CHICK MORTALITY IN EUROPEAN SHAG STICTOCARBO ARISTOTELIS RELATED TO FOOD-LIMITATIONS DURING ADVERSE WEATHER EVENTS

This study analyses the mortality of chicks of the Europcan Shag Stictocarho aris/o/elis in the Cies Islands (NW Spain). Chick mortality on this island is the main cause of the yearly variation in chick productiou. By visiting the nests after hatching occurred we determined the periods of high mortality in four years of study. The wcather conditions, especially rain and wind, during these periods were significantly more adverse than in thc periods showing no mOl1ality. There was a significant relationship between the yearly 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 physical quality of the nest sitc during the four seasons of study was similar between nests affectcd by mortality and those that were no!. In the colony we studied, feeding is almost exclusively by co-operative groups capturing sandecls, 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 present in only 22% of the pellets and during this time co-operative fishing groups were not recorded. The most important factor involved in the mortality would appear to be food limitation.

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