Stripping Down Science (17 page)

Social networking for birds? At this rate they'll all be into Twitter …

It's not just humans that like a warm winter holiday, birds do too. And luckily for them – having wings of their own – they're spared the exigencies of economy class travel. Yet although they migrate in their millions to-and-fro between far-flung corners of the globe, scientists really know precious little about this incredible feat of avian navigation. Previous statistics suggested that the average songbird might manage about 150 kilometres a day. Now, thanks to new research, this turns out to be an intellectual flight of fancy: the real rates are almost mythical by comparison.

One of the biggest problems with studying songbird migrations has been how to accurately follow the birds' progress. In the past, tracking equipment was far too bulky for tiny birds to bear without the risk of overloading them, which meant that scientists were forced to rely on marking the animals and then keeping an eye out for their arrival or even trying to pursue them by aeroplane, as one team did. As a result, most attempts to log bird migrations only ended
up recording where the animals departed from and when and where they ended up, missing the critical aspect of what happened in between.

But now Bridget Stutchbury from York University in Canada
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has solved the problem by fitting tiny geolocating bird-backpacks to 14 wood thrushes and 20 purple martins caught in their native Pennsylvania. The backpack devices are about the size and weight of a small coin, contribute less than 5% to the bird's entire weight and record light levels, so the position of a bird wearing one can be accurately pinpointed by referring to the sunrise and sunset times, which vary geographically.

Once equipped, the birds were released and embarked on their winter migrations to South America. The following year, when the birds returned to their US mating sites, some were successfully recaptured, allowing the data to be downloaded from their geolocators and the routes taken by the birds over various time periods to be retraced. ‘Never before has anyone been able to track songbirds for their entire migratory trip,' Bridget Stutchbury points out.

What was really surprising were the speeds at which the birds were covering the ground. Moving more than 450 kilometres per day, they were travelling three times faster than scientists had previously thought possible. The following spring, the return leg was even more impressive. They got back up to six times faster than during the outward leg, although they did have an agenda: they were heading home to mate and there's a significant competitive advantage to being the first back, because early arrivals have access to the best nest sites and the most food – quite similar to students returning to university at the start of the academic year, you could say.

The researchers were pleasantly surprised by the results. ‘We were flabbergasted by the birds' spring return times. To have a bird leave Brazil on April 12 and be home by the end of the month was just astounding. We always assumed they left some time in March,' Stutchbury said.

The researchers also found that prolonged stopovers were common during fall migration. The purple martins, which are members of the swallow family, had a stopover of three-to-four weeks in the Yucatan before continuing on to Brazil. Four wood thrushes spent one-to-two
weeks in the south-eastern United States in late October before crossing the Gulf of Mexico, and two other individuals stopped on the Yucatan Peninsula for two-to-four weeks before continuing migration.

There is, however, a sad note to this happy tune. Songbird numbers internationally are in dramatic decline, with some monitoring organisations reporting 70% reductions in the numbers of some species since the 1960s. Human encroachment and destruction of the animals' natural habitats have been blamed as the leading causes, alongside pesticides and poaching, but it's very hard to know how to intervene effectively in the conservation of a species if you don't know why it's threatened in the first place. This is where this study comes in.

‘Songbird populations have been declining around the world for 30 or 40 years, so there is a lot of concern about them,' points out Stutchbury. ‘Tracking birds to their wintering areas is also essential for predicting the impact of tropical habitat loss and climate change. Until now, our hands have been tied in many ways, because we didn't know where the birds were going. They would just disappear and then come back in the
spring. It's wonderful to now have a window into their journey.'

Another important question that needs answering is what do birds do about bad weather? The next step Stutchbury and her colleagues are planning is to marry up the songbird migration routes with weather reports to see how individual birds respond when faced with a storm. Will it ruffle their feathers and leave them in a flap, or will they rise above it?

More than 50 different animal species, including mammals, turtles, lobsters, fish, birds and bees are known to possess a built-in compass of some sort that allows them to plug into the earth's magnetic field so they can find their way around. Salmon, for instance, appear to use this trick to return from the ocean to reproduce in the rivers in which they were born, homing pigeons and bats both go off course if exposed to strong magnetic fields, as do bees and ants, and some bacteria even seem to know their north from south.

But despite many years of study, no one has yet discovered exactly how any of these species do this. One theory is that magnetically sensitive deposits of iron-containing compounds – like magnetite – are used, and some animals do show accumulations of iron particles in some parts of their bodies. More controversially, other scientists have suggested that some species may actually be able to see magnetic fields, although, attractive as this sounds, it's remained very much a magnetic myth because scientists haven't managed to find
any sort of chemical reaction that would be sufficiently sensitive to a relatively weak field like the earth's.

That is, until now, because Oxford scientist Peter Hore
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has recently uncovered a quantum mechanical chemical trick that some animals could be using to make magnetism visible. He proved this using a large, purpose-built molecule called CPF. At one end, this has a chemical group called a carotenoid, which consists of a long chain of carbon atoms flanked by carbon atom rings. This is linked to a large molecule called a porphyrin ring, which sits at the centre of the structure. Bonded onto the other side of this is a carbon ‘football' known as a fullerene. Seen from the side, it looks a bit like a miniature molecular weather vane, although with the important distinction that it's sensitive to magnetic fields rather than wind direction.

Molecules of CPF were suspended in liquid crystal and frozen at minus 80 degrees Celsius to lock them into position. They were then excited using light of a certain wavelength while tiny magnetic fields, on par with those produced by
the planet, were applied in different orientations. Incredibly, the molecules altered their chemical behaviour according to the direction of the magnetic field.

There were several stages to the process. First, the CPF molecule was excited by the light. This made the chemical groups at each end of the molecule (the carotenoid and the fullerene) temporarily become free radicals, which means that they each contain an unpaired electron. These electron radicals are spinning, and normally they spin in opposite directions to each other. But because the CPF molecule is quite long (as molecules go), these free radicals are separated by a distance of about 3.5 nanometres (3.5 billionths of a metre), which means that their spins can be affected by a magnetic field applied in the right direction. When this happens, one of the electrons can flip over, so the two end up spinning in parallel, rather than in opposite directions.

This quantum flip effectively unbalances the molecule, making it adopt a form called a triplet state. In this condition, before it can release the extra energy it absorbed and return to its starting state, it needs to go through an
additional reaction step, which means it remains in this altered state for slightly longer, giving the chemists an opportunity to detect it. In the eye, if a similar chemical process exists, this prolonged altered state could be picked up by some kind of signalling molecule and then relayed to the brain to make the animal aware of the orientation of the surrounding magnetic field.

Of course, the present experiments are a highly artificial situation, not least because eyes don't contain CPF. But birds' eyes do contain light-sensitive chemicals called cryptochromes, which could behave in the same way, adopting an altered ‘radical state' when the animal is aligned with the magnetic field. ‘This could trigger a chemical to change shape, which could in turn kick-start other biochemical processes to enable a bird to see the earth's magnetic field,' says Hore. So maybe there is something to saying that certain people have a magnetic personality after all …

FACT BOX

Magnetism and animals

Although scientists still don't understand how animals detect the planet's magnetic field, evidence of them doing so is everywhere to see, including, as German and Czech researchers have recently shown, on the nearest cattle farm, and even on Google Earth.

In 2008, Sabine Begall, from the University of Duisberg-Essen,
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used Google's satellite profile of the planet to find pictures of cattle grazing in fields at 308 locations on six continents: Europe, Africa, India, North and South America and Australia. These images, containing over 8500 cud-chewing cows, were analysed to see in which direction the animals were pointing in their fields. At the same time, field trips were made to observe more than 2900 deer in the Czech Republic.

Astonishingly, both the cows and the deer were arranging their bodies to point along the line of magnetic north–south. The effect
was slightly less pronounced over Africa, but this would be expected of a magnetic effect because the earth's field is slightly weaker there. Amongst the deer in the Czech Republic, the majority of the animals were found to stand (and sleep) pointing north. About a third of them arranged themselves in the reverse (south–north) direction at any one time, which might be a strategy to help them to look out for predators approaching from behind.

These findings were highly statistically significant, suggesting that the effect is real. Begall scrutinised the images looking for other reasons to explain why the animals should all line up like this, but found none. She was able to dismiss prevailing winds on the grounds that animals all over the world were doing the same thing simultaneously, yet the prevailing winds vary in direction depending upon location. Similarly, she wondered whether the animals were adjusting their sun exposure as a form of temperature control or whether they were positioning themselves to avoid being dazzled by bright light, but again, neither of these
possibilities could explain the planet-wide observation, which means only magnetism remains as an explanation.

For the moment, science is left with an interesting enigma. Why should these large, social ruminants, all with extensive ranges, feel compelled – or should that be repelled – to align themselves with the earth's magnetic field? Perhaps this is the bovine or cervine equivalent of geophysics – a way to methodically plod around a field to find food most efficiently by working in rows? Or maybe the magnetic field alters vision, making predators easier to spot? In humans, scientists have reported that the EEG (brainwave) pattern is slightly different depending upon whether someone sits facing north–south or east–west, and the rapid eye movement (REM) phase of sleep also changes subtly according to whether sleepers are slumbering in an east–west or north–south repose. But what the significance of this is, or even how it happens, no one knows. Observation and explanation, you could say, appear to be poles apart …