It was so thin that the parent's weight would crack the fragile egg shell. This made scientists interested and very worried about the problem. The falcons also have accidents. If a window is very clean, a bird could not see the window and fly into it. When the bird hits the glass, the impact could break the bird's neck or wing.
Power lines give another problem for all kinds of birds.
When they bend the wire and something touches two of them together, an electric charge is produced that is strong enough to kill the birds. The problem wasn't noticed until the 1960's. Falconry recorders noticed that less then ten percent of the nests were occupied in the country of England. This problem then spread west to the Eastern United States of America. This caused another problem for scientists to deal with in the fight to save the peregrines.