The olfactory bulbs of homing pigeons are not enlarged compared with other pigeon breeds.

The homing pigeon (Columba livia) is one of the best examples of human selection for behaviour among domesticated animals. Several brain regions purportedly differ in size between homing and other pigeons as a result of this selection, one of which is the olfactory bulbs. Olfaction is considered by many to be one of the sensory cues homing pigeons use to orient home. The intense selection for homing could therefore have driven anatomical changes in the olfactory bulbs, the first site of olfactory processing in the brain, in homing pigeons. Here, we compared the size of the olfactory bulbs, as well as size and number of mitral cells, among homing pigeons, feral pigeons, and several sporting and show breeds. Homing pigeons did not differ in absolute of relative olfactory bulb size from wild, feral, or most other pigeon breeds. Only a small show breed (figurita frill) and a meat/show breed (king) differed in absolute and relative olfactory bulb sizes from other pigeon breeds. Similarly, there were no significant differences among breeds in the absolute or relative number of mitral cells or mitral cell soma size. The lack of significant differences among most of the pigeon breeds sampled suggests neither experience nor selection for different behavioural or physical traits affects olfactory bulb size or the number and size of mitral cells. Although the results might indicate that there is not anything special about the olfactory abilities of homing pigeons, we lack data on other aspects of olfactory bulb anatomy and the acuity and sensitivity of the olfactory system across pigeon breeds. The latter knowledge gap could be addressed through genomic and transcriptomic techniques, which would provide new insights to the ongoing debate surrounding the use of olfactory cues in homing.