Predation on common tern eggs by the yellow-legged gull at the Ebro Delta

Abstract

SUMMARY: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 subcolony 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. -El Delta del Ebro mantiene una importante comunidad de aves marinas, incluyendo una colonia de charrán común (Sterna hirundo) de 3085 parejas en el año 2000, que se divide en diversas subcolonias. En la colonia de El Canalot criaron 1178 parejas en el año 1999 y 1156 en el año 2000, distribuidas en 32 y 38 subcolonias respectivamente. Estas subcolonias oscilaron entre 1 y 223 parejas y se situaron cerca de las principales colonias de gaviota patiamarilla Larus cachinnans y gaviota de Audouin Larus audouinii, que son potenciales depredadores de huevos de charrán. La depredación de huevos se estudió en 6 subcolonias en 1999 y en 27 en el año 2000. En conjunto, encontramos que el 10,6% de los nidos en 1999 y el 16,7% en el año 2000 sufrieron predación total o parcial de huevos, siendo total en el 81,1% de los eventos predatorios. La depredación fue significativamente mayor en las subcolonias pequeñas (<11 parejas): 49,4% en 1999 y 75,5% en el año 2000. Sólo se observaron ataques por parte de gaviotas patiamarillas y la conducta defensiva de los charranes fue significativamente más frecuente frente a esta especie (40,5 horas de observación), sugiriendo que en la mayoría de los casos la depredación de huevos registrada se debió a esta especie. La probabili...

An invasive gull displaces native waterbirds to breeding habitats more exposed to native predators

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Partial‐Likelihood Analysis of Spatio‐Temporal Point‐Process Data

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Artificial Ground Nest Survival in two Abandoned Farmland Habitats on Šolta Island (Croatia)

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