Shearwaters deprived of their olfactory sense before being displaced to distant sites have impaired homing ability but it is unknown what the role of olfaction is when birds navigate freely without their sense of smell. Furthermore, treatments used to induce anosmia and to disrupt magneto-reception in displacement experiments might influence non-specific factors not directly related to navigation and, as a consequence, the results of displacement experiments can have multiple interpretations. To address this, we GPS-tracked the free-ranging foraging trips of incubating Scopoli’s shearwaters within the Mediterranean Sea. As in previous experiments, shearwaters were either made anosmic with 4% zinc sulphate solution, magnetically impaired by attachment of a strong neodymium magnet or were controls. We found that birds from all three treatments embarked on foraging trips, had indistinguishable at-sea schedules of behaviour and returned to the colony having gained mass. However, we found that in the pelagic return stage of their foraging trips, anosmic birds were not oriented towards the colony though coastal navigation was unaffected. These results support the case for zinc sulphate having a specific effect on the navigational ability of shearwaters and thus the view that seabirds consult an olfactory map to guide them across seascapes.
Pigeons were made anosmic by unilateral treatment of their olfactory mucosa with a zinc sulphate solution and by plugging the contralateral nostril. In a series of releases at unfamiliar sites, 55­79 km from the home loft, the experimental birds' homing behaviour was compared with that of two control groups: unmanipulated control birds, and birds subjected to unilateral zinc sulphate treatment and equipped with an ipsilateral nasal plug. The experimental pigeons exhibited homing behaviour ­ in terms of both homeward initial orientation and homing performance ­ significantly poorer than that of both unmanipulated and treated control pigeons. In addition, the homing behaviour of the treated controls turned out to be only slightly, and not significantly, poorer than that of the unmanipulated birds. The results show that the impaired homing capabilities of the zinc-sulphate-treated birds are due to the lack of navigational information and not to non-specific brain damage caused by the experimental treatment.
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