Agricultural fields as a double-edged foraging resource: Greater use results in elevated pesticide exposure in omnivorous gulls.

Agricultural intensification is exposing wildlife to increasingly more complex mixtures of pesticides. In omnivorous birds using urban-agricultural landscapes, the spatial and environmental drivers of exposure remain poorly understood. We investigated the exposure to a suite of current-use pesticides in GPS-tracked ring-billed gulls (Larus delawarensis) breeding in the Montreal area (QC, Canada), a highly urbanized region surrounded by intensive agricultural fields. Sixty gulls were sampled during the incubation period in 2022 and 2023, and pesticide residues were analyzed in four matrices (stomach content, guano, plasma, and liver). Seven pesticides were detected, including four insecticides (three neonicotinoids and one diamide) and three fungicides (two benzimidazoles and one anilinopyrimidine). A pesticide exposure index (PEI) was calculated to express the overall gull exposure by integrating concentrations of all pesticides in the four matrices into a single standardized measure. The PEI increased significantly with the probability of presence of gulls in agricultural fields, identifying croplands as exposure hotspots. No other habitat type (i.e., industrial/residential areas, landfills, St. Lawrence River, or wastewater facilities) showed association with PEI. Clothianidin was the most frequently detected and abundant compound and showed a strong positive correlation with the probability of presence of gulls in agricultural fields for all matrices. Clothianidin concentrations significantly increased with sampling dates later in the incubation period, which was consistent with sowing and treatment schedules of corn and soybean crops. This study underscores the environmental relevance of linking foraging behavior and agricultural practices to pesticide exposure in gulls, providing an integrated framework for assessing exposure risks in agro-urban ecosystems.