A number of the key findings have also been challenged.

The deflector loft effect is shown in some cases to be a consequence of deflection of polarized light, involved in compass calibration, as anosmic birds still deflect after exposure (Phillips & Waldvogel, 1982; Waldvogel, Phillips & Brown, 1988; Waldvogel & Phillips, 1991). However, the similar findings of experiments in which winds are reversed or shielded are not challenged by this discovery. The question of whether olfactory inputs are navigational or related to motivational factors has always been a concern in interpretation (Wiltschko, 1996). In support of this, odours appear to ‘activate’ other navigational processes in young pigeons navigating by route reversal (Jorge, Marques find more & Phillips, 2009). Jorge et al. found that young pigeons, which navigate by route reversal, were unable to orient homeward if transported in filtered air, but could if transported either with access to natural odours, or artificial ‘novel’ odours. This argues that smelling ‘non-home’ odours triggers the bird to access a navigation system based on other cues. The site simulation experiments of Benvenuti & Wallraff

(1985) have also been argued to be a consequence of activation of a navigational map by non-navigational olfactory cues rather than navigational in themselves (Jorge, Marques & Phillips, 2010). Presenting non-specific odours at the false release site produced the same behaviour as access to natural odours. A subsequent test of the activation hypothesis did not support a role for

activation, however. Birds transported Selleck STI571 to a release site with access to novel odours were no more likely to orient homewards than those transported in filtered air (Gagliardo et al., 2011). However, they used higher concentrations of novel odours than those used in the previous navigation experiments, which it has been argued would make the pigeons anosmic (J.B Phillips, pers. Comm..). Nevertheless, the experiments of (Ioale et al., 1990) cannot be explained by activation, as if the benzaldehyde odour was activating MCE a non-olfactory navigational map, it would result in homeward orientation, not orientation consistent with a north-west displacement. One striking finding of the experiments on olfactory navigation in pigeons is that if olfactory navigation is correct, generally, it suggests that the view of redundancy of cues is not correct. Where olfactory deprevation effects have been demonstrated they lead to significant impairment of homing performance of pigeons at unfamiliar release sites, that is the majority do not return to the home loft. If olfactory cues are navigational, this argues that in their absence, no cues are available to take their place, which goes against the widely held view that the navigational map must be made up of redundant cues (Walcott, 1996; Wiltschko et al., 2010).