The cartwheeling tropicbirds of Christmas Island have long been a common sight in the northern Australian skies of the Indian Ocean with their elongated tails streaming in the sea breeze.
But a sharp decline in the population of one sub-species — the red-tailed tropicbird, or silver bosun — has brought it to the attention of the threatened species commission.
An estimated 3,350 individuals are left, a number that represents a 36-per-cent population drop over a 30-year period, mostly attributed to predation by cats and rats.
The silver bosun is best known for its immense tail feathers, known as streamers, that can extend for 35 centimetres behind the bird which can reach about one metre in length.
The bird's main population can be found on Christmas Island, an Australian territory that is about 1,550 kilometres off the coast of Exmouth in Western Australia.
The bird is also found at the Cocos (Keeling) Islands, Ashmore Reef and Rowley Shoals.
In the past three decades the silver bosun has stopped breeding at the Houtman Abrolhos islands off the mid-west coast and Sugarloaf Rock in the south west.
Its relative, the white-tailed tropicbird — which only breeds on Christmas Island and is referred to as the golden bosun due to its unique apricot plumage — has already garnered an endangered species listing.
Climate change a factor
Threatened Species commissioner Fiona Fraser said threats to the bird included cats, rats and yellow crazy ants.
"But there's other broad-ranging impacts to the birds," Dr Fraser said.
"Climate change is already having some impacts, most likely, and likely to have further impacts into the future.
"[This is] both directly but also indirectly through increasing frequency of cyclones, which can impact nesting habitat and availability of food sources."
Christmas Island is home to about 80,000 nesting seabirds every year and also has several land-based species including endemic species found nowhere else in the world.
Jungle warfare
Feral cats on Christmas Island have been responsible for the decline or disappearance of several species in recent years.
Forest skinks, which were once common across the island until the 1990s are now extinct, with the last-known individual dying in captivity in 2014.
Similarly, the Christmas Island pipistrelle, a tiny bat, became extinct in 2009.
Since 2015, more than 1,000 felines have been eradicated from the island, and the goal of removing all cats from the island by 2025 remains in place.
But ridding the remote island of felines, with its craggy limestone landscape that is covered in thick forests, is no easy feat.
Dr Fraser said the task of removing the last of the cats would be difficult.
"You're really at war with a really intelligent creature when you're doing this," she said.
"Somewhere like Christmas Island, it's a really dense vegetation.
"Those cats that are living and hunting there are very adapted to that environment."
Dr Fraser said current cat and crazy ant controls have already resulted in positive gains for the various tropicbird species.
Last year, Parks Australia received more than $4 million in funding for a cat eradication program.
"This program commenced in July 2021 and is achieving strong results, using new techniques — such as thermal imaging at night and sophisticated grooming traps — to target the remaining feral cats," A Parks Australia spokesperson said.
"Parks Australia's current target date for eradication is 2025 and will remain under active review as the program progresses."
The removal of cats previously raised concerns about the possible increase in rat populations.
But researchers found rat density was lower than compared to other islands and red crabs could be competing with and preying on them to keep populations down.
Public submissions regarding the listing of the silver bosun will remain open on the federal environment department's website until January 25.
