Weathering seabirds

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Sal Island in Cape Verde is home to the largest known breeding colony of red-billed tropicbirds in the western Atlantic, and possibly the world. Four breeding colonies have also recently been discovered on this volcanic island, with approximately 500 tropicbirds on Sal.

While it appears the colony is healthy, red-billed tropicbirds are in decline worldwide. As a result, researchers from Project Biodiversity and the University of Barcelona in Spain are working to better understand these overlooked birds.

It is vital to keep an eye on seabirds. “Seabirds are good ecological indicators of ocean health,” explains Albert Taxonera, project manager at Project Biodiversity. “Because they are at the top of the trophic chain, a decline in their population could signal problems with the ocean.” Seabird guano also forms a potent fertiliser for island plants found in their breeding colonies.

“We are taking blood, uropygial fat and feather samples to learn what they are eating and where, and to track organic and inorganic pollutants,” Taxonera explains. The team is also responsible for ringing and deploying tracking devices to better understand migration patterns, identify important foraging areas and learn about their breeding age, survival rate and other demographic parameters.

Research indicates around 1 000 pairs in Cape Verde. The naturally occurring cavities on Sal’s volcanic cliffs provide the ideal breeding grounds for this long-tailed bird. The tropicbirds’ preference for remote cliff faces makes them difficult to find and study.

Apart from their inaccessibility, another possible reason for the bird’s success on Sal is fish and squid availability during the breeding season. “Red-billed tropicbirds cannot travel far when breeding and require readily available food,” Taxonera says. In Cape Verde the birds seem to prefer flying fish and needlefish.

Tropicbirds. Photo: Kirsten Fairweather.

A variety of threats might have contributed to their overall decline. “Tropicbirds have evolved, breeding on islands without mammal predators,” Taxonera explains. Rats, cats and dogs have been introduced to almost every island in the world and pose a significant threat to seabirds. In early 2019 feral dogs killed 18 adults and five chicks. In addition to domestic animals, human consumption of seabirds has also hindered tropicbird numbers.

“Seabird populations are very sensitive to an increase in adult mortality,” says Taxonera. This is because of their slow breeding strategy – many seabirds, like tropicbirds, breed for the first time at five years of age. Generally the pair produces a single egg each year. Tropicbirds are monogamous – should a partner die it will take some years before the remaining partner will breed again,” he adds.

In recent years the ocean-faring frigate bird went extinct on Cape Verde and scientists are working to prevent the red-billed tropicbird following in the frigate bird’s flight path.

Written by Kirsten Fairweather, Project Biodiversity

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