A subspecies of the southern skua (Catharacta antarctica) has undergone a massive population crash in the Falklands, declining by almost 50% over the past five years according to a new study.
Also known as the Falkland skua, the ultimate cause of this subspecies’ decline on New Island, in the west of the Falkland Islands, is unknown. However, it seems to be linked with chronic low breeding success in recent years.
New Island houses the largest known colony of the Falkland skua, a large, gull-like, ground-nesting seabird.
Two surveys of the skuas nesting on New Island were conducted in 2004 and 2009 by Dr Paulo Catry of the Museum of Natural History in Lisbon, Portugal, along with colleagues from Portugal and the UK. The results are published in the journal Polar Biology.
“Although brown skuas have been the subject of many studies, virtually nothing has been done on the Falklands subspecies,” says Dr Catry.
Low breeding success
The surveys revealed that overall, the number of Falkland skua territories on New Island reduced by 47.5% in the five years between the two surveys.
The decline on New Island has raised some serious concerns. Dr Catry emphasises that “Long-lived seabirds like skuas usually change their numbers slowly and this situation cannot be considered as ‘normal’.”
Skuas are ground-nesting birds which generally have a high breeding success, with each pair raising a chick a year on average. However, the researchers found that pairs of the Falkland skua on New Island have a far lower success rate, producing as few as 0.28 chicks on average each year.
Even more surprising is that other seabirds nesting on the same island have not shown similar declines over the same period.
Exploring possible options
It is currently unclear as to whether the decline in reproductive success of the Falkland skua has been accompanied by a rise in the number of deaths of adult skuas.
Dr Catry and his team are exploring several possible explanations for the dramatic decline of this southern skua subspecies.
One possibility is that the Falkland skua is being outcompeted by the striated caracara (Phalcoboenus australis), as both species appear to feed on the same prey.
More concerning is the prospect that the recent declines in skua populations may be linked to wider problems in the marine environment.
“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.”
Read the paper published in the journal Polar Biology.
Helen Roddis, ARKive Species Text Author