Farming practices induce contrasted effects on parasitic infections in passerine birds in agricultural landscapes

Agricultural intensification has led to landscape changes and the extensive use of synthetic pesticides. Concurrently, a significant decline in biodiversity has been observed, especially for birds that frequent farmlands, which may ultimately affect community structures by altering interspecific relationships. While the contribution of pesticides to bird declines is now widely recognised, their impacts on host-parasite interactions remain largely unexplored. Pesticides may influence the presence and abundance of parasites and/or alter the host’s immune system, affecting control over pathogenic infections. We investigated, for the first time to our knowledge, the differences in wild passerines’ parasite burden, caught within 10 sites mainly surrounded by conventional farming (CF) and 10 sites mainly surrounded by organic farming (OF), as these landscapes differ in pesticide use. We estimated the number of two ectoparasites (ticks, chewing lice) and two endoparasites (blood parasites Plasmodium , coccidia) in 10 bird species. Our results highlighted a strong effect of the year and bird species on parasitic infections. Nevertheless, farming practices induced contrasted effects on parasitic burdens in birds: ectoparasite burdens were lower in CF, while blood parasite burdens were higher in this environment. This suggests that both farming practices can have a positive or negative effect on avian parasitic infections, and on bird health more generally. • Conventional farming can lead to sublethal effects on birds like immune disorders. • The burden of ectoparasites was lower in conventional farming. • Conversely, the burden of blood parasites was higher in conventional farming. • Both farming practices have positive and negative effects on parasitic infections.