Paris - At least one species of sea-faring bird uses its sense of smell to navigate over ocean waters, according to a novel experiment described in a study released on Tuesday.Temporarily deprived of the ability to smell, Scopoli's shearwaters had trouble finding their way home after embarking from the Spanish island of Menorca to forage, researchers reported in the journal Scientific Reports.The olfactory-challenged birds easily flew nearly 200km to the Catalan coast to gather food, but set off at the wrong angle on the return trip."They embarked on curiously straight but poorly oriented flights across the ocean, as if following a compass bearing ... without being able to update their position," lead author Oliver Padget, a doctoral candidate at the University of Oxford, said in a statement.Earlier experiments had produced similar results, but with a shortcoming in methodology: birds stripped of the capacity to smell were displaced long distances and then released.That left open the possibility that any disorientation might come from the displacement itself, or the inability to gather information on the outbound journey, rather than the lost sense of smell."To the best of our knowledge, this is the first study that follows free-ranging foraging trips in sensorily manipulated birds," said senior author Tim Guilford, a professor in the University of Oxford's Department of Zoology.In the new experiments, researchers split 32 birds into three groups.One was made anosmic - the technical term for "unable to smell" - by inserting zinc sulphate into the nasal passage, while another was fitted with small but powerful magnets.Odour signatureIt is also thought that migrating birds - some of which cross the globe in a single go - use Earth's magnetic field to find their way. Nothing was done to the third "control" group that might change the birds' behaviour.All three groups wore miniature GPS trackers."Precision, on-board tracking technology and new analytical methods - too computationally heavy to have been possible in the past - have made this feasible," said Guilford.The birds bearing magnets were unperturbed in their journeys, but the ones dosed with zinc had to reach land before fully recovering their bearings and rectifying their trajectories.If the study removes any doubt that olfactory faculties play a role in navigation, the question still remains: what are the birds smelling?"The short answer is that we do not know what odours shearwaters" - which fly close to the water - "might be using to navigate", Padget told AFP.Other research has suggested that albatrosses, petrels and shearwaters are sensitive to an organic chemical given off by marine algae and phytoplankton, single-cell organisms at the bottom of the ocean food chain.But rather than following a scent to its source, birds may use them like landmarks, biologists have suggested."It is more likely that different places in the environment have characteristic odour signatures, and that these signatures may become associated with particular directions back toward the home colony," Padget said.