Storks respond to scent compounds

Storks like to forage in meadows. When the grass has been mown, the search becomes considerably easier.

Because birds don’t have proper noses, they can’t smell well — or so it was thought. Now researchers are demonstrating through ingenious experiments just how excellent the sense of smell of white storks really is.

At the moment, white storks can be seen in many places foraging for prey such as snails, frogs, and small rodents on freshly mown meadows. Until now, it was assumed that the sight of short grass or the noise of mowing machines attracted the birds. Now researchers from Constance and Mainz have shown, surprisingly, that the scent of mown grass alone is sufficient.

«People simply assumed that birds cannot smell well because they don’t have proper noses», said lead author Martin Wikelski, Director at the Max Planck Institute of Animal Behavior in Constance. «Yet they have a very large olfactory bulb in their brain with many receptor molecules for scent compounds.»

The team therefore specifically investigated whether white storks use their sense of smell. «My assumption was that the storks respond to the intense smell of freshly cut grass», explained co-author Jonathan Williams. This characteristic scent comes from green leaf volatiles and is based on just a few molecules that are also used in perfumes.

Storks follow their nose

In the study, the team observed stork movements both from an aircraft and via birds fitted with GPS sensors. «We first had to rule out that the storks could hear the tractor or see the mowing taking place», says Wikelski. They therefore only included storks in the observations that were more than 600 metres away from the mown meadow and had no line of sight to it.

When mowing began, only storks that were positioned downwind of the meadow in question flew over to it. Individuals positioned upwind showed no reaction. Even when the team spread freshly cut grass on a meadow that had already been mowed earlier, storks came flying in, as reported in the journal «Scientific Reports». Even a solution of green leaf volatile compounds sprayed onto a meadow was enough to attract the storks.

The result contradicts the previously held assumption that storks primarily use their eyes to search for food. «There were storks that flew over 25 kilometres from the other side of Lake Constance to reach the mowed meadows», says Wikelski. The researchers suspect that the sense of smell may also play a major role in the foraging behaviour of other bird species. After all, birds of prey such as buzzards and red kites are also known to frequent freshly mowed meadows.

Information on flight routes

Staff at the Max Planck Institute in Radolfzell have ringed and fitted young storks with transmitters in Bühl-Oberweier. The aim is to scientifically study the migratory behaviour of the birds. The researchers want to investigate how many of the storks fly to Africa to overwinter and why some remain in Europe through the winter.