Combining GPS tracking and Stable Isotope Analysis in a synanthropic seabird: the Lesser Black-backed Gull Larus fuscus

The Lesser Black-backed Gull (Larus fuscus) has colonized and quickly proliferated along the Belgian coast during the second half of the 20th Century. The Belgian L. fuscus breeding population exploits human-conditioned nesting space and human-generated sources of food: fishery discards, industrial food waste, soil invertebrates in agricultural lands and urban garbage. This synanthropic behaviour, extensive to a large proportion of its populations along the North-eastern Atlantic coasts, has expanded the species' breeding range through the exploitation of anthropogenic resources. However, ecological trap conditions emerge as natural nesting grounds give way to urban development and food availability is affected by swift changes in human activities. In particular, the onset of a European ban on discarding by fisheries may result in a sharp and permanent decline in their key food source. Stable isotope analyses on chick feathers have revealed an overall importance of fishery discards in the food provided by local breeding parents to their chicks. However, the relative amount of fish in chick diet showed a significant degree of individual variation, which was as well observed in foraging patterns given by GPS tracking data during the chick rearing period. This variation points toward the possibility that gulls develop individual foraging strategies, which would in turn have consequences in the capacity to respond to changes in local food availability. Through cost-benefit analyses on the different observed foraging strategies of this generalist feeder, and an evaluation of the local availability and distribution of the exploited food sources, we aim at evaluating the future viability of the local L. fuscus breeding population, as well as potential changes in foraging distributions.