There has been a considerable amount of discussion regarding African Greys falling from their perches. In Greys, low dietary calcium resulting in hypocalcemia may be a cause.
Lead poisoning is the most common form of poisoning in the avian species.
Zinc poisoning produces similar tozic effects, except that with zinc poisoning, seizures are uncommon. I have been consulted on two cases of birds who fell off their perches, who were also depressed and anorexic. Zinc poisoning was the likely cause in both of these birds. He presented at the avian veterinarians after falling off his perch. The bird was depressed and not eating.