A recent increase of swift terns Thalasseus bergii off South Africa – The possible influence of an altered abundance and distribution of prey

Abstract

In the 2000s, there were large increases in the numbers of swift terns Thalasseus bergii breeding in the Western and Eastern Cape Provinces of South Africa, which are most plausibly attributed to good recruitment and to an increase in the proportion of mature birds breeding. Numbers increased coincidentally with a greatly increased abundance of two of the main prey species of swift terns, sardine Sardinops sagax and anchovy Engraulis encrasicolus, and remained high as these resources decreased. After 2005, numbers of swift terns breeding in the north and central portions of the Western Cape decreased, whereas numbers breeding farther south in that province increased. This followed displacements to the south and east of sardine and anchovy. In southern Africa, swift terns show low fidelity to breeding localities, which enables a rapid adjustment of the location of breeding to an altered availability of prey. For two seabirds that feed mainly on sardine and anchovy, but once breeding show high fidelity to colonies, African penguin Spheniscus demersus and Cape gannet Morus capensis, proportions breeding in the south and east also increased, but there were substantial decreases in overall numbers breeding in the Western Cape.