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

Wandering dragonfly

Globe skimmer travels thousands of kilometres

globe skimmers travel large distances

The dragonfly Pantala flavescens, the globe skimmer, was already known to be a migrating species that travels enormous distances. By conducting chemical analyses of the wings, Keith Hobson and colleagues once again confirm this picture.

It is well known that some butterflies migrate between widely separated regions where they spend summer and winter: monarch butterfly and painted lady. It is less known that two species of dragonflies exist that don’t refrain from undertaking a long journey either: the spot-winged glider, Pantala hymenaea, from North, Central and South America, and above all the globe skimmer dragonfly Pantala flavescens, which has an almost worldwide distribution. These species belong to the family Libelludae.

globe skimmer resting during migrationEarlier research by Daniel Troast had shown that no genetic differences exist between globe skimmer populations from North America (US), South America (Guyana) and Asia (India, Korea and Japan). This means that these populations are in contact with each other. In other words: the insects must be able to travel great distances.

Unique achievement

And they do, according to chemical analyses of the wings by Keith Dobson and colleagues. These analyses focus on the ratio of hydrogen isotopes, which corresponds to that of the water in which the dragonflies lived during their larval stage. The hydrogen isotopic composition of water bodies depends on precipitation and temperature.

Earlier, Hobson had elucidated how globe skimmers migrate annually from Northern India, or perhaps even across the Himalayas, to East Africa and back. The total distance per cycle is at least 18,000 kilometres and it takes several generations to complete it. An individual dragonfly travels up to 6,000 kilometres during its lifetime, and many individuals fly 3,500 kilometres across the ocean. That is a unique achievement in the insect world.

During flight, migrating dragonflies catch small prey from the sky. They fly at high altitudes, probably using winds that are associated with the so-called Intertropical Convergence Zone. The zone changes position during the year, causing wind and unstable weather.

Summer migrants in Japan

Now, Hobson took a look at globe skimmers that occur in Japan in summer. They can be found from April to November and occur in large numbers from June to September. Most of Japan is too cold for them in winter, so they don’t hibernate there. When they appear in spring, they come from elsewhere, thousands of kilometres away.

As the wing analyses revealed, the first specimens, in April, probably come from the southwest: South China and Southeast Asia. Later, in the summer, dragonflies arrive from the west: North China and Mongolia, or from South China, North India and the Tibetan Plateau. Might the latter trip be a continuation of the journey that the animals take from East Africa? Unfortunately, the researchers don’t mention it.

Still later, in October and November, dragonflies keep coming from the west; Hobson also found insects from Korea and the east of Russia. Only a few animals had grown up in Japan. The migration appears to be related to the wind direction, which is predominantly westerly in summer.

Rapid development

The wandering existence of the globe skimmer is partly possible because the larvae develop rapidly. While that development takes ten months in other species, the globe skimmer needs about six weeks. And then migration can continue.

This short development time also means that globe skimmer dragonflies are not dependent on areas with permanent water bodies for reproduction. Females can also use temporary water bodies from rainy periods to lay eggs in.

Willy van Strien

Photos
Large: The globe skimmer dragonfly, Pantala flavescens. Rison Thumboor (Wikimedia Commons, Creative Commons CC BY 2.0)
Small: Resting globe skimmers. Shyamal (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Another migrating insect: painted lady

Sources:
Hobson, K.A., H. Jinguji, Y. Ichikawa, J.W. Kusack & R.C. Anderson, 2020. Long-distance migration of the globe skimmer dragonfly to Japan revealed using stable hydrogen (δ 2H) isotopes. Environmental Entomology, online Nov. 21. Doi: 10.1093/ee/nvaa147
Troast, D., F. Suhling, H. Jinguji, G. Sahlén & J. War, 2016. A global population genetic study of Pantala flavescens. PLoS ONE 11: e0148949. Doi: 10.1371/journal.pone.0148949
Hobson, K.A., R.C. Anderson, D.X. Soto & L.I. Wassenaar, 2012. Isotopic evidence that dragonflies (Pantala flavescens) migrating through the Maldives come from the northern Indian subcontinent. PLoS ONE 7: e52594. Doi: 10.1371/journal.pone.0052594

Flying over sand and sea

Painted lady travels between Europe and tropical Africa

painted lady is long-distance traveller

It was discovered two years ago that the painted lady butterfly migrates to the south in autumn and reaches the tropical savannahs of Africa. Now, Gerard Talavera and colleagues proved that their offspring returns back in spring.

Most European insects spend a resting period in winter. But the painted lady doesn’t. The butterflies, which can be found almost everywhere in Europe during summer, leave when autumn arrives. At high altitudes and benefiting from downwind, they fly over Europe, including mountain areas; they cross the Mediterranean and then the Sahara, enduring extremely low and high temperatures along their way. An individual painted lady can travel four thousand kilometres within a week: an incredible achievement for these fragile-looking creatures.

Eventually, they massively arrive in the savannahs of tropical Africa south of the Sahel, as Gerard Talavera and colleagues showed two years ago. In this area, the wet season is just about to end and the vegetation is exuberant. The butterflies lay eggs and the caterpillars feed on fresh green leaves. A new generation emerges while it is winter in Europe.

Cycle

But it was unclear what happened to the descendants of the autumn migrants. After the start of the dry season, the savannahs dry out and the area rapidly becomes unsuitable for the painted lady. The butterflies disappear, but the question was: where do they go to? Perhaps the butterflies that moved into tropical Africa chose a dead-end path and go extinct. Perhaps they travel further south to find fresh green plants. Or maybe they cross the sand and the sea again to return to Europe, where they are seen in spring.

Talavera and colleagues suggested the latter possibility to be true. The painted lady population would undertake a migration flight two times a year to be able to exploit both the European summer and the green period south of the Sahara. Such a cycle would cover at least six successive butterfly generations. As yet, there was no proof that it really happened.

But now, the researchers have been able to demonstrate that painted lady butterflies indeed migrate northwards from the African savannahs in springtime. The chemical composition of the wings of a butterfly reveals where it grew up as a caterpillar. Painted ladies that are found in early spring around the Mediterranean did develop in tropical Africa, according to such chemical analysis. And they are on their way to the north.

Flying ability

So, the painted lady outperforms the monarch butterfly, a well-known long-distance traveller of North America. It covers a distance of up to about twelve thousand kilometres per cycle, the monarch covers ‘only’ ten thousand kilometres.

The monarch butterflies have a distinct wing pattern, but with the same colours: mainly black and orange. For the monarchs it was found that the more black they have on the wings and the more intense their orange colour is, the greater is their flying ability. The same might be true for the painted lady: an intriguing question for further research.

Willy van Strien

Photo: Painted lady, Vanessa cardui. Fritz Geller-Grimm (Wikimedia Commons, Creative Commons CC BY-SA 2.5)

A video made by the researchers: Vanessa’s Odyssey

Sources:
Talavera, G., C. Bataille, D. Benyamini, M. Gascoigne-Pees & R. Vila, 2018. Round-trip across the Sahara: Afrotropical Painted Lady butterflies recolonize the Mediterranean in early spring. Biology Letters 14: 20180274. Doi: 10.1098/rsbl.2018.0274
Talavera, G. & R. Vila, 2016. Discovery of mass migration and breeding of the painted lady butterfly Vanessa cardui in the Sub-Sahara: the Europe–Africa migration revisited. Biological Journal of the Linnean Society 120: 274-285. Doi: 10.1111/bij.12873
Stefanescu, C., D.X. Soto, G. Talavera, R. Vila & K.A. Hobson, 2016. Long-distance autumn migration across the Sahara by painted lady butterflies: exploiting resource pulses in the tropical savannah. Biology Letters 12: 20160561. Doi: 10.1098/rsbl.2016.0561
Hanley, D., N.G. Miller, D.T.T. Flockhart & D.R. Norris, 2013. Forewing pigmentation predicts migration distance in wild-caught migratory monarch butterflies. Behavioral Ecology 24: 1108-1113. Doi:10.1093/beheco/art037