Bird Sense (27 page)

The Emlen funnel revolutionised the study of bird migration. It consists of a blotting paper funnel about
40
cm in diameter at its widest, with an ink pad at the bottom, and a domed wire mesh top – through which the birds can see the sky. As the bird hops, the ink on its feet leaves a trace on the blotting paper which provides an index of both the direction and the intensity of migration.
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The beauty of the Emlen funnel is that it is cheap and allows researchers to test large numbers of (small) birds very quickly. Sometimes it is only necessary to place a migrant in the funnel for an hour or so to obtain a meaningful trace. Using this method, which has been verified in many different ways, we now know that small birds have a genetic programme to fly in a particular direction for a certain number of days. Although this is remarkable, on its own it is insufficient to tell us how birds navigate. Certainly, it cannot explain how a Manx shearwater on a featureless Atlantic Ocean knows how to return to Skomer or how a nightingale, resting at an oasis in the Sahara on its spring journey north, knows how to find the previous year’s territory in a Surrey woodland.

The study of how birds find their way has had a long and sometimes acrimonious history. In the mid-
1800
s, there were two main ideas for how birds like pigeons found their way home. One was that birds remembered their outward journey, a notion for which there is no evidence. The other idea was based on the relatively recent discovery that the earth behaves like a gigantic magnet and that birds possess a sixth sense, allowing them to detect the earth’s magnetic field. The novelist Jules Verne was quick to capitalise on this and the main character in his book
The Adventures and Voyages of Captain Hatteras
(
1866
) ‘. . . under the influence of a magnetic force . . . was always walking towards the north’. The notion that birds, rather than people, might use a magnetic sense to navigate came from the Russian zoologist Alex von Middendorf in
1859
but he was given short shrift by most other ornithologists, including Britain’s Alfred Newton, in the late
1800
s.
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In
1936
Arthur Landsborough Thomson, another British ornithologist, wrote: ‘No evidence of any magnetic sense has ever been obtained . . . moreover the suggestion becomes less attractive on examination, because the phenomena seem quite inadequate for the purpose.’
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Similarly, in an otherwise insightful review, in
1944
Don Griffin said: ‘no sensitivity to a magnetic field has been demonstrated in any animal, and sensitivity to as weak a field as the earth’s is made extremely unlikely by the fact that living tissues are not known to contain any ferromagnetic substances (such as metallic iron oxide . . .) which alone are capable of exerting appreciable mechanical forces in the earth’s magnetic field’.
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Not long after this, in the early
1950
s, the German ornithologist Gustav Kramer started thinking about the problem in a new way, realising that navigation comprises two steps. Birds have to know where they are at the point of release, and they also have to know the direction of ‘home’. This is how humans orientate: the first step involves studying a map (where am I?), the second involves using a compass (which way is home?). This became known as Kramer’s ‘map and compass’ model.

There are several potential compasses. The one we are most familiar with is the magnetic compass, an instrument whose magnetised needle aligns itself with the field lines, or lines of force of the earth’s magnetic field, and points north. Migration biologists have identified other types of compass that birds use to navigate, including a sun compass – used by birds that migrate during the day – and a star compass – used by nocturnal migrants.

The first evidence that birds might possess a magnetic compass emerged in the
1950
s while Frederick Merkel and his student Wolfgang Wiltschko were studying the migration behaviour of European robins in Germany. Obviously, observing the process of migration can be difficult, especially with birds like robins which migrate at night. However, by capturing robins just before they set off on migration and placing them for a few hours in a specially designed ‘orientation cage’ – a precursor of the Emlen funnel – researchers could see in which direction they hopped or fluttered, behaviour that perfectly reflects their migration direction. Using orientation cages from which the robins could see the night sky, Merkel and Wiltschko found that the birds use the stars as their compass to maintain a south-westerly heading from Germany during their autumnal migration. However, when they looked at what robins did in
complete darkness
, they found that, far from being disoriented, which is what they expected, the birds continued to hop in their usual south-westerly direction. The implications were stunning: the stars were not essential for the birds to orientate themselves correctly. There had to be something else.

To test whether this ‘something else’ was a magnetic compass, they placed robins in orientation cages surrounded by a huge electro-magnetic coil, which allowed the researchers to alter the orientation of the magnetic field. They then compared the direction of the robins’ hopping when the field was reversed or shifted to the east or west. As they hoped, the birds behaved exactly as if they were able to detect the magnetic field and altered the direction of their hopping accordingly.
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Studies of other species subsequently produced similar results and by the
1980
s it was generally agreed – despite earlier scepticism – that birds do indeed possess a magnetic sense that allows them to read compass directions from the earth’s magnetic field. In other words, these birds
do
possess a magnetic compass.

Remarkably, birds also possess a magnetic
map
that allows them to identify their location – like a GPS system, but, rather than using satellite signals, birds use the earth’s magnetic field.
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Migratory birds are not unique in this respect: a magnetic sense has been detected in non-migratory birds like the chicken, as well as in mammals and butterflies, presumably to help them navigate over more modest distances.
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One reason why a magnetic sense had once seemed so improbable was that birds do not obviously possess a specific organ capable of detecting a magnetic field. For senses such as vision and hearing, the eye and the ear are very obviously designed to detect light and sound, respectively, directly from the environment. Magnetic sensations are different because, unlike light and sound, they can pass through body tissues. This means that it is possible for a bird (or other organism) to detect magnetic fields via chemical reactions inside individual cells throughout its entire body.

There are currently three main theories as to how animals, including birds, detect magnetic fields. The first is referred to as ‘electromagnetic induction’ and may occur in fish, but birds and other animals seem to lack the highly sensitive receptors necessary for this mechanism. The second involves the magnetic mineral known as magnetite (a form of iron oxide), discovered in certain bacteria in the
1970
s, which is responsible for bacteria aligning themselves with a magnetic field. Further research revealed that other species, including honeybees, fish and birds, also possessed minute crystals of magnetite. Microscopic crystals of magnetite were detected around the eye and in the nasal cavity of the upper beak – the latter inside nerve endings – of pigeons in the
1980
s. As we’ll see, these were promising locations if the crystals were to play a part in navigation.
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The third theory comprises the interesting possibility that a magnetic sense might be mediated by a chemical reaction.

In the
1970
s it was discovered that certain types of chemical reaction could be modified by magnetic fields, but at the time no one imagined that such a process might help migrating birds find their way. Even more remarkably, it seems that these particular chemical reactions are induced by light, prompting a group of researchers in the United States to speculate that birds might be able ‘see’ the earth’s magnetic field.
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This unlikely idea prompted Wolfgang Wiltschko and his wife, Roswitha, to investigate. They knew from the research of others that free-flying pigeons with one eye covered by an opaque patch were better at homing if they could see with their right eye, rather than with their left. Significantly, this better right-eye performance was most pronounced under cloudy conditions (when the sun was not visible). This meant, of course, that the birds could not be using a sun compass, but suggested that perhaps they were using a magnetic sense somehow linked to the right eye. It sounds unlikely, but the Wiltschko team also knew that birds’ brains are highly lateralised, and the pigeon result was consistent with the left brain (which receives visual information from the right eye, as we saw in chapter
1
) being better at processing information relating to homing and navigation. To test this idea directly, the Wiltschkos turned to their favourite study species once again, the European robin.

With both eyes uncovered, the robins hopped in their normal migratory direction. But when the magnetic field was experimentally switched through
180
° (as in their earlier experiments), the birds also switched their hopping direction by
180
°. The robins were then tested with one eye covered by an opaque patch. With the right eye exposed to light (i.e. a patch over the left eye), the birds’ orientation was exactly as it had been with both eyes receiving light. But with the right eye covered and only the left eye receiving light, the robins were unable to orientate – implying that they could not detect the earth’s magnetic field. This extraordinary result suggested that only the right eye can sense the earth’s magnetic field.

How does this right-eye/left-brain process work? Is it simply that the right eye is more sensitive to light? To find out, the Wiltschkos conducted a further test, putting the equivalent of contact lenses on their robins. Both ‘lenses’ allowed the same amount of light into the eye, but one lens was frosted, giving a fuzzy image of the world, while the other was clear. The results were once again startling. The right-eye/left-brain effect remained, but when the robins viewed the world only through a frosted lens over the right eye they were unable to orientate. With a clear lens on the right eye, they orientated with precision, as before.

What this means is that light itself is not crucial, but what matters is the clarity of the image. It appears that it is the robin’s ability to see contours and edges in the landscape that provides the appropriate signal to trigger the magnetic sense. Extraordinary! As one of my colleagues said: ‘You couldn’t make this stuff up.’

Where does this visually induced chemical reaction leave the magnetite idea that I mentioned earlier? They do not seem to be alternatives, but, rather, two separate processes that might work in unison in the same animal: the chemical mechanism based in the eye provides the
compass
, while the magnetite receptors in the beak provide the
map
. The compass may detect the
direction
of the magnetic field while the map detects the
strength
of the magnetic field, and by integrating both types of information birds can find their way home, whether it is across a featureless ocean or crossing large land masses.
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The fact that a magnetic sense in birds was once considered impossible, and that discoveries about the senses of birds are still being made, is extraordinary. It is discoveries like this that make science buzz.

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Emotions

The greeting display of a pair of northern gannets – what do partners feel on being reunited?

Many scientists appear to be uncomfortable about using the term emotion when referring to animals, for fear that they automatically imply anthropomorphic assumptions of human-like subjective experience
.

Paul, Harding and Mendl,
2005
, ‘Measuring emotional processes in animals: the utility of a cognitive approach’,
Neuroscience and Behavioral Reviews
,
29
:
469
–
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