Only natural local odours allow homeward orientation in homing pigeons released at unfamiliar sites.

According to the olfactory navigation hypothesis, birds are able to exploit the spatial distribution of environmental odourants to determine the direction of displacement and navigate from non-familiar locations. The so-called "olfactory activation hypothesis" challenged the specific role of olfactory cues in navigation by suggesting that olfactory stimuli only activate a navigational system that is based on non-olfactory cues, predicting that even artificial odourants alone are sufficient to allow unimpaired navigation. In this experiment, we compared tracks of experimental birds exposed to different olfactory stimuli before being made anosmic at the release site prior to release. One group of pigeons was exposed to purified air enriched with artificial odourants, while a second group was exposed to environmental air. The birds stimulated with artificial nonsense odourants displayed several behavioural differences from both untreated controls and anosmic pigeons exposed to environmental air prior to release: nonsense odourants birds were unable to determine the home direction, they mostly flew within a space outside the homeward oriented quadrant, and they flew shorter distances on the day of release. Our data failed to support a mere activational role of olfactory stimuli in navigation, and are consistent with the olfactory navigation hypothesis.