The use of certain pesticides, especially DDT was not good for the animals. It could be shown that in areas where DDT was used, the thickness of the shells of their eggs was reduced. This caused a dramatic decline in their numbers, in certain countries. Since the use of DDT has been forbidden in many countries, their numbers are increasing again. This recovery was helped because their nesting places were protected in many countries; some countries also bred these falcons in captivity and released them into the wild.