A study by Maltese researchers has shed new light on the little-known pre-laying behaviour of Scopoli's Shearwaters, revealing that the seabirds undertake lengthy journeys across the Mediterranean before returning to nest on the Maltese Islands.
The research, recently published in the international ornithological journal Ardeola, marks the first time scientists have tracked the movements of Scopoli's Shearwaters (Calonectris diomedea) during the critical weeks leading up to egg-laying.
The study was conducted by ornithologists Marie Claire Gatt, Benjamin Metzger, Paulo Lago and Martin Austad, who used lightweight GPS devices to monitor the birds' movements in May, just before the breeding season begins.
Scopoli's Shearwaters, known in Maltese as Ċief, are endemic to the Mediterranean Sea, with around 3,000 breeding pairs nesting across Malta and Gozo. However, populations have been declining, raising concerns among conservationists.
Researchers discovered that female shearwaters leave their nesting sites for more than 20 days at sea while developing their single annual egg. Unlike many bird species, Scopoli's Shearwaters lay only one egg each year and cannot replace it if it is lost.
"The pre-laying period is crucial because females need to gather enough nutrients to produce their large egg and prepare for the demanding breeding season ahead," the researchers explained.
The GPS data revealed that females spend much of the nutrients they acquire during these lengthy foraging trips on egg production. Male birds also embark on extended feeding expeditions lasting around 15 days, but return with increased body reserves that enable them to undertake the first incubation shift, which can last up to a week without feeding.
This division of labour allows females to return to sea shortly after laying their egg to rebuild their depleted energy reserves.
The study also highlighted the vast distances travelled by the birds. Foraging routes extended hundreds of kilometres beyond Malta's territorial waters, with important feeding grounds identified off the coasts of Tunisia and Libya, as well as in Italian waters.
The findings underscore the international nature of seabird conservation, as protecting Malta's breeding population depends on environmental policies and conservation measures implemented across multiple Mediterranean countries.
While at sea, the birds face threats including accidental capture in fishing gear, overfishing and marine pollution. On land, invasive rats pose a significant danger to their eggs.
Researchers found that females often return to lay their egg before their male partner has completed his foraging trip. Forced to leave again soon after laying due to depleted energy reserves, females may leave eggs unattended for short periods, creating opportunities for rat predation.
Conservationists warn that rat populations can be boosted by human-generated litter, increasing pressure on native wildlife species that evolved in the absence of such predators.
The researchers say the findings reinforce Malta's international responsibility to monitor and protect its breeding seabird populations.
Data for the study was collected between 2010 and 2020 during two EU LIFE-funded conservation initiatives led by BirdLife Malta: the LIFE+ Malta Seabird Project and the LIFE Arċipelagu Garnija Project. The projects involved collaboration with BirdLife partners the RSPB in the United Kingdom and SPEA in Portugal, as well as Transport Malta and Malta's Ministry for the Environment.