There are no mountains or other visual landmarks in the middle of the Atlantic Ocean, and if you find yourself without a map or GPS, chances are it's not going to be easy finding your way home. However, for a Cory's shearwater, navigation in this featureless environment is just another run-of-the-mill exercise. These birds spend most of their lives wandering the oceans, but every year they will return to the same small island to breed. Although it's like finding a needle in a haystack, these birds have no trouble. So, how do they do it? It's a question that fascinates Anna Gagliardo, from the University of Pisa, Italy. She explains that there are two potential cues that could aid their long-haul flights – their sense of smell or the natural geomagnetic field. But which cue is it? In collaboration with the researchers from her own university as well as the University of the Azores, Portugal, the Max Planck Institute for Ornithology, Germany, and the CNRS/CEFE, France, Gagliardo visited a colony of breeding Cory's shearwaters on the remote Portuguese Azores archipelago to find out how shearwaters made it back to their colony after being displaced (p. ).
During 2 years out in the field, the team captured 24 shearwaters, only catching birds that had just returned after a foraging trip to relieve their partner from egg incubation duty. Some of the birds were then given a GPS tracker and a magnet, as Gagliardo explains: ‘We manipulated their magnetic sense by using very strong cylinder-shaped magnets. The cylinder rolled inside a box that was glued on the head of the animal so the magnetic field masked the geomagnetic field and in addition it was not constant because the magnet moved.’ Other birds were given satellite transmitters and rendered temporarily anosmic (without a sense of smell) by lavaging their nostrils with a zinc sulphate solution. The birds were then placed on board a cargo ship heading east back to Portugal's capital, Lisbon.
After 24–39 h at sea the birds were released about 800 km away from their colony. Unaware that the team was looking after their eggs in their absence, all the birds set off, eager to get back to their nests. The team was then able to monitor what routes the birds took home either by GPS or continuous satellite updates. All the birds with magnets and a full sense of smell returned home. They were clearly able to successfully orientate themselves and navigate their way back to the nest, with all birds taking very similar and closely spaced routes. However, it was a different story for the birds without a sense of smell, recalls Gagliardo: ‘The results are very clear; the anosmic birds were unable to pinpoint the colony, they wandered around the ocean for thousands of kilometres and they were completely confused. Some of them did come back, but only after a long and winding trip.’
Gagliardo acknowledges that the results weren't entirely unexpected and that she always suspected that sense of smell was key to oceanic navigation in shearwaters. She admits this as she explains why only anosmic birds were given a satellite transmitter: ‘GPS [units] must be recovered in order to download the data from the log, but they are cheaper so we thought we'll take the risk, because it's very unlikely that these birds [with magnets] won't come back.’ It seems the risk paid of and it's clear that Cory's shearwaters use their large noses to find home, although what exactly they smell is the next mystery to be solved.
