Maltese research reveals the long sea journeys of Scopoli's Shearwaters before egg-laying

Researchers in Malta have uncovered the wide-ranging movements of Scopoli's Shearwaters in the crucial period before the birds lay their eggs, tracking journeys that reach far beyond the country's own waters.

Using lightweight bird-borne GPS devices, ornithologists Marie Claire Gatt, Benjamin Metzger, Paulo Lago and Martin Austad followed the seabirds in May, just before the females laid, and published their findings in the international ornithological journal Ardeola.

Scopoli's Shearwaters (MT: Ċief; Calonectris diomedea) are a seabird found only in the Mediterranean. Around 3,000 pairs nest on the Maltese Islands, but here and elsewhere this ocean-wandering bird has been falling in numbers.

Scopoli’s Shearwaters at sea (Photo: Benjamin Metzger)

In shearwaters and related species, females lay only one egg a year. If that egg is lost, they cannot lay again until the following year, which makes the pre-laying period vital: it is when the female gathers the nutrients she needs to produce the egg and prepare for a long breeding season.

The study found that female Scopoli's Shearwaters leave the nest for a foraging trip of more than 20 days at sea, during which they develop their single, large egg. Producing the egg appears to use up most of the nutrients the female takes in over that time.

Despite its importance, the pre-laying movements of Scopoli's Shearwaters had never been tracked until now. The same group of researchers had earlier published the first pre-laying movements of the vulnerable Yelkouan Shearwater (MT: Garnija; Puffinus yelkouan).

Their male partners also make long trips, lasting around 15 days, but they come back to the nest fattened up and ready to take the first week-long shift sitting on the egg while fasting. This frees the females to return to sea and rebuild their body reserves.

These foraging trips cover hundreds of kilometres beyond Malta's waters. The seas off Tunisia, Libya, and Libya are key feeding areas, so protecting this wide-ranging bird depends on multiple countries' policies, complicating conservation efforts. At sea, threats include fisheries bycatch, overfishing, and pollution.

GPS tracks of ten female Scopoli’s Shearwaters, each lasting over 20 days, just before returning to their nest in Malta to lay (Photo: Benjamin Metzger)

On land, invasive rats can eat their eggs if left unattended. Researchers found that females often return to lay eggs while the male is away foraging. Drained of energy, females then must head out to sea soon after, leaving the eggs at risk until the male returns.

Rats were not part of the landscape these seabirds evolved in, and litter left by people raises rat numbers and, with it, their impact on native wildlife.