There is a correlation between brain size and IQ in humans, but it doesn't apply to individuals. Much of it is caused by malnutrition stunting both growth and mental development. And besides, the smaller your brain, the quicker you can think.

Bird brains are highly efficent because of weight constraints caused by flying. Parrots also can live many decades because they so have time to lean many tricks in their lives. Everyday a cockatoo comes and eats figs off my parents' tree. Normally they come in flocks and are very noisy. However, he always comes alone and is very quiet as he doesn't want the other cockatoos in his group to know about the figs he's found.