Brain tissue is matebolically as active and hence as expensive as muscle, and it is active day and night. Our brain accounts for only 1.5 percent of our body weight, but it demands about 20 percent of our energy supply. At any one time, this energy is used mainly by those neurons that are active...
Animals have evolved to minimize energy use whenever possible. Large, metabolically expensive brains would only have developed and be maintained for very compelling reasons.
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[B]rain mass in any one individual is not constant. Recent research with some birds shows that the mass of the hippocampus, the portion of the brain devoted to the specific functions of singing and food caching, increases and decreases seasonally with use. Birds can grow and shrink brain tissue as needed, thereby avoiding expensive maintenance of tissues not in use.

- Heinrich, Bernd, *Mind of the Raven: Investigations and Adventures with Wolf-Birds* (New-York: HarperCollins, 1999), pp. 327-330.