The California condor is another accidental victim of human activity. These huge birds with bald heads and big beaks weigh up to fourteen kilos, have a wingspan of up to two metres and can live as long as sixty years. They play an important role in the ecosystem by feeding on dead animals, thus preventing the spread of disease. Reintroduced into the wild after becoming extinct, other than in captivity, they are now under threat once more because of chemical pesticides introduced into the food chain and lead poisoning from the ammunition used to kill the animals they feed on.

The word they in paragraph 3 refers to ________.