Sensory mechanisms of long-distance navigation in birds: a recent advance in the context of previous studies.

Displacement studies have clearly shown that experienced avian migrants are able to perform true navigation, i.e., they can find the correct direction leading to a target destination from unfamiliar sites. The sensory mechanisms of true navigation remain poorly understood, though some remarkable progress has been made in the last 10-15 years. There are two primary hypotheses explaining the sensory nature of navigation: (1) a magnetic map hypothesis proposes that birds use parameters of the geomagnetic field that are predictably distributed on the globe. As for the sensory nature of this hypothesis, it has been assumed by some researchers that the magnetic receptor cells reside in the upper beak (the so-called 'beak organ'), and transmit information via the trigeminal nerve to the brain; (2) an olfactory map hypothesis assumes that birds can smell their position by taking advantage of odours distributed in the atmosphere. There are a growing number of studies supporting both of the hypotheses mentioned though in different avian model species. In this review, an attempt is made to provide an overview of the evidence for different navigational cues proposed thus far, with the main focus on the recent studies addressing the magnetic and olfactory navigation hypotheses. Also, a list of key open questions, together with possible experimental approaches, is proposed. [ABSTRACT FROM AUTHOR]