Birds largely rely on their sense of smell to navigate on their long migration routes. Indeed, the “third sense” has been shown to be a more important for them than orientation based on the sun and the earth’s magnetic field. Exactly how birds navigate on their migration routes has not yet been fully clarified. How does a bird develop an “internal map”? How does it find its way back to last year’s nest?

Adult migratory birds are clearly able to remember migration routes. Even if they end up flying over unknown terrain, they still manage to find their way to the right place. Researchers at the Max Planck Institute for Ornithology in Radolfzell and their colleagues at the universities of Princeton, Pisa and Copenhagen have established in a field study that odours considerably facilitate bird migration and act as a more important navigational cue than the sun and the earth’s magnetic field.

Researchers already know from the fledglings of many bird species that they use an inherited species-specific compass point to reach their wintering grounds on their first migratory journey. If they are moved a significant distance away from their starting point, they fail to reach their destination.

As opposed to this, adult migratory birds are able to remember routes that they have flown just once, and to correct their flight direction following a change of location and find their way back to their wintering locations. This is proof of real navigation performance and, based on this, scientists are trying to identify the factors and mechanisms that enable the animals to find their locations.

Researchers working with Richard Holland and Martin Wikelski from the Max Planck Institute for Ornithology in Radolfzell and the University of Konstanz began by studying small songbirds in their natural environment. To this end, they captured 24 adult and 24 juvenile catbirds (Dumatella carolinensis) in the field station of Princeton University (New Jersey). The researchers then manipulated the olfactory sense in eight birds from each group by applying a saline solution to the birds' nasal mucous membranes. As a result, the birds were no longer able to smell properly; however, their olfactory cells were not permanently damaged. The researchers also manipulated the magnetoreception capacity in another eight birds from each group by means of strong magnetic impulses. The sensory perception of the birds in the control group remained unimpaired.

Simultaneous to this, 19 adult catbirds were captured in Illinois -- in the middle of the American continent -- transported overnight to New Jersey and divided there into the same three groups. The scientists then fitted all of the birds with a radio transmitter weighing 0.9 grams and released them. This enabled the ornithologists to observe the flight of the birds from the both the ground and the air.

In the course of their autumn migration, catbirds usually fly through New Jersey in a south-west direction along Delaware Bay. If -- like their fellow species members in Illinois -- they flew a strict southern course, they would arrive in Cape May. This would mean that they would cross Delaware Bay at its widest point or would have to fly northwards up the coast again until they reach a narrow point in the bay. For this reason, adult birds usually avoid the direct southern route.

The non-smelling adult birds selected a different flight route than the experienced control group and the animals whose magnetoreception had been manipulated. The birds, whose olfactory capacity had been impaired, were unable to find their bearings and flew south. Like the juvenile birds, they had to fall back on their endogenous direction-finding skills because they could no longer rely on their sense of smell. The animals from Illinois with impaired olfactory perception also flew in a southerly direction while the control group tried to compensate for the change in location by flying in a south-westerly or westerly direction.

In contrast to the olfactory manipulation, the magnetoreception manipulation did not affect the orientation in either the adult or juvenile birds. "Other field studies have also failed to produce clear proof of any effect of the distortion of magnetoreception," says Richard Holland. Therefore, the question arises for the researchers as to how important the magnetic field actually is for avian navigation over large distances. "Nevertheless, we do not assume that the failure of the magnetic pulse treatment to trigger any effect indicates that magnetoreception does not play any role in the migration of adult birds." The results of the study enabled the scientists to reach the initial conclusion that the olfactory sense is a significant component of the birds' navigational chart. In addition, the experiment also provides a reliable method for future field studies aiming to examine the role of environmental factors in bird migration.