the researchers compared each year’s estimated arrival date in Australia with monthly readings of the Southern Annular Mode, a clear pattern emerged: the birds arrived later in years in which the weather in New Zealand was milder towards the end of their breeding season. In other words, the harshness of the New Zealand winter, historically the very reason for making the dangerous crossing, appears to be fading as a trigger for migration.

The pace of this change is what worries the scientists most. Bashar Jarayseh, first author of the study and a PhD student at the Max Planck Institute for Biological Intelligence, explained that most of the banded dotterels which migrate come from harsher inland alpine habitats, escaping the New Zealand winter for the milder conditions found across the Tasman. But the ocean crossing itself is dangerous, and if the time spent in Australia continues to shorten, the benefits of migration may shrink while the risk remains constant. Within a few decades, he suggested, migrating individuals may find themselves at a disadvantage compared with their resident counterparts. The researchers predict that this could eventually lead to what they describe as a behavioural extinction of migration, in which the entire population abandons the crossing and remains in New Zealand year-round. Such an outcome would leave the species more vulnerable to local shocks, for instance a single, unusually harsh winter striking the whole population at once.

The finding also carries broader scientific significance because of where it comes from. A great deal of what is known about climate change and bird migration originates in the northern hemisphere, where the dominant signal has been birds arriving at their breeding grounds earlier and earlier in spring as the planet warms. The banded dotterel adds a case from the far less studied southern hemisphere, and the direction of the shift is different: rather than advancing towards their breeding grounds, these birds are reaching their wintering grounds later each year. Luke Eberhart-Hertel, a co-author of the study at the Max Planck Institute for Biological Intelligence, emphasised that the observations of birdwatchers made it possible to detect this pattern across an entire continent, something that would be nearly impossible to achieve with traditional fieldwork alone. The team is now following individual dotterels using satellite tags, light loggers, or simply unique combinations of coloured leg bands, to study how their movements change from year to year.

That individual-level work addresses one of the deepest questions raised by the study: how does a population-wide shift in migration timing actually come about? There are several possibilities. Individual birds might each adjust their departure dates flexibly from one year to the next in response to local conditions. Alternatively, the change could come about through the composition of the population shifting, with more individuals opting to stay in New Zealand and fewer making the crossing, or with some birds switching their strategy entirely from being migrants to becoming year-round residents. Bart Kempenaers, who heads the Department of Ornithology at the Max Planck Institute for Biological Intelligence, noted that it remains to be seen whether there could be a tipping point beyond which the Tasman crossing is abandoned altogether. Understanding whether human-driven climate change disrupts the seasonal cues that individual birds use to time their journeys, and how such disruption affects their survival probability, is now a central goal of the ongoing research.

The answers may matter well beyond ornithological curiosity. The banded dotterel is already under pressure in New Zealand from introduced predators, and any additional stress on the population could complicate conservation efforts for the species. If migration is indeed a behaviour on the verge of disappearing in this population, conservation managers will need to know whether the remaining residents can cope with the environmental extremes that migrants once escaped, and whether the loss of the migratory habit reduces the genetic and behavioural flexibility of the species as a whole. The study also demonstrates the growing power of citizen science: millions of casual sightings, carefully aggregated and analysed, can reveal continent-scale ecological changes that no single research team could document on its own. As climate change continues to reshape the seasons of the southern hemisphere, the banded dotterel may become a textbook example of how quickly a long-standing migratory tradition can begin to unravel, and a warning that behavioural responses to warming do not always take the form scientists have come to expect from the northern hemisphere.

For now, the dotterels still make their crossing each autumn, lifting off from New Zealand’s beaches, braided rivers and volcanic uplands and striking out west over one of the stormiest stretches of ocean on the planet. Whether future generations of birdwatchers in Australia will continue to greet the double-banded plover each winter is an open question, one that the coming years of tracking and observation at the Max Planck Institute for Biological Intelligence are designed to answer. What the eBird record already makes clear is that the timetable of this migration is shifting fast, four days later with every passing year, and that the mild New Zealand autumns brought by a changing southern climate are quietly rewriting one of the region’s great seasonal spectacles.

Subject of Research: Climate-driven delay in the autumn migration timing of New Zealand banded dotterels to Australian wintering grounds

Article Title: A migration running out of time? New Zealand banded dotterels reach Australia later each year

Article References: A migration running out of time? New Zealand banded dotterels reach Australia later each year. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: banded dotterel, double-banded plover, migration, climate change, Southern Annular Mode, eBird, citizen science, New Zealand, Australia, Tasman Sea, shorebirds, behavioural extinction