Bird Sense (26 page)

Perched
70
m above the sea, I lean forward with my guillemot hook, my arms stretched to the limit, the birds edging further away, dancing around the hook, determined not to get caught. After thirty minutes, I give up and climb back up the rope to where, out of sight of the birds, my expectant colleagues are waiting. They are disappointed at my failure, as am I, and Ben offers to have a go.

He disappears over the edge until all we can see is the top of his climbing helmet, and occasionally the end of the pole, as he patiently edges closer to the birds. When the birds are on high alert like this, the only hope is that there is some kind of distraction, like a fight or a bird arriving from the sea with a fish. That’s exactly what happens – a fight (I can hear the aggressive calls) – and I see Ben move the pole in a purposeful manner. Suddenly he’s up the rope and, with a huge grin, hands me a guillemot bearing a distinctive green geolocator.

We carry the bird another
70
m up to the cliff top where Guilford’s students are waiting. The geolocator – still on the bird’s leg – is plugged into the laptop and the data downloaded. Satisfaction isn’t guaranteed: sometimes the devices fail. But not this one. Within minutes of capture, like magic the bird’s previous
370
days appear on the computer screen. Lying on the grass, we cluster around the laptop, shading the screen from the sun. A map of the world appears, pinpointing every ten-minute fix, until the bird’s entire year of travel emerges.

This is what we see: soon after the end of the previous breeding season last July, the bird set off south for the Bay of Biscay, spending a few weeks there before flying
1
,
500
km north to spend much of the winter off north-west Scotland. Then, back down to Biscay again in the weeks before the start of the current breeding season and then back on to this very ledge on Skomer.

This is instant gratification, a year’s worth of unique data delivered in a few moments on a computer screen. It seems miraculous, and, indeed, the new tracking technology – geolocators, satellite trackers and so on – has resulted in a revolution in the study of bird movements, migration and navigation.

We later recover the geolocators from several other guillemots and, reassuringly, they all show similar patterns, providing us with a dynamic picture of the huge distances these birds move during the winter months while away from the colony.

This is novel guillemot information. These few results completely change our view of their movements based on decades of ringing recoveries. Ben and I are delighted; for years we had fantasised about where our guillemots might go outside the breeding season. But, compared with some of the recently completed studies on other species, ours is a modest success. Geolocators have recently been used on birds as small as red-backed shrikes and nightingales, tracking their migratory movements from northern Europe to Africa and back. In terms of distances travelled, however, the most spectacular results come from shearwaters, albatrosses and Arctic terns, all of which undergo monumental oceanic journeys, and particularly impressive is the bar-tailed godwit’s eight-day,
11
,
000
-km non-stop flight from New Zealand to Alaska.
1

As we sit on the Skomer cliff top in the sun, the map on the computer screen in front of us raises an important question. How, with nothing but an oceanic horizon, do guillemots know which way to fly to find their breeding colony, or, indeed, to find their feeding areas in Biscay or off northern Scotland? How do those godwits traversing the entire Pacific Ocean know where to go? How birds find their way – not just during migration, but during their everyday lives – is a question that has been asked many times in the past millennia.

Two of the many people who asked this same question were David Lack and Ronald Lockley in the
1930
s. Lack was then a schoolteacher at Dartington in Devon, studying robins in his spare time, later famous for his book
The Life of the Robin
(
1945
), and somewhat later as the most renowned ornithologist ever. Ronald Lockley was an amateur ornithologist, who, in
1927
, at the age of twenty-six, set up home with his wife Doris on the uninhabitated island of Skokholm, five kilometres to the south of Skomer. Over the next few years Lockely studied the island’s seabirds, including its most numerous and mysterious species, the Manx shearwater. Around
150
,
000
pairs of shearwaters breed on the adjacent islands of Skomer and Skokholm, some
40
per cent of the world population. The birds are nocturnal, to avoid predatory gulls, and come ashore between March and September only to breed, spending the rest of the year at sea. Lockley’s investigations of the bird’s breeding biology broke new ground for, at that time, very few seabirds had been studied in any detail.

In June
1936
, Lack took a group of schoolchildren to Skokholm where they camped alongside Lockley’s tiny whitewashed cottage. One evening, as dusk fell, Lack and Lockley began to talk about how birds found their way, speculating about what might happen if Lack were to take a shearwater back to Devon with him: how quickly would it return to Skokholm? The children listening in loved the idea so when Lack and the children left Skokholm on
17
June they took with them three shearwaters, each bearing a unique ring. Sadly, two birds died en route, but the third, a bird that Lockley had named Caroline (he was unashamedly anthropomorphic about his ringed birds), was duly released from Start Point, southern Devon, at
2
p.m. on
18
June, some
225
miles (
360
km) by sea from Skokholm. Communication between Devon and Skokholm was limited to the postal service, which could take several days, so Lockley was unaware that two of his precious birds had died. Not expecting any bird to return until
19
June at the earliest, Lockley nonetheless decided to check their burrows just before midnight on the evening of
18
June. To his amazement Caroline had returned and was incubating her egg, just nine hours and forty-five minutes after release. Ecstatic, Lockley wrote: ‘It is clear . . . that Caroline knew the way. She had no time for searching. She recognised in what direction Skokholm lay and made for it. Our success with Caroline was provocative. It suggested further experiment.’
2

To establish whether the shearwaters had a true navigational sense, Lockley and Lack realised that they had to be released from locations they could not possibly have visited previously. Accordingly, there followed a succession of ‘releases’ from a variety of increasingly ambitious locations, including an inland site in Surrey in the UK, from Venice in Italy and from Boston in the USA. The rapidity with which some of the birds returned to Skokholm reaffirmed a strong navigational sense.
3

Lockley’s pioneering studies were continued and developed by Geoffrey Matthews, an ornithologist at the Wildfowl Trust in Slimbridge, Gloucestershire, in the UK, who elevated the operation to a rather more scientific level in the early
1950
s. Matthews released birds from a variety of locations, including the top of the Cambridge University library tower, noting their direction of departure and being careful not to release the next bird until the previous one had flown out of sight (to avoid the possibility of them influencing each other). The majority of birds left the library tower in a westerly direction and, flying across country, returned directly to Skokholm, ‘providing the first unequivocal evidence of a true navigational ability in a wild bird’.
4
I wonder what any birdwatcher, oblivious to the great experiment, might have thought had he chanced to see one of these shearwaters winging its way westwards so many miles from the sea.

Not only did Lockley start the shearwater navigation studies, but he was the first to examine the bird’s breeding biology, establishing that its incubation period was fifty-one days; the fact that partners incubated in alternating six-day shifts; and that the slow-growing chick spent no less than ten weeks in the burrow before fledging. Lockley left Skokholm in
1939
just before the outbreak of the Second World War, but in the early
1960
s there was renewed interest in Skokholm’s shearwaters when Mike Harris started his PhD studies there. Harris began ringing large numbers of shearwater chicks in an effort to understand better the birds’ biology, and between
1963
and
1976
ringed – with the help of what were affectionately known as the ‘shearwater slaves’ – a staggering
86
,
000
birds. A fortuitous spin-off from all this ringing was a number of recoveries that provided a glimpse of where the shearwaters went outside the breeding season. It was already known that Manx shearwaters occasionally appeared in the southern hemisphere; the great seabird biologist Robert Cushman Murphy had seen one off the coast of Uruguay in
1912
, but it was assumed that such sightings were of birds whose breeding colonies were at the very south of their range in the Azores. Confirmation that Skokholm birds sometimes travelled the
10
,
000
km to South America first came from a ringed bird found dead on the coast of Argentina in
1952
. But, of course, one swallow (or in this case one shearwater) makes neither a summer nor a convincing case for regular long-distance movements.

That Manx shearwaters do indeed regularly winter off the coast of South America was elegantly confirmed in the
1980
s when Mike Brooke and his former PhD supervisor Chris Perrins decided to look at the
3
,
600
recoveries of ringed shearwaters accumulated over the previous twenty years. Using ringing recoveries to infer movement patterns of seabirds is a bit like trying to ascertain the summer holiday locations of British tourists from the police stations where their lost passports are handed in – crude at best and subject to all sorts of biases. The recoveries suggested that the shearwaters leave their breeding colonies on Skokholm and elsewhere in Britain in the autumn, fly south past the Bay of Biscay, on past Madeira, the Canary Islands and West Africa, and then somewhere near the equator cross to South America, arriving off the coast of Brazil. The return journey the following spring starts with birds heading out into the central South Atlantic and then heading back to Britain via a slightly more westerly route than their southward migration.
5

In August
2006
, Tim Guilford and his colleagues placed geolocators on the males and females of six pairs of Manx shearwaters breeding on Skomer Island. Because shearwaters nest in burrows, they are much easier to recapture than guillemots. The following spring, soon after the female had laid her single egg, all twelve birds were recaptured. The geolocator analysis confirmed the broad pattern of movement previously deduced from fifty years of accumulated ringing recoveries, but provided some unexpected information as well. First, the birds wintered further south than ringing recoveries suggested: off the coast of Argentina, south of the Rio del Plata in an area of mixed ocean currents that presumably provides rich fishing for the birds. Second, it was previously thought, on the basis of the occasional very rapid ringing recovery – including a bird that was found on the coast of Brazil just sixteen days after ringing – that shearwaters fly directly to their wintering grounds. The geolocator information showed that such rapid, direct flights are not typical: rather, the birds have frequent stopovers, much as terrestrial migrants do, presumably to refuel. In some cases, shearwaters remained at their stopover locations for a couple of weeks.
6

While this new technology has extended and refined our view of the huge global distances some birds travel, so far at least it has not provided many new insights into
how
birds make these journeys and how they find their way.

Paradoxically, perhaps, it has been through the study of
captive
birds that we have gained most understanding of navigational mechanisms. In the early
1700
s casual observers of caged songbirds like the nightingale noted how their birds began an agitated hopping each autumn and spring when they would normally be migrating. Two hundred and fifty years later, in the
1960
s, biologists were finally able to capitalise on this so-called migratory restlessness through the use of an ingenious device known as an Emlen funnel, after Steve Emlen who invented it.
7