Prey delivery to a breeding colony of Caspian Terns Hydroprogne caspia on the west coast of South Africa

Abstract

The Caspian Tern Hydroprogne caspia is widely distributed, but it is rare and little studied in southern Africa. We recorded the prey delivered by Caspian Terns to a breeding colony near the Berg River Estuary on the west coast of South Africa over two seasons (2013 and 2014) using a non-invasive photo-sampling approach. A total of 1 541 prey were identified (90% of prey photographed). At least 18 fish species were recorded, including both marine and estuarine species, indicating that breeding terns foraged in the Berg River Estuary, 7 km from the colony, as well as in St Helena Bay. Southern Mullet Liza richardsonii were the main prey in both seasons (57.6% overall), followed by gobies (15.6%, including Pelagic Goby Sufflogobius bibarbatus), Elf Pomatomus saltatrix (4.7%) and Cape Silverside Atherina breviceps (3.2%). The standard length of mullet ranged from 30 mm to 215 mm (mean ± SD: 101 ± 36 mm). Parents delivered significantly smaller mullet to small chicks than large chicks and partners. The terns lost 3.1% of prey to kleptoparasitism by Kelp Gulls Larus dominicanus (2.6%) and conspecifics (0.5%). Attacks occurred while in flight or as they landed to deliver their prey. Some adults ‘pseudo-swallowed’ prey items while pursued in flight to deter kleptoparasitic attacks. Our study confirms that Caspian Terns exploit a wide range of prey and it is unlikely that the abundance of suitable prey limits their population in the region.