The number of sparrowhawks decreased in Europe in the 1950s. Chemicals to kill unwanted insects ("pesticides") were put on seeds before they were planted. Small birds ate the seeds and then sparrowhawks ate the small birds. The amount of chemicals in the food chain increased. This affected the sparrowhawks. Some were poisoned and others laid eggs with shells that were too thin. The shells broke before the chicks hatched. The pesticides were banned and the sparrowhawk population increased again. It is now one of the commonest birds of prey in Europe.