Scientists study Saba tropicbirds

The Red-billed Tropicbird, a unique seabird that largely nests on Saba, direly needs more protection from predators such as feral cats and rats. Scientists Mardik Leopold and Michiel Boeken from the Netherlands were on Saba for four weeks to study and ring the tropicbirds.

Many people may not be aware of this, but Saba is very important to this particular seabird. More than 35 per cent of the Caribbean population of this tropicbird species nests on Saba and St. Eustatius. The birds that nests on Saba and St. Eustatius represent about 17 per cent of the world-wide population of Red-billed Tropicbirds.

Sea bird specialist and marine biologist Leopold of Wageningen University and independent ecologist Boeken came to Saba in December to carry out further research on the tropicbird colonies, where the birds have their nest in the crevices between the rocks at Great Level, Tent Bay, Old Booby Hill and Pirate Cliffs. There are also some nests in Cove Bay and Kelbey’s Ridge, and a few more scattered around the island.

The vast majority of the nests are at Old Booby Hill, probably the largest colony in the world. The scientists found several hundred nests of which about 150 were being occupied by adult tropicbirds, ready to lay or guarding their single egg or chick. Leopold and Boeken have counted the numbers of adult birds, chicks and eggs at the different colonies. They weighed and measured the adults, the eggs and the chicks.

Adults and chicks were ringed, the latter only if they were large enough. The scientists ringed 200 birds with a stainless-steel band, which lasts much longer than the aluminum bands used in the past. Leopold and Boeken also analyzed the types of fish that the tropicbirds eat. The birds’ diet mostly consists of flying fish, but several other species were found as well.

Comparing the data of past four weeks with those of previous research carried out by Boeken in 2011/2012, they found a dramatic decrease in number of breeding pairs in the colonies at the southern coastline. By comparing egg size and chick growth of both study periods, they showed that food at sea is not the limiting factor.

The good news is that the remaining birds are reproducing again, whereas in 2012 no chick grew older than four days, due to predation by feral cats. It is clear that the culling of cats has had a positive effect. At Old Booby Hill, the number of breeding pairs seemed at least to be stable, but predation of chicks (feral cats) and eggs (rats) had increased clearly.

Leopold and Boeken will write a scientific article about their findings on Saba. They carried out the research on their own initiative. “We still know so little about these birds. That is why more research is needed. We hope to give more body to this through Wageningen University,” said Leopold.

The chicks remain on the nest for three months until they are big enough to fly. Remaining in the nest for this period, along with the fact that the nest is located on the ground makes the chicks vulnerable for both rats and feral cats. These cats are a huge problem for the tropicbirds and eradication is necessary in order to preserve the tropicbird colonies on Saba.

Adult tropic birds return to the location where they were born to lay their eggs and raise their chicks. Exchange between colonies is extremely rare, and has never been observed to occur between islands, despite extensive ringing on both Saba and Statia.

This isolation makes this species vulnerable for local extinction, as has happened with the White-tailed Tropicbird on Saba during the first decade of this century. Continued monitoring is important to signalize further changes in the population.

Leopold and Boeken shared their findings during a well-attended presentation at Long Haul Restaurant in the Windwardside on Thursday evening. The scientists thanked the Saba Conservation Foundation (SCF) and the Public Entity Saba for their cooperation. They thanked the Saba people for their support.

The Daily Herald