Goodwin D: Crows of the World. 2nd ed., British Museum, London, 1986.
Humphrey NK: The social function of intellect (Eds. Bateson PPG, Hinde RA: Growing points in ethology). 303-317, Cambridge, Cambridge University Press, 1976.
Byrne RW, Whiten A: Machiavellian intelligence: social expertise and the evolution of intellect in monkeys, apes and humans. Oxford, Clarendon Press, 1988.
Emery NJ: Cognitive ornithology: the evolution of avian intelligence. Phil Trans R Soc B 361: 23-43, 2006.