Abstract
The Ebro Delta holds a large seabird community, including a common tern ( Sterna hirundo ) local population of 3,085 pairs in 2000 which breeds scattered in several colonies. At El Canalot colony, 1,178 (1999) and 1,156 pairs (2000) of this species bred distributed in 32 and 38 sub-colonies respectively. These sub-colonies varied in size from 1 to 223 pairs and were placed near the main breeding colonies of yellow-legged gulls (Larus cachinnans) and Audouin´s gulls (L. audouinii), which are potential egg-predators of terns. We studied egg predation during 1999 (6 sub-colonies) and 2000 (27 sub-colonies). Overall, we found that 10.6% of the nests in 1999 and 16.7% in 2000 suffered partial or total egg predation, being total in 81.1% of the predatory events. Predation was significantly higher in small sub-colonies (< 11 pairs): 49.4% in 1999 and 75.5% in 2000. Only attacks from yellow-legged gulls were observed, and defence behaviour of terns was significantly more frequent against this gull species (40.5 hours of observation), suggesting that in most cases the egg predation recorded was due to this species. Probability of egg predation was significantly and negatively correlated with distance to the nearest yellow-legged gull sub-colony, although this relationship was no more significant after adjustment for sub-colony size. On the other hand, distance to the nearest Audouin´s gull sub-colony did not show any effect. Our results suggest that the impact of large gulls (at least yellow-legged gulls) upon smaller seabirds breeding in the area might be important, especially when they are breeding in small sub-colonies. Further studies are needed to analyse the general impact of large gulls upon the breeding populations of other colonial bird species in the area.
Highlights
Most seabird species breed in colonies, resulting in conspicuous aggregations of adult individuals, eggs and/or chicks, which are a readily exploitable source of food for many predators
Of this species bred distributed in 32 and 38 sub-colonies respectively. These sub-colonies varied in size from 1 to 223 pairs and were placed near the main breeding colonies of yellow-legged gulls (Larus cachinnans) and Audouin’s gulls
We studied egg predation during 1999 (6 sub-colonies) and 2000 (27 sub-colonies)
Summary
Most seabird species breed in colonies, resulting in conspicuous aggregations of adult individuals, eggs and/or chicks, which are a readily exploitable source of food for many predators. Avian species which prey on seabird colonies are usually natural predators, and their effects are commonly weak (Burger and Gochfeld, 1994). Environmental changes induced by man, have favoured the proliferation of some avian predators, especially large gulls Terns seem to be highly sensitive species owing to their small size (Burger and Gochfeld, 1994), and because their traditional breeding areas are not uncommonly placed close to large gulleries Many studies have reported adverse effects of large gulls on tern species, including kleptoparasitism (Stienen and Brenninkmeyer, 2002), nest site competition (Kress et al, 1983; Sadoul et al, 1996), and predation on eggs and chicks Many studies have reported adverse effects of large gulls on tern species, including kleptoparasitism (Stienen and Brenninkmeyer, 2002), nest site competition (Kress et al, 1983; Sadoul et al, 1996), and predation on eggs and chicks (e.g. Saliva and Burger, 1989; Burger and Gochfeld, 1991; Becker, 1995, Guillemette and Brousseau, 2001)
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