Flying high

Great reed warbler avoids being roasted by the sun

Great reed warbler flies extremely high during migration

During migration, the great reed warbler may climb to extreme altitudes, Sissel Sjöberg and colleagues show. Presumably, that is to control their body temperature.

Imagine: the great reed warbler, a medium-sized songbird, climbs up to 6 kilometers above ground level during migration. The discovery of Sissel Sjöberg and colleagues also surprised the researchers themselves.

The birds breed in Europe and overwinter in tropical Africa; in both locations, they stay in reed beds. So, they make a big trip twice a year. They typically fly at night and use the day to rest and eat.

Nonstop

But when great reed warblers cross the Mediterranean, they cannot land. And when they fly over the Sahara Desert, it makes no sense to go down, because there is nothing edible there. Across such barriers, they continue to fly during daytime, traveling more than 30 hours non-stop if necessary.

To find out more about flight behaviour during migration, Sjöberg equipped several birds with data loggers that record various data during migration: light, ambient temperature, and altitude. In addition, the data loggers register the movements of the birds: whether they are flying, resting, or moving on the ground in search of food.

The birds fly at night at an altitude of 2400 meters on average, it turned out, which is already quite high. But if they prolong their fly into daytime, they go more than twice as high. At dawn they quickly climb to about 5400 meters, up to more than 6000. At dusk, they descend in an equally short time.

Why do they do that?

From an altitude of 1500 meters on, temperature and wind speed are the same between day and night; the temperature at 2400 meters is about 14 °C. So, that is no reason for going higher during daytime.

Heat radiation

A difference between day and night is the presence of the sun. The great reed warblers, the bodies of which are already warm due to their flapping flight, cannot stand the solar heat radiation, Sjöberg thinks. It puts them in danger of getting overheated. At 5400 meters it is 9 °C below zero; there, they will not be overheated by solar radiation. So, when the sun is shining, the birds better fly much higher.

An additional advantage may be that during the day, the birds have a better view of the landscape when flying higher. Also, they are out of reach of birds of prey, especially Eleonora’s falcon, which hunts up to 3500 meters.

The biennial migration is a great achievement. The long flying periods in which the great reed warbler goes extremely high during daytime make it extra impressive. Hats off.

Willy van Strien

Photo: Great reed warbler in breeding habitat. Zeynel Cebeci (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Source:
Sjöberg, S., G. Malmiga, A. Nord, A. Andersson, J. Bäckman, M. Tarka, M. Willemoes, K. Thorup, B. Hansson, T. Alerstam & D. Hasselquist, 2021. Extreme altitudes during diurnal flights in a nocturnal songbird migrant. Science 372: 646-648. Doi: 10.1126/science.abe7291

Escape scene

Young aphids try to hitch a ride on adults

young pea aphid hitches a ride on an adult

Young pea aphids demand a piggyback ride on the back of an adult when they have to walk on the soil, Moshe Gish and Moshe Inbar report. But they don’t always get what they want, because the adults try to remove them.

When their host plant starts shaking and a warm, moist air is blowing over it, sap feeding pea aphids (Acyrthosphon pisum) are in danger: a mammalian herbivore is approaching. Just before the plant is eaten by the grazer, they massively drop off the plant to the ground to escape. But they are not safe there either; they can be trampled, dessicate, starve or fall prey to predators that hunt on the ground, like spiders. And so they start walking in search of a new plant.

That is not easy for young aphids (the nymphs). The ground is bumpy with clumps of soil, cracks, stones, fallen twigs and leaves. The young critters are moving much slower than adults. But there is a solution, Moshe Gish and Moshe Inbar discovered: piggybacking on a large aphid.

Tiny drama

In a series of lab experiments, the researchers simulated escape scenes. They placed about ten female aphids on a fava bean plant. Next morning, young aphids had been born; pea aphids can reproduce parthenogenetically, females giving birth to daughters without having been mated. By tapping the plant and exhaling over it, the researchers induced the aphids to drop off. At some distance from the fava bean plant, a circle of lentil plants was placed to offer a destination, the bottom was covered with soil.

A tiny drama takes place in such a case, as the researchers observed. Immediately after landing, young aphids try to climb on an adult. They also climb on green plastic beads and on dead aphids that they happen to stumble upon, but from these, they quickly disembark. If a living aphid stays immobile after they climbed on, they get off after a while too. But when their carrier starts walking, they ride to a new plant, where they arrive faster than when they would have walked the distance on their own.

But adult aphids are not really willing to help young ones.

Nuisance

On the contrary: passengers seem to be a nuisance to them. When a nymph tries to climb on an adult aphid, this aphid will often raise its body to make it difficult, or it runs away. If there are already nymphs on its body, it often stays motionless, waiting until the passengers leave. Or it repeatedly lowers its head or posterior end to the surface to get rid of them. The back is the best place for a nymph to sit on. Eventually, an adult aphid will start walking with one passenger at most.

Once it has left, after a delay, it will reach a new plant as soon as an aphid without a hitchhiker, so the walking speed is not affected by the burden. But it is energetically costly to bear it.

Willy van Strien

Photo: © Stav Talal

Also these tadpoles try to catch a ride

Sources:
Gish, M. & M. Inbar, 2018. Standing on the shoulders of giants: young aphids piggyback on adults when searching for a host plant. Frontiers in Zoology 15: 49. Doi: 10.1186/s12983-018-0292-7
Gish, M., A. Dafni & M. Inbar, 2010. Mammalian herbivore breath alerts aphids to flee host plant. Current Biology 20: R628-R629. Doi: 10.1016/j.cub.2010.06.065

In its own bubble

Alkali fly manages to stay dry in very wet water

Alkali fly can enter the alkaline Mono Lake

Each insect would drown in Mono Lake, a saline soda lake in California. Each insect, except for the alkali fly, which has unique adaptations to survive the extreme environment, as Floris van Breugel and Micael Dickinson show.

Only bacteria, algae and brine shrimp tolerate the saline water of Mono Lake in California- and the alkali fly Ephydra hians, which flourishes here. It is an amazing critter. The larvae develop in the water, feeding on the algae. Adult flies, which occur in great numbers along the shores, regularly crawl into the water to feed on algae or to lay their eggs. They are the only adult insects that are able to dive into the briny water and emerge alive, Floris van Breugel and Michael Dickinson report.

Wax

Several insect species exist that survive submersion in the water of lakes or streams, thanks to a water-repellent layer of hydrocarbons (waxy substances) on their cuticle and tiny hairs that trap a layer of air. But they would be wetted and drown in Mono Lake. That is because the water of this lake contains a large amount of sodium carbonate, a salt known as baking soda, the presence of which renders the insects incapable of keeping the layer of air intact; such water is ‘wetter’ than pure water and penetrates into the layer of air. Sodium carbonate is one of the substances that make the water strongly alkaline.

But the alkali fly easily dives into the alkaline water and when it emerges, it is completely dry. The researchers show that this is possible because the diving fly possesses a very dense mat of hairs and a layer of superhydrophobic hydrocarbons which, combined, prevent wetting. Upon entering the water, a stable air bubble forms around the fly, which enables it to spend 15 minutes underwater; the bubble protects it from the hostile water and offers a supply of oxygen.

Limestone columns

In the past 60,000 years, Mono Lake became more salt and more alkaline because it has no outlet, and as water is evaporating, the concentrations of mineral salts gradually rise. Limestone columns, named tufa, developed, along which the flies descend below water surface.

The alkali fly’s ancestors had to enter the lake to forage in a time when the lake was still fresh and algae were the only food available, the authors hypothesize. While the lake gradually became more alkaline, the fly adapted to the new conditions. Nowadays, it can safely graze underwater, as no fish predators occur. The flies are preyed upon by many birds that forage near the lake, like gulls.

When oil, from decaying organic matter or sunscreen and other cosmetics, is floating on the water, even the alkali flies are wetted and drown, in spite of their unique adaptations.

Willy van Strien

Photo: Alkali fly, pictured under water inside its protective air bubble. Floris van Breugel/Caltech

Source:
Van Breugel, F. & M.H. Dickinson, 2017. Superhydrophobic diving flies (Ephydra hians) and the hypersaline waters of Mono Lake. PNAS, online Nov. 20. Doi: 10.1073/pnas.1714874114