Birds’ sense of direction still up in the air
Talk about going off-course. Scientists are heading back to the drawing board after a controversial new study undercut a widely held theory about birds’ ability to detect the Earth’s magnetic fields in order to navigate.
Many researchers had believed that a strange population of iron-rich cells in the upper beaks of pigeons that appeared to resemble neurons were the birds’ long-sought magnetoreceptors. But a study published online this week in the journal Nature found that these cells are probably just macrophages, a type of immune system cell that fights off infection.
David Keays, the neuroscientist at the Research Institute of Molecular Pathology in Vienna who oversaw the new study, discussed the results and their implications.
First off, why pigeons?
Pigeons are amazing. One of the things that they do really well is they home — they’ve been selectively bred for hundreds of years to home to their loft. Before we had the telegraph, you’d deliver the pigeon post.
The idea is that having an ability to detect magnetic fields helps the pigeon get home, and this ability is being selected for through breeding. So we believe they’re a good model to investigate magnetoreception.
How do scientists think this works?
It has been claimed that pigeons had magnetoreceptor cells located at the front of their beaks, and researchers thought there’d be some really cool molecules in these cells. I thought if I could find these molecules, I could find the magnetoreceptors.
To do that, I had to confirm the results of previous work, which asserted that these were iron-rich cells that were neurons, and that they were in six very distinct locations [in the beak].
But what we show is they’re not in these locations — they’re all over the place. And they aren’t in fact neurons, which was quite a disappointment. Most people, like ourselves, believed these original reports.
How’d you test for these special cells?
So we took pigeons and we sectioned them very finely, into 10-micrometer sections. In total, we looked at about 200 pigeons and generated more than 250,000 sections.
That’s a lot of pigeon slices.
Yeah, my lab is full to the brim of pigeon sections. I don’t think there’s anyone in the world who has as many as I do.
We stained them with a chemical stain that turns iron-rich cells bright blue. Then we mapped the location of these cells onto the pigeon beak. Much to our surprise, there was a huge [variation] among pigeons.
So each pigeon was different?
In Pigeon No. 203, we saw 108,000 cells in the beak, but Pigeon No. 200 had about 200 cells. That doesn’t make sense if it’s a genetically encoded magnetoreceptor.
How did you solve this mystery?
Fortunately, Pigeon No. 199 had an inflammatory lesion — maybe it was injured, maybe one of the other pigeons had a bit of a go at it, maybe it lost a battle over a female pigeon. This was in the upper beak, in the region where we were looking for the magnetoreceptors. And this inflammatory lesion was surrounded by 80,000 of these iron-rich cells.
We thought maybe these cells were macrophages, a type of immune cell, and had nothing to do with magnetoreception.
Once we found these cells in the beak and concluded they were macrophages, we predicted that we would find them elsewhere on the pigeon. And that is in fact the case — you can take skin from the wing or the abdomen, the scalp, and you find identical cells.
Where does this leave researchers?
It’s clear that these blue cells are a red herring. So we’re not going to be spending any more time chasing them.
But it’s also apparent that animals do detect magnetic fields and there must be a way they do it. So we’re trying to find this population of cells using the latest tools in molecular biology: imaging and genetics.
It’s not like trying to find a needle in a haystack; it’s like trying to find a needle in a haystack of needles. At the moment we’re focusing on a region of the pigeon called the olfactory epithelium.
How have scientists reacted to your paper?
I presented some of this data at a conference last year and there was part of the audience who thought it a fantastic study. There were also some members of the audience who were incredulous. So it’s clearly a controversial study that’s challenging the current dogma. But we’re absolutely confident about our results and the conclusions that we’ve reached.
What about bird-lovers outside the scientific community?
So it’s very funny: The guy who looks after our pigeons is a fanatical pigeon racer. These people selectively breed pigeons for their homing abilities. I think one of the reasons this guy is eager to help us is he thinks we’re going to tell him how to make his pigeons fly faster.
I hate to break it to him that all we’ve done so far is find out that these iron-rich cells are macrophages, and we’re no closer to finding the true magnetoreceptors.
This interview has been edited for space and clarity.
