Blog post by Steffen Oppel, Senior Conservation Scientist, RSPB Centre for Conservation Science
What could be hard about living on a remote island in the tropics? Most of us will envision blue skies, white sandy beaches, and no care in the world while relaxing on a beach, eating coconuts and sipping Piña Coladas. Life for seabirds breeding on those islands is quite different, as they must find enough fish in the surrounding waters to feed their chicks. New research shows that tropical seabirds breeding on a remote island have to fly almost three times as far to make ends meet than birds of the same species nesting near the coast of Africa.
Red-billed Tropicbird on Egg Island (Saint Helena) with a GPS logger © Edward Thorpe
Red-billed tropicbirds – as their name suggests – are seabirds with a big red bill that live in the tropics around the world. They plunge into the water to catch small fish, but how far they fly from their nest site to find food was poorly known. Researchers from the University of Barcelona, the Saint Helena Government, and the RSPB have attached small GPS loggers to tropicbirds at two colonies in the Atlantic Ocean over the past years, and published their results in the journal Marine Ecology Progress Series in December.
The two study colonies were located on a small island along the coast of Senegal, and on the remote island of Saint Helena, a UK Overseas Territory in the South Atlantic. The researchers found that the tropicbirds on St Helena flew on average about 600 km out to sea to find food while their partner was incubating the egg, while the birds in Senegal flew only 150 km. These differences were most likely because the waters around Senegal have much higher productivity, and therefore more food for seabirds, than the waters in the middle of the Atlantic Ocean. For the tropicbirds on St Helena that means that quickly nipping out to sea to grab some fish entails a one-week round trip of roughly 1500 km.
Map of the foraging tracks of Red-billed tropicbirds from a colony in Senegal (left) and on St Helena (right). Red lines are from birds whose partner was incubating an egg at the time of tracking, black lines are from birds that were feeding small chicks and therefore had to return home sooner.
So how do the tropicbirds find the fish? Unlike Masked Boobies or Ascension Frigatebirds, which fly into all directions from their breeding colony, the tropicbirds on St Helena only flew to the north. The waters to the north of St Helena are characterised by a greater diversity of large predatory fish, which may drive smaller fish to the surface where tropicbirds can catch them. Thus, knowing what other marine creatures help them to catch food may allow the tropicbirds to live in waters that have overall much less food available than the rich waters around the coast of Africa.