While the olfactory navigation hypothesis is by far the most extensively tested when considering pigeon homing, it has rarely been considered when discussing true navigation in migrating birds. Stable odour gradients such as would be necessary for a bi-coordinate map have not been demonstrated to exist beyond approximately 200 km (Wallraff

& Andreae, 2000). This makes it difficult to explain the majority of displacement experiments on migrants by the use of olfactory navigation. Nevertheless, two experiments on homing of migratory birds in the breeding season found a deficit in performance after olfactory deprivation (Fiaschi, Farina selleckchem & Ioalé, 1974; Wallraff et al., 1995). More selleck kinase inhibitor surprisingly, a recent experiment demonstrated that adult catbirds displaced 1000 km east from Illinois to Princeton in the US, subjected to olfactory deprivation by zinc sulphate treatment and then radio-tracked from a light aircraft were unable to correct for the displacement in the way that controls were (Holland et al., 2009). If this finding

is borne out by further experimental support and shown to be a deficit based on the removal of navigation cues, then it may require a re-analysis of the bi-coordinate map theory for true navigation. It appears to be hard to explain how stable olfactory gradients could exist over the 1000 km necessary to explain this behaviour navigationally. Homing pigeons have not been shown to use olfactory cues beyond 700 km, and then only if they had access to environmental air during the displacement (Wallraff, 1981). With regard to the use of olfactory signals by migrants, an interesting parallel finding from a neurobiological study of migratory restlessness is that both visual and olfactory areas of the brain become more active at night during the migratory period, while they are most active during the day outside this time (Rastogi et al., 2011). This suggests

that olfaction plays a significant role in migratory behaviour, but it is still an open question as to what role this is. A recent hypothesis proposes that in fact the primary role of olfaction across organisms (and thus reason for its evolution) is navigation (Jacobs, 2012). If it does indeed turn selleck products out to be the case, then theories of true navigation based on a bi-coordinate map made stable environmental gradients may need to be significantly reconsidered, because olfactory cues do not seem to fit easily into this paradigm. The intensity of the Earth’s magnetic field was proposed as a cue for bird navigation over a century ago (Viguier, 1882). The Earth’s magnetic field is stronger at the poles than at the equator and it therefore has the potential to indicate latitudinal position. However, this is only functional over a relatively coarse scale (Bingman & Cheng, 2006).