For many birds, migration is a major component of life. You'd expect think that migration would have a whole cascade of effects on those birds, including the nervous system. But which way?
On the one hand, migration might be correlated with large brains to handle the the complex navigation tasks. On the other hand, migration might be correlated with small brains that are energetically efficient.
But what these authors are really trying to figure out is not the correlation, but the direction of causation. Do lineages become migratory because their small brains allow it, or does migration cause brains to become smaller, as neurons as jettisoned over evolutionary time for efficiency?
And here, there's a lot of math. It's not easy going for someone who doesn't do these kinds of models routinely, and I don't. Sol and colleagues argue that the models they tested indicate that migration causes brain size to reduce, rather than the other way around. Again, however, a good chunk of variation appears to be due to factors that were not included in the model. They also suggest that some brain areas may be more subject to selection pressures, and it's more complex than just the overall brain size alone.
When people think about the evolution of brains, the most common thing that leaps to mind is brains getting larger. Maybe that's just because that's such a major aspect of human evolution. This work is a nice example of selection pressure working to reduce brain size.
Sol, D., Garcia, N., Iwaniuk, A., Davis, K., Meade, A., Boyle, W., & Székely, T. (2010). Evolutionary Divergence in Brain Size between Migratory and Resident Birds PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009617
Picture from user philip.bitnar on Flickr. Used under a Creative Commons license.