Sketch of the olfactory navigation hypothesis according to Wallraff's idea that atmospheric odours are distributed along gradients (see ‘The structure of the olfactory map’ for further explanation). Small dots of different colours represent different odour compounds distributed along gradients, the direction of which are indicated by the arrows on the inset drawings. For simplicity, only three odours are represented. (A) Learning phase: at home birds learn windborne odours in association with wind direction. A pigeon inside an aviary open to the winds is represented for simplicity. However, this model is valid also for free-flying birds. (B) Operant phase: at the release site the bird gathers information about local odours; the more abundant presence of the ‘blue’ odour in comparison to the other two odours indicates that the bird has been displaced northwest. In fact, the bird has learned that the northwest wind was richer in ‘blue’ odour compounds. The bird orients southeast to fly home. The graphical representation is also consistent with the early view of Papi's ‘mosaic map’, if one assumes that different ratios of different compounds give a unique bouquet so that different areas are characterised by different smells.
Orientation of pigeons exposed to odorous air currents. The birds were kept in a corridor-like aviary and exposed for some hours per day to odorous air current. Only one odour was delivered at a time. One group of birds (triangles) was exposed to olive oil from the south and turpentine from the north; another group of birds (diamonds) was exposed to olive oil from the north and turpentine from the south. The circular diagrams report the initial orientation of the birds when released from two sites (home direction and distance are indicated near the small outer arrows). Before being released the birds were exposed to one of the two odours (olive oil, A and B; turpentine, C and D) learned in association with the artificial wind direction and oriented consistently to the expected direction (large outer arrow) predicted by the artificial odour current. The mean vectors of the initial orientation distributions are represented by the inner arrows (filled, open and half-filled/open symbols refer to the two different release sites and to the pooled distribution, respectively). The mean vector lengths can be read on the scale in A. Data are from Papi et al. (Papi et al., 1974).