When a pigeon fancier sends his birds on their first homing run, he has a stressful wait; if they can't navigate, they'll be lost forever. But most birds return successfully against the odds. How birds `home' on their first outing remains a topic of hot debate, but Anna Gagliardo, Martin Wild and colleagues present strong evidence in this issue of the journal that inexperienced aviators could navigate using an olfactory map(p. 2888).
The mechanisms that inexperienced birds use to navigate have been a controversial subject for more than three decades. One possible mechanism,proposed by Floriano Papi in the early 1970s, suggests that naive homing pigeons navigate according to an olfactory map. He suspected that the young pigeons picked up scents blown to their aviary from all directions, and eventually learned to associate these smells with the direction that they came from. Although this map is probably relatively imprecise, it would be accurate enough for displaced pigeons to recognise which direction they had been moved before release, allowing them to set the correct bearing home. But Anna Gagliardo from the University of Pisa explains that this idea has always been challenged, with other scientists suggesting an equally controversial theory;that the birds may home using a magnetic map.
Whether or not naïve pigeons home by navigating from a magnetic or olfactory map is still not clear, but in 2004, Cordula Mora, Martin Wild and colleagues from the University of Aukland, New Zealand, used precise surgical techniques to show that the pigeons can detect magnetic fields, and the trigeminal nerve is essential for this sixth sense. They also suggested that if the birds were navigating with a magnetic map, experiments designed to test for olfactory navigation could have inadvertently damaged the trigeminal nerve and disrupted the bird's magnetic sense, preventing them from accessing their magnetic map for navigation, but leading the researchers to suspect that the birds' lost sense of smell was responsible for their lost sense of direction.
For Gagliardo, this was a challenge. She realised that there was only one way she could show which map-reading sense was essential for inexperienced navigators to find their way home; that was to release inexperienced birds,whose trigeminal or olfactory nerves had been severed, into the wild and see whether or not they could find their way home. And there was only one person who could do the tricky surgery with the necessary precision: Martin Wild. Gagliardo invited him to Italy.
Accepting the invitation, Wild travelled to Pisa for 12 days, where he worked round the clock operating on young adults that had never flown outside their loft. Severing the olfactory nerves in one group, and the trigeminal nerves in another, the team were ready to release the animals more than 50 km from home and record which direction they chose to head. Driving half of the birds to release sites north and south of their home loft, Paolo Ioalérecorded the animals' release directions, while Maria Savini waited patiently in the birds' loft to record when they returned.
Amazingly, all but one of the birds that had lost their magnetic sense returned to the loft within a day of release, but only 4 of the 24 birds that had lost their sense of smell returned home, and these took longer than a day while the rest were lost forever. The pigeons didn't seem to need their magnetic sense to find home, but they certainly needed their sense of smell.
