Scientists have been long time puzzled by how can birds follow their route during the migration and why pigeons can turn back home from large distances (pigeons are the only domestic birds able to fly, that's why this trait was more obvious on them).
The researchers have not known for sure what sense empowers the birds with their navigational abilities: hypotheses included smell (but birds generally have weak smell, except condors, albatrosses and their relatives), sun position or a star map during the night.
The discovery in 2004 of tiny deposits of a mineral called magnetite (lodestone) in the beaks of pigeons and bobolink
(a North American songbird) biased the debate towards the hypothesis that birds can read Earth's magnetic field (image).
The sensory cells containing the magnetite were wired to the brain, just like in the case of migratory fishes like trout, salmons and stingrays.
Making use of a peculiar error in the Earth's magnetic field, a research team has been able to prove how pigeons, released off hundreds of kilometers from home, employ magnetic field for returning home.
"We are now confident pigeons do use the intensity of the Earth's magnetic field to determine position during homing," said Dr. Todd Dennis, of the University of Auckland, lead researcher of the study.
The experiment's homing pigeons, fitted with global positioning system (GPS) tiny devices, were set free in New Zealand's Auckland Junction Magnetic Anomaly, a zone where the Earth's magnetic field presents a natural anomaly provoked by a cluster of big rock slabs deep below the surface in Mt Roskill.
These rocks inflict a detectable spike in the geomagnetic field.
The team checked this way how a distortion in the magnetic field could impair the pigeons' sense of orientation.
59 out of 92 released pigeons made a loop, flying up to 4km in the wrong direction following the variations in strength of the local magnetic field, before redirecting themselves to the right direction.
As soon as the birds got out of the magnetic distortion, they redressed their direction.
"This showed the birds kept track of gradients in the magnetic field to navigate." said Dennis. Image credit: Peter Reid.