Year- and Age-Related Variation in the Survival of Adult European Shags over a 24-Year Period

Over 13,000 chicks and 1,800 adult European Shags (Phalacrocorax aristotelis) were banded at a colony in southeast Scotland between 1963 and 1987. Survival estimates for adults (birds three or more years old) were calculated from subsequent retrapping of these birds back at the colony and recoveries of birds found dead in the colony and elsewhere. The mean annual survival for the period 1967-1992 was estimated at 0.878 (95% C.I. = 0.859, 0.897). European Shags exhibit considerable annual variation in several breeding parameters, but there was no evidence that survival was lower in years when breeding was late or reproductive output reduced. Survival in years when the number of nests in the colony showed a dramatic decline was not significantly lower than normal years. An age-related effect was found indicating that survival declined significantly in birds older than 13 years.