Long-term changes in the size, structure and location of whiskered tern Chlidonias hybrida [L.] nests in deteriorating environmental conditions of a North African lake

Long-term changes in wetland habitats have been lately reported all over the world. Global warming and, in particular, irrigation and eutrophication of shallow lakes cause changes in vegetation, often in directions differing from the natural ecological succession. As a result, from the viewpoint of waterbirds, their nesting habitats deteriorate, which leads to changes in their breeding biology and distribution. In this paper the nesting of Whiskered Tern Chlidonias hybrida (L.) was studied in changing habitat of Lake Tonga, the El-Kala National Park, NE Algeria in 1996 and 2005–2006. Since the 1960s, there has been a clear trend of extensive development of emergent and floating-leaved vegetation, with up to 80% of the lake surface being currently covered during summer. Nesting colonies of Whiskered Terns are mostly connected with beds of the white water-lily (Nymphaea alba L.). The average distance of breeding colonies from the shore line increased from 176 m in 1966 to 786 m in 2005 and 933 m in 2006. Over the study period, average nest size got twice smaller. Nest diameter was on the average 470 mm in 1996 and 236 mm in 2005–2006. The nests also changed shape and composition. The size and shape of nests were correlated with measures of their composition. Variables characterizing size, shape and composition of nests indeed differed in response to ecological variation of habitat between and within years. Our results suggest that the main function of Whiskered Tern nests is providing a firm support for eggs, incubating adults and young nestlings, which would correspond to the nest support hypothesis of Collias and Collias (1984). Nest size was influenced by ecological conditions in two ways: directly, by changing the availability of construction materials, and indirectly, by changing the distribution of colonies and, thus, physical conditions for nests. We also suggest that some components of the changes in nest size, shape and composition may be linked with the social and sexual signaling system of Whiskered Terns.