OLFACTORY-BULB SIZE AND NOCTURNALITY IN BIRDS

Recent evidence shows that, despite earlier beliefs, many birds have a functional sense of smell. There is also considerable variation in olfactory-bulb size among bird species, yet the evolutionary significance of this variation has remained elusive. We argue that birds living under low-light conditions, where vision is less efficient, should have evolved or maintained an increased olfactory ability and, hence, larger olfactory bulbs. Using a family-level comparative analysis to control at least partially for taxonomic artifacts, we show that none of a series of ecological variables (diet, nest type, development, nest dispersion, and migratory behavior) accounts for variation in olfactory-bulb size once the effects of body size and brain size (measured by cerebral-hemisphere length) have been controlled. Activity timing, however, accounts for significant variation even after the removal of these other variables. We discovered 13 independent cases in which nocturnal or crepuscular lineages have evolved a diurnal habit, or vice versa, and compared relative olfactory-bulb sizes between each branch pair. In all but one case, nocturnal or crepuscular birds have larger olfactory bulbs than their diurnal counterparts. We therefore demonstrate a widespread relationship between ecology and the evolutionary development of a part of the brain.