Animal populations often host individuals that differ in terms of foraging behaviour. It is still largely unknown what causes and maintains these differences, and whether they have fitness implications. In this research project, the causes and consequences of foraging generalism versus foraging specialisation were studied in GPS-equipped Herring Gulls (Larus Argentatus) breeding along the Belgian coast. Foraging specialisation was quantified both in terms of resource specialisation and spatial specialisation, during three distinguished periods of the breeding season. Morphological traits or sex could not be assigned as predictors of foraging differences. While drivers of foraging specialisation thus remain unknown, it is suggested that levels of specialisation may rather be due to behavioural differences. Contrary to expectations, energy expenditure of spatial specialists was higher than that of spatial generalists during the chick rearing period. Spatial specialists traversed longer distances during the other two periods of the breeding season. Furthermore, no effects of foraging specialisation on the reproductive success of individuals were found. In the absence of such fitness implications, the co-existence of multiple foraging strategies may result in populations that are more resistant to environmental stochasticity.
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