There is more to the eyes of the Robin

We are used to see Robins all year round, and indeed, although is a large resident population in the UK, they are migratory birds in much of their range, with northern European birds migrating every winter to Southern Europe and North Africa. Many of these migratory Robins pass through or winter in the UK. Robins migrate at night, and much research has been devoted to find out how they manage to navigate during migration. Human navigation uses a 'map and compass' system, to find out where we are and what direction to go. Recent discoveries on bird navigation have shown that they rely on an internal magnetic sense. Indeed, although many migratory birds are known to have a magnetic sense, the first demonstration of a magnetic sense in animals came from experiments on caged Robins. Experiments to document migration orientation usually involve using what is called an Emlen funnel, a round cage in the shape of a funnel covered on blotting paper on the sides, a clear top, with an inkpad at the bottom, something like this:

Emlen funnel 

 Caged migrant birds become restless at night during the migration season and jump more often in the direction where they intend to migrate, making little ink prints on the paper. Caged robins changed their heading accordingly when the magnetic field was artificially altered using strong magnets. Migratory birds magnetic sense is extraordinary as it relies on light as detected by the eyes - in particular the right eye -, which is interpreted in a magnetic way. Photons exciting a particular molecule (called 'cryptochrome') in the retinal cells form radical pairs sensitive to the magnetic field. The information reaches the brain through the optic nerve and the bird is able to form a map of intensity of the magnetic field overimposed to their visual field. The intensity of the magnetic field is region specific, helping the bird to navigate even in low light conditions. Recently, Katrin Stapput and collaborators found out that this magnetic sense depends of the bird having a clear image on their right eye as Robins that had their left eye covered with a frosted goggle - which prevents them from forming a clear image -could navigate, while if the right eye did not form a clear image they got disorientated (see their result below).

Robin wearing goggles (from Stapput et al 2010)

Orientation of European Robins during Spring Migration with the Local Geomagnetic Field as the Only Cue. (A) In the binocular control tests without goggles (Bi), the birds preferred their normal northerly migratory direction. (B) Birds tested binocularly in a radio frequency field (BiRF) were disoriented, indicating that the orientation was based on radical pair processes. (C) When the right eye was covered with a clear foil and the left eye covered with a frosted one (Rclear), the birds were oriented in their normal migratory direction. (D) When the left eye was covered with a clear foil and the right eye covered with a frosted one (Lclear), the birds were no longer oriented. The triangles at the periphery of the circles indicate mean headings calculated from three tests of the individual birds; the arrows originating from the center represent the grand mean vector drawn proportional to the radius of the circle = 1. The two inner circles are the 5% (dashed circle) and 1% (solid circle) significance borders of the Rayleigh test. (From Stapput et al 2010).  

 Next time you see the lovely bright eyes of the Robin spare a thought for the amazing things they can do with them.

More information
Wiltschko, R., & Wiltschko, W. (2006). Magnetoreception BioEssays, 28 (2), 157-168 DOI: 10.1002/bies.20363
Stapput, K., Güntürkün, O., Hoffmann, K., Wiltschko, R., & Wiltschko, W. (2010). Magnetoreception of Directional Information in Birds Requires Nondegraded Vision Current Biology, 20 (14), 1259-1262 DOI: 10.1016/j.cub.2010.05.070