Christmas Island tropical birds have long been a common sight in the skies north of Australia’s northern Indian Ocean with their elongated tails flapping in the sea breeze.
- A Christmas Island bird known for its spectacular tail feathers could soon join the list of vulnerable species
- The Indian Ocean red-tailed tropicbird, or silver bosun, has seen a population decline of 36% in 30 years.
- The public presentation of the vulnerable list of the silver bosun is open until January 25.
But a sharp decline in the population of one subspecies, the red-tailed tropicbird or silver bosun, has drawn the attention of the endangered species commission.
An estimated 3,350 individuals remain, a number that represents a 36 percent population drop over a 30-year period, attributed primarily to predation by cats and rats.
The Silvery Bosun is best known for its immense tail feathers, known as streamers, which can extend for 35 centimeters behind the bird which can reach around a meter in length.
The main population of the bird can be found on Christmas Island, an Australian territory that lies about 1,550 kilometers off the coast of Exmouth in Western Australia.
The bird is also found on Cocos (Keeling) Islands, Ashmore Reef, and Rowley Shoals.
In the past three decades, the silvery bosun has stopped breeding on Houtman Abrolhos Islands off the Midwestern coast and Sugarloaf Rock in the Southwest.
Its relative, the white-tailed tropicbird, which only breeds on Christmas Island and is known as the golden bosun because of its unique apricot plumage, has already garnered an endangered species list.
Climate change a factor
Threatened Species Commissioner Fiona Fraser said threats to the bird included cats, rats and yellow crazy ants.
“But there are other far-reaching impacts for birds,” said Dr. Fraser.
“Climate change is already having some impacts, most likely, and is likely to have more impacts in the future.
“[This is] both directly and indirectly through increased frequency of cyclones, which can affect nesting habitat and the availability of food sources.”
Christmas Island is home to around 80,000 nesting seabirds each year and also has several terrestrial species, including endemic species found nowhere else in the world.
Feral cats on Christmas Island have been responsible for the decline or disappearance of several species in recent years.
Forest skinks, 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 small bat, became extinct in 2009.
Since 2015, more than 1,000 cats 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 cats, with its rugged limestone landscape covered in thick forests, is no easy task.
Dr. Fraser said that the task of eliminating the last of the cats would be difficult.
“You’re really at war with a really intelligent creature when you’re doing this,” he said.
“Somewhere like Christmas Island, it’s 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 various tropical bird species.
Last year Parks Australia received over $4 million in funding for a cat eradication programme.
“This program started in July 2021 and is achieving good results, using new techniques, such as nighttime thermal imaging and sophisticated grooming traps, to target 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 the cats raised concerns about possible increases in rat populations.
But the researchers found that the rat density was lower than on other islands and red crabs could be competing with them and taking advantage of them to keep populations low.
Public filings regarding the listing of the silver bosun will remain open on the website of the federal department of the environment until January 25.