The keen eyes of an eagle, the extraordinary hearing of an owl – the birds’ senses are perfectly adapted to find their food. Now, an experiment has shown that storks use a further purpose for this: They can sniff freshly mown meadows and thus the areas where they can find frogs or small rodents particularly well. In general, the sense of smell for birds could play a greater role than previously thought.
It’s a familiar picture for the farmers on Lake Constance: When they start mowing their meadows, storks often appear out of nowhere next to the tractors. The white storks live in the humid areas around the lake and feed on snails, frogs and small rodents that find shelter in high meadows. If these meadows are mowed, the small animals are easy prey.
What role does the sense of smell play?
How the storks find out where a meadow is currently being mowed has not yet been clear. Because it was believed that birds mainly rely on their eyes and ears. But with these senses, freshly mown meadows are often difficult to spot. But how then? A research team led by Martin Wikelski from the Max Planck Institute for Animal Behavior in Radolfzell got to the bottom of the question. “My guess was that the storks react to the intense smell of the freshly cut grass,” says co-author Jonathan Williams from the Max Planck Institute for Chemistry in Mainz. The typical smell is produced by so-called green leaf fragrances and consists of only three different molecules. It is therefore conceivable that the storks could sniff them.
The storks have the prerequisites for this: “It was assumed that birds cannot smell good because they don’t have real noses,” explains Wikelski. “They have a very large olfactory bulb in the brain with many receptor molecules for fragrances.” The team investigated whether the sense of smell actually leads the storks to freshly mown meadows. Observed sensors of tagged animals. “First we had to rule out that the storks could hear the tractor or see the mowing process,” says Wikelski.
Therefore, the researchers only included storks in their observation that were more than 600 meters away from the mown meadow and had no direct visual contact. They also made sure that the storks did not become aware of the mowing process through the behavior of conspecifics or other birds.
Always follow the leaf scent
And indeed: the observations confirmed that the storks’ sense of smell guides them to their source of food. It was shown that when mowing began, only those storks flew to the meadow in question, which were located downstream in a downdraft cone of around 75 degrees – even from long distances. “There were storks that flew over 25 kilometers from the other side of Lake Constance to the mown meadows,” says Wikelski. The conspecifics, however, who were upwind and therefore could not perceive the smell of grass, did not react.
To make sure that the smell of the cut grass alone attracted the storks, the scientists then switched to a meadow that had been mowed two weeks earlier. “The grass in this meadow was still very short. That is why it is of no interest to the storks for foraging, ”explains Wikelski. To deceive the birds, however, the team distributed grass on this meadow that had only recently been mowed on another, more distant meadow. A short time later the first storks flew in again – evidently attracted by the smell. In the last test, Wikelski and his colleagues mixed a solution from the leaf fragrances and sprayed it on another test meadow. The result: This meadow with the artificially applied scented cocktail also attracted storks from the area.
“This proves that storks find their way to feeding sites through smells in the air,” says Williams. This finding contradicts the previous assumption that birds mainly use their eyes and ears to forage. The researchers suspect that the sense of smell could also play a greater role than previously assumed when other bird species search for food. For example, birds of prey such as buzzards and red kites, which are regularly observed over freshly mown meadows.
Source: Max Planck Society, Article: Scientific Reports, doi: 10.1038 / s41598-021-92073-7