Thielcke, G.; Krome, M.: Experimente über sensible Phasen und Gesangsvariabilität beim Buchfinken (Fringilla coelebs). Journal für Ornithologie 130 (4), pp. 435 - 453 (1989)
Scientists studying a rare population of tool-using chimpanzees have found that aging can have a profound impact on their abilities—but not all chimps decline equally.