The Swift Tern paradox

In the Benguela region, four species of seabirds depend upon Cape Anchovy Engraulis capensis and Southern African Sardine Sardinops sagax for food. Three species from this community have declined over the past four decades (the endemic Cape Cormorant Phalacorocrax capensis, Cape Gannet Morus capensis and African Penguin Spheniscus demersus). This trend is thought to be due to a combination of factors, including environmental forcing in combination with competition with purse-seine fisheries.

While these birds are decreasing in South Africa, the population of the fourth species from this community – the Swift Tern Thalasseus bergii – has remained stable or even increased strongly since the early 2000s. The reasons for this particular trend are poorly understood. Contrary to the other seabirds from the community which display a strong attachment to traditional breeding grounds, Swift Terns may choose an entirely different breeding island from one year to the next. Hence, where the other species are exclusively dependent on anticipated resources that may or may not be available in the vicinity of their colony at time of breeding, Swift Terns may adopt a more nomadic pattern, presumably deciding which is the best breeding locality based on cues which involve knowledge of geographical variability in food abundance. Such flexibility, in conjunction with a high juvenile survival rate due to a strong dispersal capability and an increase in fish resource on the South Coast of South Africa since the early 2000s, has been proposed as the main factor explaining the positive trend of this species. 

The first aim of this research project is to better understand why the South African population of Swift Terns is increasing when other species from the same guild are decreasing. The second objective is to increase our knowledge of this species which, although it has a large distribution range, has received surprisingly little attention from research. Studies on Swift Terns in South Africa have been relatively scarce and have been mostly limited to counts, life history traits, dietary analyses or the link between food abundance and number of breeding pairs. Elsewhere in the world, there are few records of research on the ecology of Swift Terns, with only one existing comprehensive study, on an Australian population. This study is a collaboration with Richard Sherley and Peter Ryan and is the research project of an MSc student, Davide Gaglio.

Swift Tern with Cape Anchovy (© Richard Sherley).

Breeding Swift Tern (© Richard Sherley).