Brown skuas coastward shift puzzles scientists

More than three decades after South Africa celebrated the successful eradication of feral cats from Marion Island, the gradual recovery of native burrowing petrels might have been expected to support an increase in brown skua breeding territories inland.

Instead, researchers from the University of Cape Town (UCT) and Nelson Mandela University have discovered that the breeding distribution of brown skuas – large, formidable seabirds that sit close to the top of the island’s food web – has shifted in the opposite direction. Their breeding territories are increasingly concentrated along the coastline, despite modest increases in some of the burrowing petrel populations on which inland-breeding skuas traditionally prey.

The unexpected finding, published in Polar Biology, illustrates that ecological recovery following the removal of an invasive predator can be complex and may not follow simple predictions.

“Our findings show that even after one of the world’s most successful invasive species eradication programmes, an ecosystem does not necessarily return quickly or predictably to its former state. Predators can respond in surprising and sometimes counterintuitive ways,” said Michelle Risi, the study’s lead author and a PhD candidate at UCT’s FitzPatrick Institute of African Ornithology.

Marion Island, one of South Africa’s remote sub-Antarctic territories, has long served as a natural laboratory for studying ecological change. Brown skuas are important predators and scavengers, feeding on penguins, petrels and carrion. Changes in their abundance, breeding distribution and productivity can provide useful indicators of broader ecological change.

Domestic cats were introduced to Marion Island in 1948 to control mice around the research station. Instead, they multiplied into a feral population of about 2 000 animals, killing an estimated 450 000 burrowing petrels every year before the last cat was eradicated in 1991. The programme has since become one of the world’s landmark conservation successes.

Study findings

Scientists believed that the recovery of petrel populations would eventually lead to more skuas establishing breeding territories inland, where petrels historically formed an important part of their diet. The new study revealed that this prediction has not materialised.

During the most comprehensive island-wide survey in more than three decades, the research team documented 983 brown skua breeding territories in 2023 – surpassing the 878 territories recorded during the benchmark survey conducted in 1987 and 1988. Yet, while the overall breeding population has marginally increased, the most striking finding was the change in where the territories occurred.

Breeding territories along the coast have increased by 39% since the late 1980s, while inland territories have declined by 31%. More than 90% of territories are now found in coastal lowlands, clustered around major penguin colonies. “This shift surprised us,” said Risi.

“The skuas appear to be concentrating around prey populations that are themselves declining.”

Burrowing petrel populations have shown modest recovery since cats were eradicated, though this progress has been slowed by invasive house mice which prey on petrel eggs and chicks. Meanwhile, several penguin populations – the skuas’ primary coastal prey – have declined during the same period.

“The shift towards the coast is unexpected because the skuas appear to be concentrating around prey populations that are themselves declining,” said Risi.

The findings suggest that brown skuas continue to rely heavily on penguins, rather than switching back to burrowing petrels as scientists had anticipated. Factors such as prey accessibility, strong loyalty to established breeding sites and uneven recovery of different petrel species may all be contributing to the shift.

“Despite these changes, the population appears resilient,” noted Risi. Researchers found slightly more breeding territories than in the late 1980s, while fledgling production has remained relatively stable over the past decade. Average fledgling production measured 1.10 chicks per breeding pair between 2010 and 2022, only slightly below the 1.26 recorded in the late 1980s.

But the future remains uncertain. Marion Island is entering another period of ecological change. Climate change is reshaping Southern Ocean ecosystems. Highly pathogenic H5N1 avian influenza has already reached the island, killing dozens of skuas, and South Africa is preparing an ambitious programme to eradicate invasive house mice – another major threat to burrowing seabirds.