Synthetic pesticides used in conventional agriculture may have hazardous effects on wildlife. Still, most studies assessing the effects of pesticides are conducted under controlled conditions and include one to three different substances, neglecting the real exposure conditions of wild organisms to pesticide mixtures. In a recent study, we analysed the blood of 35 wild Montagu’s harrier (Circus pygargus) nestlings to assess their pesticide contamination loads among 116 substances, including herbicides, fungicides and insecticides. This bird of prey sets its nest on the ground amongst cereal crops, exposing chicks to pesticide mixtures during their growing period. We therefore investigated whether the contamination loads of these nestlings could influence their growth and physiological stress by assessing the stress hormone corticosterone.
The Montagu’s harrier (Circus pygargus) is a specialist bird of prey, inhabiting agricultural lands of Europe during its reproduction. Credit Sorenza Phelippeau
We found 18 different substances (9 herbicides, 5 insecticides and 4 fungicides) in the blood of these Montagu’s harrier chicks, of which three were banned at least 10 years before sampling took place. On average, nestlings had five different substances in their blood with a total concentration of 780 pg.mg-1. The nestling with the highest contamination load had nine different substances and a total concentration of 3473 pg.mg-1 in its blood. The pesticide concentrations influenced three different traits assessed in the study: the tarsus growth (reflecting the skeletal growth), the body condition (reflecting the fat reserves), and the baseline corticosterone (reflecting the level of stress without disturbances) of chicks. Depending on the sex and age of the nestlings, higher contamination loads were associated with delayed tarsus growth, lower body condition and lower baseline corticosterone secretions. This indicates that pesticides may interfere with hormone secretions, alter food intake and disrupt gut microbiota. This affects nutrient absorption and bone growth, which may ultimately impair nestlings’ survival. Indeed, nestlings with poor overall condition at fledging have fewer chances to survive the migration to their wintering areas.
These results suggest that nestlings may face adverse effects of pesticide mixtures on their health. More studies are needed to confirm if these findings are relative to other farmland birds and contexts.
*Fuentes, E., Moreau, J., Millet, M., Bretagnolle, V., & Monceau, K. (2025). Pesticide mixture effects on physiological stress and morphology of growing wild nestlings. Journal of Hazardous Materials, 139346. https://doi.org/10.1016/j.jhazmat.2025.139346
Elva Fuentes has a PhD in Environmental Biology, Populations and Ecology from La Rochelle Université. During her PhD at the Centre d’Études Biologiques de Chizé, she sought to determine the actual levels of pesticide contamination in wild birds, the factors that can influence these levels of contamination, and the effects on bird health that can be associated with this contamination, through the study of Montagu’s harrier (Circus pygargus) chicks. The research is part of the emerging field of wild ecotoxicology, which combines concepts and methods from ecotoxicology, ecophysiology and conservation biology to improve biodiversity conservation actions in anthropized environments.
