https://phys.org/news/2018-07-homing-pigeons-local-natural-odors.html

The first group was transported in open crates and released without manipulation. Pigeons in group two were carried and kept at the release site in airtight containers ventilated with natural air before they were released. The third group also travelled in an airtight container, with the air filtered to remove most of the natural odorants it might contain. During their travels and before their release, the birds in this group were exposed to the scents of lavender, eucalyptus, rose and thyme. Prior to their release, the nasal cavities of the birds in the second and third groups were washed with a zinc sulphate solution in order to make them temporarily unable to smell.

There was a noticeable difference between each group in terms of how well the pigeons could navigate. As expected, most of the unmanipulated control pigeons found their way home successfully. By contrast, the majority of the pigeons in each of the other two groups was impaired at homing. However, an interesting difference emerged between the two groups released under anosmic (smell disrupted) conditions. The birds exposed to environmental odour cues performed significantly better than the birds stimulated with artificial smells.