Birds population are declining in many world regions. The main pressures originating such decline are suspected to be land use change or pollution by pesticides, that can impact them directly or indirectly by reducing their food resource. However, the relative contributions of these different anthropogenic pressures on bird decline remain unknown and can differed depending on birds life history traits.

This study aimed to fill this gap by focusing on French bird populations, shown to have decreased by 25% since the 90's. The study took place between 2008 and 2021, using the French Breeding Bird Survey where birds are surveyed in more of 1500 sites during this period. We analysed both the mean abundances and temporal trends of birds' populations depending on their diet (insectivorous, granivorous and generalist) and their migratory strategy. We related these mean abundance and temporal trends to spatio-temporal changes in pesticide use accounting for their toxicity toward birds and bird's food resources (insect or plant), as well as land use change.

Our results showed that land use but not pesticide use had strong effect on birds abundance on the French territory depending on their traits. Insectivorous birds' abundance increased with grassland amount while granivorous and generalist birds increased with urban area independently of their migratory strategy. On the contrary, bird's temporal trends was more related to change in pesticide use than land use. We found a strong negative effect of pesticide use on sedentary insectivorous birds and to a lesser extent on sedentary granivorous birds but no effect on birds with a generalist diet or migratory birds.

The present study provides evidence of strong negative effect of pesticide use on birds population trends, especially insectivorous one. Our results suggest that this negative effect of pesticide goes through both direct effect on birds, but also throughout food resources (insect) and support of this food resources (plant). Our results point out the reduction of pesticide use as a key conservation measure if we are to stop current bird decline.