Chick mortality in European shag Stictocarbo aristotelis related to food limitations during adverse weather events.

This study analyses the mortality of chicks of the European Shag Stictocarb o aristotelis in the Cies Islands (NW Spain). Chick mortality on this islan d is the main cause of the yearly variation in chick production. By visitin g the nests after hatching occurred we determined the periods of high morta lity in four years of study. The weather conditions, especially rain and wi nd, during these periods were significantly more adverse than in the period s showing no mortality. There was a significant relationship between the ye arly mortality rate in chicks and the number of windy and rainy days during the growth period of the chicks (May) during four years of study. The phys ical quality of the nest sire during the four seasons of study was similar between nests affected by mortality and those that were not. In the colony we studied, feeding is almost exclusively by co-operative groups capturing sandeels, with this prey accounting for over 70% during the breeding season . During a period of heavy rains in 1997, the sandeels were found to be pre sent in only 22% of the pellets and during this time co-operative fishing g roups were not recorded. The most important factor involved in the mortalit y would appear to be food limitation.