Latitudinal trends, population size and habitat preferences of the Damara Tern Sterna balaenarum on Namibia's desert coast

The Damara Tern Sterna balaenarum is a rare, near-endemic breeder along the southwestern coast of Africa. Found predominantly in Namibia, its world population was once believed to be 4000 birds. Population surveys in the northern Namib Desert have recently estimated that 12,000 adults exist. The aims of this study were to (1) determine the population status along the southern coast, thereby completing Namibian surveys, (2) assess latitudinal trends (over 10̀) in relation to ocean upwellings for the entire coast and (3) determine breeding habitat preferences. This was completed in 1994 and 1996 using random sampling techniques. Survey squares of 1 km², randomly plotted on 1:50,000 maps, were located in the desert with a Global Positioning System (GPS) unit and searched for breeding terns. A total of 1780 km² was sampled in the southern desert and extrapolations indicated that about 1450 adult terns occur in this region. This gives a global population estimate of 13,500 adult Damara Terns. Significant habitat preferences were apparent with terns nesting most often on gravel plains in the north but switching to salt pans in the south. Lower salt pan temperatures in the south may account for these trends. The peak density was in central areas (23|dGS) with a decreasing density north and south along Namibia's 1470 km-coast. This is parallel to recent findings for linear shorebird densities and macro-invertebrate density. This suggests that primary productivity of the Benguela is highest in the central regions and not where the upwellings occur 400 km south. We propose a delayed blooming effect to account for these trends and conclude that this diminutive tern is influenced in many aspects of its life history by Namibia's cold offshore upwellings.