Australia’s most intense cyclone wiped out 90% of seabirds on an island

A new study has revealed a chilling consequence of climate change: the devastation of seabird populations by extreme weather events. Tropical Cyclone Ilsa, which struck Bedout Island, Australia in April 2023, is estimated to have wiped out 80-90 per cent of the island’s seabird population.

The paper, published in journal Nature Communications, noted that the cyclone Ilsa that crossed Bedout Island on April 13, 2023, wiped out thousands of seabirds of three species, turning the island into a mass graveyard.

Cyclone Ilsa was reported to be category 5 cyclone — a severe tropical cyclone with sustained winds of 230 kilometres per hour.

Located in the Timor Sea off the remote north coast, the entire island suffered extensive damage, with the coastal vegetation stripped away. 

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The study found that three species, particularly Masked Booby (Sula dactylatra bedouti), Brown Booby (S leucogaster) and Lesser Frigatebird (Fregata ariel) perished in the storm, with only 40 breeding Masked Booby, a putative endemic subspecies, recorded on the island 15 weeks later.

Seabirds are an important part of the ecosystem, contributing to the health of islands and the reefs that surround them. 

The birds connect the marine and terrestrial ecosystems by transporting marine-derived nutrients to terrestrial breeding, roosting, and nesting areas via guano deposition and other means.

“The frequency and intensity of such storms are likely approaching a threshold beyond which Bedout’s seabirds cannot readily recover, with cyclones hitting the island on average every seven years in recent decades,” it noted.

The authors identify intense winds and heavy rains as the main hazards associated with tropical cyclones. Every year, Australia experiences an average of 11 tropical cyclones, and their intensity and frequency are increasing, as are cyclones around the world. 

The Bedout Island, where the study was conducted, and Ashmore Reed are critical bird and biodiversity areas, accounting for nearly two-thirds of the world’s Kuyangarti/Lesser Frigatebird Fregata ariel and Purralyakura/Brown Booby Sula leucogaster populations.

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An aerial survey conducted by the researchers found no living seabirds of any species immediately after the cyclone passed on April 17. 

The researchers counted as many as 26,235 Brown Booby and 6,941 Masked Booby carcasses during a series of visits and surveys in June. While in July, the estimates for the former were 10,272 and 3,887 for the latter. 

“These estimates often exceed the known breeding population because of the limited sampling opportunity available and over dispersed data. We are therefore confident that nearly all Masked and significant numbers of Brown Boobies from Bedout Island died during Cyclone Ilsa,” the study said.

The scientists also discovered some unusual behaviour in the birds. Given that long-lived birds should prioritise personal survival over current reproductive success, the adult birds were expected to abandon their nests on the island. 

However, contrary to previous cyclone records, seabirds on Bedout Island did not flee. 

Carcasses of species such as the Lesser Frigatebird have been discovered in their nesting sites, for unknown reasons, as these birds are known to forecast cyclones based on wind, barometric pressure, and other factors and alter their behaviour. 

The cyclone hit the island when Boobies were incubating both young chicks and eggs, the study said. The strong bond between adult and chick may have contributed to adults’ reluctance to abandon their children and eggs. 

But it still does not explain the behaviour entirely, as Frigate birds would have possibly been in the early stages of egg laying during April. 

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According to researchers, these figures are conservative and could be higher because they allow them to determine how many eggs and chicks died as a result of sediment erosion during the cyclone. 

In the last 15 years, the island has seen at least three cyclones: George, Stan, and Islan, each separated by a seven-year gap. According to the researchers, the time between extreme weather events and their return has an impact on population recovery.  

“Like Bedout’s Masked Boobies, the critically endangered Abbott’s Booby (Papasula abbotti) breeds on a single island off Australia’s northwest coast. Severe storms have been linked with the destruction of up to 30 per cent of nests and fledglings in a single event and a decline in the number of adults attempting to breed in subsequent years,” it noted.

It warned that major losses in seabird colonies can significantly impact island habitats and species complexes. Extremely severe events such as cyclone Ilsa could alter ecosystem function or result in slow island recovery, they pointed.