Before the American naturalist Audubon used vultures to do experiments on bird smell, buried a rotten dead pig under a sugar cane stalk, tested whether the vultures would be attracted by the putrid smell, it turned out that no vultures looked for the smell, and later he threw a piece of odorless deerskin full of fillings outside, but attracted a vulture, so he guessed that the vultures did not rely on smell to feed. Birds have long been thought to have a less sensitive sense of smell, and textbooks have written about it. But a recent experiment will give people a new understanding of the sense of smell in birds.
There has always been an interesting phenomenon, the white stork always goes to the field to forage for food after the farmer mows the grass, but how does the white stork know that the farmer has cut the grass? To answer this question, German zoologist Professor Martin Wikelski flew a helicopter to monitor dozens of white storks with GPS. The findings were published in a research paper titled "Smell of green leaf volatiles attracts white storks to freshly cut meadows" in the journal Scientific reports. The study confirmed through two interesting experiments that white storks were lured to foraging in farmland by the smell of freshly mowed grass.
European white storks congregate on fields where farmers cut grass (Figure 1). Until now, it is unclear whether storks are also close to being cut in pre-industrial times. Interestingly, farmers are often confused by the fact that storks only approach them during mowing activities.
Aerial and ground observations
The study is based on the assumption that storks may smell the smell that drifts with the wind and will only approach farmland if they get these olfactory cues. Using 11 flights lasting 2-4 hours, the researchers identified the locations of 70 white storks within a radius of about 6 kilometers (Figures 2 A, B) that could be associated with agricultural activities. That is, to know where the white stork perched in the upper and lower winds. To determine whether storks use only olfactory cues, the researchers had to rule out all sensory effects that can sense food, such as visual, auditory, or social cues. Visual cues can be ruled out because trees and terrain obstruct the white storks cannot visually touch the tillage site, and the auditory cues of farming activities are also excluded because they are too far away, because the white storks have the best hearing in the 0.5-15 kHz range. However, experiments have shown that it is not possible to record any auditory information about agricultural activities from background noise of more than 300 m. To ensure that storks cannot use the same species of birds or other birds that are also flying as an indication, the data is recorded only when no other birds in the air can provide a mow signal (black and white dots in Figure 3).
Natural farming experiments
The researchers flew planes hovering over the trimmed farmland to see if storks were moving closer to the farm from any direction. As soon as a stork starts flying, record the time and direction, as well as the number of storks in the flock. Subsequently, record whether storks or stork herds approach farmers and when they are looking for food on this newly cut land.
The ground observation is calibrated with the GPS footage on the white stork to ensure consistency between ground observation and aerial monitoring. Wind speed and direction in the air are measured with the Cessna GarminG1000 instrument and wind speed and direction on the ground using a standard anemometer.
The results show that when the air from the farmland reaches the nose of the white stork, they will fly only a few meters above the farmland ground (Figure 3B). In the case of stork "Zozu" (Figure 2), GPS data confirmed the results of aerial observation and showed that no visual or auditory information was available at a distance of about 4 kilometers. Since Zozu was the first bird to reach the farm, no other bird could point to this potential feeding site. When examining natural tillage events, the researchers observed that storks living in the windward direction of the newly cultivated fields did not approach the experimental area (Figure 3). In contrast, only storks located in a downwind cone of about 75° approach tillage activity (black dot in Figure 3A). The farther the wind direction of storks deviates from the centerline, the slower they approach the experimental area (Figure 3B). This indicates that the white stork only comes to the field to forage for food when asked about the smell of mowing.
Two pretend experiments
To experimentally determine whether storks only use olfactory information to approach potential foraging sites, the authors conducted two experiments: One was to move the grass that had just been cut in the distance to the experimental site, trying to attract the white stork to smell the smell of the mowed grass and come to feed. The second is to spray a chemical solution that represents the smell of grass on grasslands without agricultural activities to see if they can attract white storks.
Experiment 1: Pretend to mow the grass
Because if the grass is too high, they have no way to touch the food on the ground, so they usually take advantage of the farmer's mowing to forage in the field. So the researchers arranged for farmers to cut a whole truckload of fresh grass about 15 kilometers away and immediately transport the grass to about 2.5 kilometers from ground activity within 45 minutes (travel time) (orange dot in Figure 3). The farmer then distributes the freshly cut grass to a field where the grass passed about two weeks ago, that is, the grass there is still very short and there are no white storks to feed for for a while. Then observe from the air if there are white storks approaching.
After the fresh grass cut from elsewhere was placed on the field without mowing activity, 3 individual individuals and some white storks began to search the field for food, but after about 10 minutes, they realized that there was no food available in the cut grass and left (Figure 2), and the experimental results also apply to the above 75° downwind cone theory. This suggests that the white stork is attracted to the smell of grass cut from elsewhere to feed.
Experiment 2: Chlorophyll volatile spraying
The ultralight plane (the green dot in Figure 3) flew at a low altitude of about 20 meters, spraying a three-component mixture (simulating the smell of mowing) onto untreated grass-covered fields. The chemical mixture used to mimic the scent of fresh mowing consists of three green leaf volatiles, namely (Z)-3-hexenal, (Z)-3-hexenol, and hexyl acetate. These volatile organic compounds have been shown to be the main VOC compounds released when the leaves are mechanically injured. Observe again in the air to determine if any storks will approach.
Chlorophyll volatile spraying experiments showed that grass fragrance attracted 9 individual groups of white storks and some white storks from 0.5 to 1.2 km away (Figures 4A, B), but the control group that sprayed water did not cause white storks living in the same location to approach (Figure 4C). Storks that are not within the 45° range of the wind cone under chlorophyll volatilization ± are also not close to this area (n=7). This is consistent with the angle at which they naturally approach farmland (Figure 3A). The results of this experiment are also applicable to the above 75° downwind cone theory. This means that white storks will go to their fields to forage for food as soon as they smell the smell of this three-component chlorophyll volatile compound.
epilogue
The study found that storks approached pastures in the wind only if they could perceive atmospheric information from newly cultivated fields, without relying on auditory, visual or social cues. The study proved that the cut grass, or even the three main olfactory components of the cut grass (green leaf volatiles), were enough to attract storks close to the hypothetical foraging site. This shatters the idea that most birds primarily use visual signals to find food. However, previous experiments have shown that the New World vultures use scent to find corpses. Similarly, some seabirds inhabit upwelling areas of dimethyl sulfide (DMS), where there are large numbers of marine microorganisms and vertebrates that congregate on the surface of the sea, facilitating frequent group feeding by seabirds. While birds feed primarily through sight or hearing, it has become clear over the past decade that olfactory cues may be more important to birds than previously expected.
Original link:
https://doi.org/10.1038/s41598-021-92073-7
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