Avian Magnetic Field Sensing Visualizes Direction

A hermit thrush. Songbirds may be a familiar sight, but studying their migration patterns is difficult. They travel at night — thousands of feet in the air — defying scientists' attempts to track them. Image Credit: U.S. Fish and Wildlife Service

Avian Magnetic Field Sensing Visualizes Direction

Here on planet Earth, many forces exist that aid in our collective experiences. Animals of every stripe have developed specialized senses on which to take advantage of these forces in their own special way.

It is suspected that the Hammerhead Shark evolved the way it did, eyes perched at each end of a wide and flat front nose end, so that it could better sense the low level electronic field of living animals the shark considers food – a hunting strategy.

It turns out that migratory birds have a tool in their eye that allows it to sense the geo-magnetic field of the Earth so that flocks could migrate to where food is plentiful through the seasons and survive – a different type of hunting strategy.

Avian Magnetic Field Sensing - Neuronal tracing reveals that Cluster N receives input through the thalamofugal visual pathway. Schematic side view of the bird's brain indicating the locations of tracer application. Retrograde tracer (BDA, shown in green) was iontophoretically applied into Cluster N (shown in magenta). Anterograde tracer (CtB, shown in red) was injected into the vitreous of the contralateral eye. Image Credit: Image courtesy of PLoS and article authors, Heyers D, Manns M, Luksch H, Gu¨ ntu¨ rku¨n O and Mouritsen H.

This from a study submitted to the Public Library Of Science via Science Daily -

Do Migratory Birds 'See' The Magnetic Field?
A visual pathway links brain structures active during magnetic compass orientation in migratory birds.
Science Daily - September 26, 2007

Every year millions of migratory birds fly towards their wintering quarters and come back in next year´s spring to breed. Behavioral 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 fulfill 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 behavior.

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.

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.

Citation: Heyers D, Manns M, Luksch H, Gu¨ ntu¨ rku¨n O, Mouritsen H (2007) A Visual Pathway Links Brain Structures Active during Magnetic Compass Orientation in Migratory Birds. PLoS One 2(9): e937. doi:10.1371/journal.pone.0000937
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