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Swift Tern Population Dynamics and Foraging Behaviour

Swift terns Thallasseus bergii are single-prey-loaders, usually carrying one fish back to the nest at a time (© Richard Sherley)
The swift tern is a single-prey loader, usually bringing one fish back to the nest at a time.
Image © Richard B. Sherley.
The swift (or great crested) tern Thalasseus bergii represents something of a paradox today in South Africa. In the Benguela region, four species of seabirds depend upon anchovy Engraulis capensis and 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 prey shortages driven by changes in the marine environmental and pressure from local purse-seine fisheries.

 While these birds are decreasing in South Africa, the population of the fourth species from this community – the swift tern – has remained stable or even increased strongly since the early 2000s. The reasons for this particular trend are poorly understood. Terns are usually considered to be species which are particularly sensitive to changes in their environment, but in contrast to the penguin, gannet and cormorant 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. 
swift tern breeding colony on Robben Island.
Image © Richard B. Sherley.

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 distributional 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 the species, with only one comprehensive study in existence, on an Australian population.

This project is a carried out in collaboration with and funded by the Percy FitzPatrick Institute of African Ornithology. The PhD student, Davide Gaglio, working on the project is co-supervised with Dr Timothée Cook and Prof. Peter Ryan.