Fig. 3. Illustration of the four nonverbal cognitive tools displayed by corvids and apes, which are proposed as the basis for complex cognition: causal reasoning (New Caledonian crow and chimpanzee tool use), imagination (insight in ravens and role taking in chimpanzees), flexibility (western scrub jays' flexible memory for degraded and fresh food items and tactical deception in apes), and prospection (western scrub jays recaching food and chimpanzees carrying stone tools). Illustration of the four nonverbal cognitive tools displayed by corvids and apes, which are proposed as the basis for complex cognition: causal reasoning (New Caledonian crow and chimpanzee tool use), imagination (insight in ravens and role taking in chimpanzees), flexibility (western scrub jays' flexible memory for degraded and fresh food items and tactical deception in apes), and prospection (western scrub jays recaching food and chimpanzees carrying stone tools). These cognitive tools interact in different ways to produce complex cognition. [Drawing by C. Cain] Nathan J. Emery, and Nicola S. Clayton Science 2004;306:1903-1907 Published by AAAS