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On the fly

Research into butterfly migration proves rewarding

Rebecca Nesbit 30 November 2008

www.lablit.com/article/442

Suspended animation: a specimen flies on, oblivious to its lack of relative motion

Research into insect movements has never been so important: as our climate changes, so will migration patterns

Four delicate wings and 1,500 km to fly: the journey begins in Africa. Each spring butterflies emerge from their chrysalises and take their chances on incredible migrations. The summer in Africa is too dry for the caterpillars’ food plants to grow; the butterflies must find cooler climates to breed in. And so Britain receives a beautiful array of butterfly migrants each year, from the striking red and black of the Red Admiral to the delicate Long-tailed Blue, a rare visitor from the Mediterranean. Moths too make migrations, often in far larger numbers than the butterflies. They make use of high altitude wind currents, but it would be wrong to think that they are at the mercy of the wind. Just like birds they choose the direction they travel in; they don’t end up on our shore by chance.

One of our most fascinating butterfly migrants is the Painted Lady, with long, slender wings mottled orange and tipped with white. The spring migration brings the Painted Lady to Europe from the desert fringes of North Africa; these butterflies may have been to places most of us can only dream of. In good years they cover Buddleia bushes in gardens right across England.

It is the extraordinary migration of the Painted Lady which leads to sightings of an otherwise normal looking girl (me) running round Buddleia bushes and thistle patches, stalking butterflies with my net. The butterflies can often be caught off guard while they’re feeding; one swish of the net and they’re mine. Despite carrying the official title of ‘student’, I am no longer blessed with a 3-month summer holiday. For the last three summers I have been entirely at the mercy of the weather and the butterflies, as I collected all the data I needed for my PhD. I graduated from the University of Durham in 2005 and alarmed many of my friends by committing to four years of chasing butterflies. But I have never looked back. Setbacks such as two summers of very poor weather (2007 and 2008) and an appalling year for Painted Ladies (2007) are all part of being an ecologist.

I have spent each spring in Gibraltar. Migrating butterflies choose the shortest sea crossing from Africa, and many stop and feed at the first land they find. Birds use the same migration route; eagles, vultures and storks fly above our heads as we work. Working at a site with sea views across three countries and two continents makes up for any experiments going wrong (thankfully, because they regularly did). The tapas bars across the border in Spain helped as well.

Having caught the butterflies, I attach them one at a time inside our ‘flight simulators’. The unglamorous exterior consists of a white plastic barrel on top of a small table. But inside is a delicate mechanism that allows us to record a butterfly’s flight behaviour. A tungsten rod protrudes into the barrel and the butterfly is attached to it, with glue. The rod is rigid, so the butterfly is suspended in mid-air and flies on the spot. The experiment works because the butterfly is completely oblivious to the fact that it’s not getting anywhere, so just keeps flying. The rod moves on a low friction bearing and the butterfly is able to turn freely. The turning is detected by a computer, so we can always see in which direction the butterfly is heading, allowing us to plot out the path it would have taken if it was flying free. We let each butterfly fly for 15 minutes; in the wild a butterfly could cover an incredible 1.5 km in that time.

Recording migratory flights in this way gives us insight into butterfly migration. We have shed some light on an enigma that has long puzzled naturalists. The spring Painted Lady migration is very visible: butterflies are often recorded flying north in large numbers to breed in northern Europe. However, their offspring are rarely seen migrating south in the autumn. Could it be that the flight to the UK was wasted? Do the descendents of the spring arrivals simply die when the weather gets cold?

The author and a colleague running the flight simulators

I think not. In the autumn, we have seen butterflies fly south in the flight simulators. We have an idea about why we don’t see this autumn migration in the wild. Radars have detected butterflies flying hundreds of metres above the ground, taking advantage of fast winds. Perhaps the butterflies we see migrating at ground level are just a small part of the story, and many butterflies return to Africa after all. So actually the Painted Ladies haven’t got it all wrong, flying north as Englishmen flock south to join the mad dogs in the Mediterranean sun. By migrating to Europe and back they can ensure that they’re always in the right place to breed – a fresh and juicy thistle patch.

I have also looked at how the butterflies know which way to fly. Migrating birds use the Earth’s magnetic field, the stars and the sun to guide them, but these complex systems of navigation do not seem possible in an insect. Our research suggests that butterflies use a sun compass. If we block the sun from view, the butterflies no longer head in the correct direction.

It may seem harsh that a butterfly, having made it so far across land and sea, should end up tethered inside a flight simulator. But the small number of butterflies that we catch has no impact on the population; it is just a tiny fraction of the number of butterflies lost at sea or eaten by birds. Research into insect movements has never been so important: as our climate changes, so will migration patterns. Understanding butterfly migration will help us work out how their populations will change. Some species will spread north, while other species will suffer losses. This is important because some butterfly species may be threatened with extinction, but also because some caterpillars are agricultural pests. My work is a vital piece in the incredible puzzle of butterfly migration. Wildlife has always fascinated me, and the more I learnt the more I realised nature’s complexity. I am fortunate to be working on a project that catches many peoples’ imaginations, including my own. The challenges of studying butterflies outside in the sun make my work both trying and fulfilling.