A team of German researchers set out to test how White Storks locate freshly mown fields so rapidly, finding that the birds were homing in on the scent of newly cut grass.
When pastures are cut by farmers, the wealth of insects and rodents that are disturbed or caught up in the blades provides a valuable feeding opportunity for birds such as storks and raptors.
White Storks are well known for arriving on the scene extremely quickly, as if from nowhere, but how they detect just-mown fields has been a mystery. There are a number of possibilities, with the birds conceivably using visual, social, auditory or olfactory cues to locate these feeding opportunities.
White Storks fly towards newly cut grass as soon as air from the field reaches them. Farming activity provides easy access to insect and rodent prey (Tom Hines).
Martin Wikelski from the Department of Migration at the Max Planck Institute of Animal Behaviour, based at the University of Konstanz, led a team of researchers seeking to find out how storks located freshly mown pastures.
The team observed a local population of around 70 White Storks from a plane over the course of 11 flights, each lasting between two and four hours. They recorded the activity of storks with a 6-km radius of farming operations, carefully excluding the possibility that visual cues were leading storks to the location of a mower by taking note of obstructions between the bird's starting position and the machinery.
The use of auditory cues was excluded as, time and time again, no storks moved towards farming equipment that was in operation but not mowing. For good measure, they excluded observations of storks moving towards a mower that was less than 600 m from their position.
Social cues were harder to exclude, but the team recorded storks moving towards mowing activity when there were no other storks or raptors indicating the presence of farming operations. Where circling birds of prey or storks could have given away the position of a field being cut, the farming event was excluded from the dataset.
They also logged the position of each stork in relation to farming activity, so that they knew which were stationed upwind or downwind of the field being cut. Storks that did not react to the activity were logged alongside those that flew towards the field in question.
As another part of the study, freshly cut grass from 15 km away was transported to a field cut two weeks previously, and a leaf alcohol mix simulating the smell of freshly cut grass was sprayed onto an untouched field. In both cases, separate groups of storks from downwind of the fields made an approach and searched for food for a short while before giving up. Storks that were not downwind (within 45°) of these fields did not fly towards them.
This is similiar to observations of 'natural' farming events, where fields were being cut by farm workers. Storks upwind of a freshly worked field did not approach the area, whereas those positioned downwind of the activity flew in rapidly. The further off the direct downwind line the birds were, the slower they were to fly towards the area being cut.
The researchers say that their results show that storks only fly towards cut pastures when air carrying the scent of freshly cut grass reaches them, and that the birds do not rely on auditory, visual of social cues. Even the experimental placement of an imitation of the scent was enough to draw birds positioned downwind in.
They said: "This unambiguous demonstration of the use of atmospheric odours to learn about foraging opportunities breaks with the notion that storks, like most birds, primarily use vision to find food."
It is widely considered that most birds rely on sight to find food, with the exception of New World vultures and some seabirds, but this study opens up the possibility that a wider variety of species might use scent to locate feeding opportunities.
Wikelski, M, Quetting, M, & 7 others. 2021. Smell of green leaf volatiles attracts white storks to freshly cut meadows Scientific Reports. DOI: https://doi.org/10.1038/s41598-021-92073-7