Decline of the Southern Skua in the Falkland Islands
The Falkland Islands, a British overseas territory in the South Atlantic Ocean near the coast of South America, is home to several unique indigenous bird species. One of them, Catharacta Antarctica, also known as the Southern Skua or the Falklands Skua, is in serious decline. Over the past five years, their population has gone down almost 50 percent. The exact reasons are unknown, but some experts suspect the decline is due to low breeding success and increased competition for resources. Some fear that the problems with the skua are linked with an unhealthy Patagonian marine ecosystem.
The Falkland Skua is a subspecies of the Great Skua, a carnivorous gull-like species. It lives on the Falkland Islands and travels north when not breeding. It feeds on fish, small mammals, and eggs. Often, it scavenges for carrion and employs kleptoparasitism, which means it saves energy by stealing food from another predator. They nest on the ground because the Falklands have no native trees, only grasses and shrubs.
Two surveys of the skuas nesting on New Island, home of the largest population to west of the Falkland main islands. The surveys were conducted in 2004 and 2009 by Dr. Paulo Catry of the Museum of Natural History in Lisbon, Portugal as well as other researchers from Portugal and the UK. "Although brown skuas have been the subject of many studies, virtually nothing has been done on the Falklands subspecies," says Dr Catry.
The surveys, spaced five years apart, showed a dramatic decline of 47.5% on New Island. This has caused worry because the Falkland Skua has a long life span. Such a dramatic change in so short a time means the situation is extremely abnormal.
One theory is that the decline is due to low breeding success which affects the birth rate. Typically, the Falkland Skua has a high rate of breeding success. Each breeding pair raise one chick a year on average. The researchers found that the new rate on New Island is only 0.28 chicks per year. This is despite the fact that other nesting seabirds on New Island have not shown similar declines in breeding rates.
A higher death rate of adult Skuas is suspected of compounding the low birth rate. Dr. Catry's team believes the species may be struggling to compete with another avian predator, the Striated Caracara, a.k.a. the Johnny Rook, which feeds on the same prey. Part of the Falconidae family, this bird is extremely aggressive, attacking everything from baby lambs to sick sheep. It will even steal red pieces of clothing hanging out to dry because they are the same color as meat. The Johnny Rook is a formidable foe for the Skua and a thorn in the side of Falkland sheep farmers.
The greatest fear, however, is that the Skua decline is related to a decline in the marine ecosystem. "Falkland skuas are top predators of marine ecosystems. They will take fish, squid, crustaceans, and they are also important predators of other seabirds," says Dr Catry. "If something is not well with them, it may mean that something is not well with the rich Patagonian shelf ecosystem."
The study has been published in the journal, Polar Biology.
Link to published article: http://www.springerlink.com/content/y51776v813542706/
Photo credit: Paul Gale