Birdscope has an interesting and topical article on migrant songbirds and brain size that I found worth noting: Brainy Birds Stay Behind, or ‘Resident birds have bigger brains than migrants, study finds.’
Resident songbirds have larger brains than long-distance migrants, according to the study by McGill University researcher Daniel Sol and colleagues (Proceedings of the Royal Society, June 2005). The researchers also found that the resident birds were more inventive foragers, suggesting a possible link between bigger brains and the ability to find food in northern winters.
That’s a completely counter-intuitive finding, I would say. I had thought that the apparent memory requirements of migrants for their routes and destinations would include not just specialized brain structures, but added brain mass. On the other hand, a sizable amount of research suggests that migration is governed not by cognitive brain function, but by subconscious instinct. Similarly, resident songbirds might require greater brain-power for foraging in areas with scarce food supply during colder months, and for roving semi-nomadically (without instinctual destinations) in search of food.