Increased pesticide use has been a key cause of Grey Partridge decline

Population declines in many farmland bird species have been documented during the last quarter of the 20th century in parts of Europe and North America where agricultural intensification has been most marked. 'Intensification' is multivariate, difficult to define precisely, but taken to subsume all those advances that have allowed farmers to increase primary productivity. Increased pesticide use on arable land has been a key cause of Grey Partridge Perdix perdix decline through its impact on the availability of insect foods to chicks, and hence on reproductive success.

Source: J.D. Wilson et al. (2005). British Ornithologists'Union, Ibis, 147, 453-463
http://www.ncl.ac.uk/biology/assets/MWhitt_pdf/05wilson_ibis.pdf