Frigatebird returns to nest on Ascension for first time since Darwin

Endangered species may be saved from extinction after eradication of feral cats that had been eating its chicks
Robin McKie, science editor The Observer Saturday 8th December

One of the world's rarest seabirds has returned to remote Ascension Island in the Atlantic 150 years after its colony was wiped out by feral cats. Last week ornithologists spotted two nests containing eggs being guarded by Ascension frigatebirds, the first of the species to breed there since Charles Darwin visited the island in the early 19th century.

Ascension frigatebirds only survived in a small colony on a nearby rocky outcrop where they were considered to be highly vulnerable to outbreaks of disease and oil spills. But now they have returned to the island after which they are named, raising hopes that the vulnerable bird may be rescued from extinction.

The news marks the success of a project which has cost UK taxpayers more than £500,000 and has involved the eradication of hundreds of feral cats that had been eating frigatebird chicks.

"We are absolutely overwhelmed," said Derren Fox, a conservation officer based on Ascension. "We thought it would take decades for the Ascension frigate to come back and breed after we had got rid of the island's feral cats. But we have already succeeded after only a few years. This suggests we have a real chance of saving the Ascension frigate."

The project's success also raises hopes of saving colonies of other species threatened by feral animals. These include populations of seabirds and amphibians on Montserrat, Gough Island and South Georgia, which are all ravaged by rats, mice and other wild creatures.

In the early 19th century, Ascension Island was home to more than 20 million seabirds, mainly masked boobies, black noddies, brown noddies and Ascension frigatebirds. The frigatebird was considered to be the most important because it was unique to the island. Adults are about 30 inches in length while males have distinctive red sacs on their chests which they inflate during courtship.

Around 1800, rats – accidently introduced by settlers – began to kill off chicks. Cats were imported to kill the rats but instead joined in the killing of frigatebird chicks. "By the time Darwin visited the island in 1836, there were only a few frigatebirds left and the last few were killed off not long after he left," said Clare Stringer of the RSPB, which has played a key role in re-establishing the bird on Ascension. Only a small colony of around 10,000 survived on Boatswain Bird Island, a rocky outcrop off Ascension's east coast which could not be reached by cats.

In 2002, the RSPB – backed with funding from the Foreign Office – launched a programme to eradicate Ascension's feral cats. "It was slightly tricky," said Stringer. "We had to avoid killing islanders' pet cats and kill only feral animals. Owners were told to collar and microchip their pets. Then traps were laid and feral cats caught in these were put down."

In 2006, Ascension was declared to be free of wild cats. "It has taken six years to get frigatebirds to start to recolonise the island since we got rid of the feral cats and frankly it could have taken much longer," said Fox, who – with fellow conservation officer Stedson Stroud – has been monitoring the island for signs of the frigatebird's return. "We now have two nests being tended by parent birds and that should encourage a lot more to settle here in future."

http://www.guardian.co.uk/environment/2012/dec/08/frigatebird-returns-to-ascension

Seabird recovery after cat eradication on Ascension

Ratcliffe, N., Bell, M., Pelembe, T., Boyle, D., Benjamin, R., White, R., Godleya, B, Stevenson, J. & Sanders, S. (2010). The eradication of feral cats from Ascension Island and its subsequent recolonization by seabirds. Oryx, 44(1), 20.

The introduction of mammal predators to islands often results in rapid declines in the number and range of seabirds. On Ascension Island the introduction of cats in 1815 resulted in extirpation of large seabird colonies from the main island, with relict populations of most species persisting only in cat-inaccessible locations. We describe the eradication of feral cats from this large and populated island. The campaign had to minimize risk to humans and maintain domestic animals in a state that prevented them re-establishing a feral population. Feral cat numbers declined rapidly in response to the strategic deployment of poisoning and live trapping, and cats were eradicated from the island within 2 years. During the project 38% of domestic cats were killed accidentally, which caused public consternation; we make recommendations for reducing such problems in future eradications. Since the completion of the eradication campaign cat predation of adult seabirds has ceased and five seabird species have recolonized the mainland in small but increasing numbers.

Breeding success of seabirds at Ascension was low compared to that of conspecifics elsewhere, and the roles of food availability, inexperience of parent birds and black rat predation in causing this warrant further investigation. It is likely that the low breeding success will result in the rate of increase in seabird populations being slow.

Ascension frigatebird - the return continues

... For over a century, these seabirds have been confined to breeding on the small outlying islet, Boatswainbird Island, after taking refuge there from the feral cat population on Ascension’s mainland. Cats were introduced onto the island in the early 19th Century to control introduced rats and mice, but instead made a significant dent on the seabird populations. The RSPB initiated a project in 2002, supported by funding from the Foreign and Commonwealth Office, to remove feral cats from the island that proved successful, with Ascension being declared feral cat free in 2006. The seabirds began to return to nesting on the mainland immediately, starting with the masked boobies, but it wasn’t until 2012 that the island’s most iconic seabird began its return.

From RSPB 

Recovery of seabirds after the extirpation of cats

Hughes, B., Martin, G. R., & Reynolds, S. J. (2008). Cats and seabirds: effects of feral domestic cat Felis silvestris catus eradication on the population of sooty terns Onychoprion fuscata on Ascension Island, South Atlantic. Ibis,150(s1), 122-131.

The population of Sooty Terns Onychoprion fuscata breeding on Ascension Island in the Atlantic Ocean was monitored over 17 years (1990–2007). This period spanned the programme of feral Domestic Cat Felis silvestris catus eradication from the island, which commenced in 2001 with the last Cat recorded in 2004. We report on the abundance of Sooty Terns and Black Rats Rattus rattus before and after Cat eradication.

The Sooty Tern breeding population in the 1990s averaged 368 000 and Cats were killing Terns at an average rate of 33 adults per night. Following Cat eradication, adult Terns are no longer predated. However, egg predation by both Rats and Common Mynas Acridotheres tristis has continued with Mynas destroying more eggs than Rats. Unexpectedly, we observed a change in Rat predatory behaviour.
Following Cat eradication, Rats have become a major predator of Sooty Tern chicks. Despite this change, the Tern population has shown a season-on-season increase since Cat eradication, 48.8% in 2005, 8.2% in 2006 and 6.1% in 2007, and the breeding population increased to 420 000 birds in 2007. Incubation success improved from 66.0 to 84.4% during Cat eradication, before dropping down again to 67.9% after Cats were eradicated and Rat control measures were introduced. Index traplines were set for Rats and Rat numbers fluctuated widely immediately after Cats were eradicated but there were no significant differences that could be attributed to changes in Cat numbers. Ascension Island Sooty Terns breed every 9.6 months and juveniles defer breeding for seven seasons. Hence 2008 is the first year in which an increase in the breeding Sooty Tern population directly attributable to Cat eradication is likely to be detected. We conclude that long-term monitoring is essential to guide conservation practice even in this relatively simple predator–prey system.

Stamps celebrate seabird return to Ascension island

Stamps celebrate seabird return