How pigeons are able to detect magnetic fields to navigate their way home remains a mystery, say researchers.
Biologist Dr Jeremy Shaw, from the University of Western Australia, and colleagues, report their findings today in the journal Nature.
"At this stage it's still that needle-in-a-haystack problem," says Shaw.
Behavioural studies show that pigeons are able to detect magnetic fields.
Finding the cells that are responsible for this magnetic sense has eluded scientists.
But in 2003, German-led researchers reported iron-rich magnetoreceptors in six locations of the pigeon's beak.
The team reported finding nerve cells rich in magnetite and haematite, which are highly magnetic.
Shaw and colleagues carried out detailed histological examinations in 270 pigeons but could not confirm these findings.
"We've done a very robust detailed study of the entire beak region and found nothing of the sort," says Shaw.
Instead, the researchers found different forms of iron that are only weakly magnetic. They also found the iron was present in macrophages, not nerve cells.
Both findings mean the structures identified by the German research are "highly unlikely" to be magnetoreceptors, says Shaw.
"We're not saying there isn't a magnetoreceptor there somewhere, but we're saying the cells that had been previously described as magnetoreceptors are not them."
Pigeons for science
Shaw emphasises the animals sacrificed in the study were killed in a humane way.
He says the research is important because of what it can reveal about the "sixth sense" of magnetic field detection.
"It's one of those fundamental questions in biology that has persisted for many many years now," says Shaw.
He says the research could help understand problems such as the stranding of whales, which also detect magnetic fields.
Zoologist, Professor Mike Walker of the University of Auckland in New Zealand has about 30 years studying the ability of animals to detect magnetic fields.
Walker says the original German research was faulty and should not have passed peer review. By contrast, as one of the reviewers on the new research, he is absolutely convinced by its findings.
He says animals like birds are full of iron in the blood and it is very difficult to distinguish magnetite in this context.
"It's a classic magnetic needle in a haystack full of iron," says Walker.
Walker says magnetite is so magnetic that an animal would only need a tiny amount of it for a magnetoreceptor.
"Half a dozen cells containing the magnetite would be enough to give a whale a very adequate compass," he says.
Walker says his own research previously found magnetite in the snout of rainbow trout, using more robust methodology than the German research.
"We've looked a lot of different fish and found it pretty much in the same place every time," he says.
Walker says it is likely that magnetoreceptors are also likely to be in the same place in birds since fish evolved before birds, and he'd like to test his theory.
"I just need some warm bodies and money," he says.