Bird navigation has puzzled animal scientists for a long time, despite decades of research, not least because each species seems to have its own flight manual and designing experiments to unlock these secrets is incredibly challenging for us, on the ground.
Scientists, in the 1960s, first posited that birds locked into the Earth's magnetic field for navigation in flight, but as this 2021 review explains, experimental design left a lot to be desired, and replicating results also proved difficult. It's now fairly well accepted that migratory birds in particular use complex systems of internal magnetoreception and optical biochemistry to guide them in the air, but a lot of questions remain unanswered.
Now, in what may very well prove to be a landmark discovery, scientists at the Max Planck Institute of Animal Behavior in Germany believe they've cracked the code behind how homing pigeons can fly hundreds of miles and then return home with precision. Regardless of how you feel about these (rather unfairly) maligned birds, it would appear to possess a deceptively sophisticated way of staying on track in the air, that even involves their immune cells.
In a first, the scientists found that pigeons tap into the Earth's magnetic field through iron-rich immune cells only found in their livers.
These cells, or macrophages, primarily break down old red blood cells. During this process, they become enriched with iron, turning them into quantum powerhouses that connect with magnetic fields like an internal compass. Without these cells, the birds lose their way completely.
“We didn’t expect immune cells to act like sensors for magnetic fields at all," says Professor Christian Kurts, Director at the Institute of Molecular Medicine and Experimental Immunology at the University Hospital Bonn, and one of the study’s co-senior authors."Our results reveal a previously unknown mechanism for magnetic perception in animals."
“What looks like a ‘gut feeling’ in bird navigation may actually have a physical basis,” adds co-author Professor Martin Wikelski, Director at the Max Planck Institute of Animal Behavior.
As touched on earlier, while we know migratory birds and other long-distance flyers use the Earth's magnetic fields, but precise understanding of the tools they have on board in order to do this has eluded scientists. In this study, the researchers wanted to work out just how pigeons were also harnessing the power of the planet.
Earlier research, even studies from the trailblazing husband-and-wife duo Wolfgang and Roswitha Wiltschko (pioneers of unlocking the mystery of bird navigation), believed that magnetite-based receptors found in avian beaks were also steering pigeons in the right direction. In this latest research, the scientists used vibrating-sample magnetometry (VSM) and magnetic cell separation – tools used to first screen organs and then isolate groups of cells that could be involved.
Of course, they analyzed pigeon eyes, beak and brain – all of which have been implicated in prior avian navigation studies – but their results pointed them in a very different direction.
“We had some clues that the liver and spleen have magnetic properties, because they break down red blood cells and so store much iron in the body,” says first author Dr. Clivia Lisowski, from the University of Bonn and the University Hospital Bonn.
They found that the liver contained the highest concentration of iron, making it the strongest candidate for housing the bird's magnetite-based receptors.
“Iron is crystallized in oxide nanoparticles making the cells superparamagnetic and reactive to magnetic fields," says Professor Ulf Wiedwald, from the University of Duisburg-Essen. "We found by far the strongest magnetic response in liver tissue."
Further analysis confirmed that liver macrophages in the liver powered this species internal GPS system.
The ornithological team then put their findings to the test on pigeons that had been trained to fly 20 km (12 mi) away from and return to their aviary at the research institute. When the birds' macrophages were removed, the pigeons got lost – but only on overcast days. On days when the sun was not obscured by cloud cover, the pigeons without the macrophages still made it back to the aviary.
This suggested that the pigeons possessed a multifaceted navigational system that, if the internal GPS failed, they could also use solar clues to find their way home.
Further experiments with eletron microscopy found that these specialized, iron-rich macrophages are found in close proximity to nerve fibers – suggesting that signals from the cells could be transmitted to the brain, keeping pigeons on track in the air.
“These findings provide the first concrete evidence of how the Earth’s magnetic field can be perceived within the body and passed on to the brain to guide movement," says Lisowski.
“Animal navigation is one of the most fascinating phenomena in nature,” adds Wikelski. “If immune cells are part of how birds sense direction, it would fundamentally change how we understand navigation.”
The study was published in the journal Science.
Source: Max Planck Institute of Animal Behavior via EurekAlert!
