Temporal and age-related dietary variations in a large population of yellow-legged gulls Larus michahellis : Implications for management and conservation

There was an extraordinary increase in the numbers of European gulls during the twentieth century which has been linked to higher availability of food derived from human activities. At Berlenga island (Portugal), the population of yellow-legged gulls Larus michahellis increased from 2600 individuals to a peak of 44,698 gulls (1974–1994), after which control measures have been put in place. Despite the management effort, little is known about the feeding ecology of this population. To investigate temporal and age-related variations in the diet of yellow-legged gulls at Berlenga, 1668 adult pellets and 145 chick regurgitates were collected and analysed between 2009 and 2012. Contradicting the generally accepted idea that these birds depend mainly on human-related food, adult gulls relied substantially on a locally abundant natural prey, the Henslow’s swimming crab Polybius henslowii. Nevertheless, large amounts of refuse and fish were consumed in periods of apparent lower availability of swimming crabs. Despite the large temporal shifts in diet and feeding areas (change from marine to terrestrial prey), adult gulls consistently provisioned their chicks with a fish-based diet and chick condition remained constant. These results not only highlight the great resilience of this population to changes in food availability but also indicate that food from different human activities remain highly accessible. With the implementation of recent EU legislation regarding the reduction of fishery discards, and the increase of urban populations in the mainland, the monitoring and appropriate management of gull populations will be decisive for the healthy conservation of coastal systems used by these gulls.Fundação para a Ciência e a Tecnologia (FCT