Experimental anthropogenic food restrictions drive short-term foraging and immuno-haematological changes in sympatric breeding gulls

Fishery discards and landfills provide major subsidies to scavenging seabirds, shaping their foraging behaviour and population dynamics. However, few studies have compared the behaviour and health of individuals with and without access to such predictable anthropogenic food subsidies (PAFS). We assessed the foraging behaviour and immuno-haematological condition of incubating Audouin's (Ichthyaetus audouinii) and yellow-legged gulls (Larus michahellis) in response to an experimental restriction of access to fisheries and landfill food subsidies in southern Portugal. In response to food restrictions, gulls from both species increased their foraging distance from the colony, likely relying on alternative prey, and/or foraging in association with other fishing fleets in Portuguese or nearby Spanish waters. This was supported by a decrease in the number of yellow-legged gulls observed at both the landfill and the Culatra fishing harbour during the food restriction trials. These changes were followed by alterations in the leucocyte profiles of yellow-legged gulls, characterised by a predominance of heterophils over lymphocytes, i.e. an indicator of physiological stress. Our findings show that limited access to PAFS can influence gulls' foraging behaviour and physiological condition, highlighting their reliance/dependence on these predictable subsidies. These short-term responses suggest that PAFS may function as ecological traps for opportunistic seabirds during the breeding period. Understanding such responses is critical to predict long-term ecological consequences and to develop waste and fisheries management policies aligned with EU strategies.