Garden Warbler (photo: Garth Peacock).
Every year millions of migratory birds fly towards their wintering quarters and come back in next year's spring to breed. Behavioural experiments have shown that the Earth's magnetic field is the main orientation cue on their journeys. Nevertheless, surprisingly little is known about the neuronal substrates underlying these navigational abilities. In recent years, it has been suggested that sensing of the magnetic reference direction involves vision and that molecules reacting to the Earth's magnetic field in the birds' eye form the molecular basis for a vision-dependent compass mechanism.
Cryptochromes, which fulfil the molecular requirements for sensing the magnetic reference direction, have recently been found in retinal neurons of migratory birds (Mouritsen et al., PNAS, 2004). Furthermore, studies investigating what parts of a migratory bird's brain are active when the birds use their magnetic compass showed that the cryptochrome-containing neurons in the eye and a forebrain region ("Cluster N"; Mouritsen et al., PNAS, 2005; Liedvogel et al., EJN, 2007) are highly active during processing of magnetic compass information in migratory birds. Sensory systems process their particular stimuli along specific brain circuits. Thus, the identification of what sensory system is active during magnetic compass orientation, provides a way to recognize the sensory quality utilized during that specific behaviour.
In the current study the research group from Oldenburg, Germany and their collaborators traced the neurons from the eye and from Cluster N. The results "link" the recent findings by demonstrating a functional neuronal connection between the retinal neurons and Cluster N via the visual thalamus.
Garden Warbler (photo: Steve Oakes).
Thus, the only two parts of the central nervous system shown to be highly active during magnetic compass orientation are linked to each other by a well-known visual brain circuit, namely by parts of the so-called thalamofugal pathway. For the first time, clear neuroanatomical data suggest which specific brain pathway processes magnetic compass information in migratory birds. These findings strongly support the hypothesis that migratory birds use their visual system to perceive the reference compass direction of the geomagnetic field and that migratory birds are thus likely to "see" the geomagnetic field.
The paper is published in the online, open-access, journal PLoS ONE and is freely available to read in full here.