Influence of Food on Numbers Breeding, Colony Size and Fidelity to Localities of Swift Terns in South Africa’s Western Cape, 1987-2000

• Published in: Waterbirds (De Leon Springs, Fla.) Fla.), 2003-03, Vol.26 (1), p.44-53
• Main Author: Crawford, Robert J. M
• Format: Article
• Language: English
• Subjects: abundance and distribution of prey; Anchovies; Biomass; Breeding; Breeding seasons; Breeding sites; colony size; fidelity to colony; Food availability; Ostriches; proportion breeding; Sardines; Sea birds; Sterna bergii; Swift Tern; Swifts
• ISSN: 1524-4695; 1938-5390
• DOI: 10.1675/1524-4695(2003)026[0044:IOFONB]2.0.CO;2

During 1987-2000, the numbers of Swift Terns (Sterna bergii) breeding in South Africa’s Western Cape varied between 1,449 and 5,668 pairs, distributed at 13 localities. Numbers were significantly related to the combined biomass of Anchovy (Engraulis capensis) and Sardine (Sardinops sagax), two of the main food items. In years when food was scarce, numbers breeding decreased by up to 67%. The sizes of breeding aggregations of Swift Terns were significantly related to the combined biomass of Anchovy and Sardine. Numbers of Swift Terns breeding at a locality ranged from 1-4,192 pairs, with an average of 750. Numbers breeding in discrete colonies ranged from 1-3,000 pairs, with an average of 513. Swift Terns showed low fidelity to breeding areas, 61% of which were used just once. This is attributed to their requirement to breed near to fish prey, and its variable distribution from year to year. Breeding by other seabirds probably often indicated safe sites for nesting. Swift Terns bred by themselves in 17%, with Hartlaub’s Gulls (Larus hartlaubii) in 77%, and with Crowned Cormorants (Phalacrocorax coronatus) in 18% of breeding attempts.