Migration mysteries

woodcock_migration_by_rodger_mcphail

Recent research has revealed some truly fascinating findings about woodcock migration, says Owen Williams. But there is still much we do not know about our most enigmatic wader.

On October 29 last year, I caught and ringed my first woodcock of the winter. The first is always an exciting moment and so I posted a notice of this on the Woodcock Network's Facebook page. A day later, a photograph was posted of a woodcock being tagged in Japan. Yoshika had actually originally contacted me, asking for advice on how to catch woodcock, so I described our technique and to my delight he soon posted that he had just caught and ringed his first ever woodcock. Yes, the passion for woodcock is truly global!

The news that Yoshika was ringing the same species as myself, but on the other side of the world, caused me to marvel at the widespread distribution of this enigmatic bird. Its breeding range extends across a wide swathe of boreal forest that covers much of northern Europe, Scandinavia and Russia. When winter comes, much of this habitat becomes too cold and hostile, so the birds migrate to warmer latitudes to overwinter. The spread of this migration is truly remarkable with eastern breeders flying as far south as Thailand, Vietnam and parts of India. Central breeders head for northern Iran, Iraq, Lebanon, Egypt and Libya, while western breeders migrate to all of the Mediterranean countries including Morocco, Algeria and Tunisia. And with the warming effect of the Atlantic, we see woodcock heading in a more westerly direction to overwinter here in Ireland, the UK, France, Spain, and Portugal. Interestingly, there is also a sedentary population of woodcock in the Azores and Canary Islands. 

woodcock_migration_3It is likely that the North African wintering destinations were established during the last ice age, which ended approximately 12,000 years ago. At this time – when much of Europe was covered in ice and snow – the only areas with suitable boreal woodland habitat were Portugal, Spain, Italy and Greece. During these times, the onset of winter saw woodcock cross the Mediterranean to the warmer landmass of North Africa, where the coastal fringes offered a wintering habitat of rainforest and grassland. Nobody knows how woodcock arrived on the Azores; it may be that small numbers were driven there during the ice age, but due to persistent headwinds, they were unable to leave. 

After the ice age, southern Europe warmed and the boreal forest retreated northwards and continued to do so until today; its northern extent now lies across the north of Norway, Sweden and Finland, rather than the north of Spain, Italy and Greece. Although woodcock have to fly long distances to and from their breeding grounds, the risk associated with migration is worth it when weighed against the reality that this habitat offers their young an abundant food supply over the short summer. 

The harsh winter conditions in these areas also reduce disease and parasites, giving an additional advantage to woodcock broods. Despite catching over 1,500 woodcock, I have never encountered feather lice or any other avian parasite.

But not all woodcock make such long migrations. Considering the species across its range, we see a large variation in migration strategies depending on where they breed. In the late 19th century, Lord William Percy embarked upon a study that involved ringing woodcock chicks on his estate at Alnwick. This was the first research conducted involving bird-ringing in the UK. Recoveries of his special monogrammed silver rings over successive seasons showed that most of their woodcock didn't migrate at all. In contrast, recoveries from woodcock ringed in the far north of their breeding range shows that these birds migrate up to 3,800 miles to overwinter.

woodcock_migration_1We also know from Russian ringing data that juvenile woodcock set off from the breeding grounds on their first migration before the adults. A very high proportion of my early catch in November tend to be juveniles, with adult numbers gradually building through December, although a picture is now emerging that if the winter is mild, the adults take longer to arrive. 

I find it fascinating that juveniles make these migrations without any previous experience. Indeed, when we look at the pattern of juvenile migration, it is so consistent that it is difficult to believe that this is just a random process; there must be some degree of genetic hardwiring passed on from generation to generation that determines where an individual woodcock will fly to in the autumn. As we have already discussed, long after the ice age, woodcock still migrate to North Africa, despite flying over miles of perfectly suitable wintering habitat in southern Europe. Could it be that this is so genetically hardwired that, despite the passing of 13,000 years, it still impels woodcock to make this long journey? It would be fascinating to research how many of these birds are hardwired juveniles, and whether in following years as adults they moderate their migration urge to use less energy and overwinter on the north side of the Mediterranean. 

Recent developments in tagging technology have provided biologists with a much clearer picture of bird migration. This has resulted in the publishing of many scientific papers on this complex subject. I recently came across an excellent blog by Professor Gary Ritchison of East Kentucky University, which summarises some of the latest research – it makes fascinating reading. 

woodcock_migration_sparrowFor example, in one study by Thorup et al., migrating American white crowned sparrows – which usually migrate down the west coast of America – were intercepted and relocated to the east of the country. This revealed that, whereas adults made a course correction to accommodate their new location, juveniles were unable to do so and set off again on the same heading as they used on the west coast, thus taking them to the wrong wintering location. 

It has long been known that captive migratory birds demonstrate a tendency to fly towards a particular wall of their aviary at migration time. This has enabled scientists to experiment by changing various potential navigational factors such as star maps and magnetic fields in a planetarium style aviary. Research (Wiltschenko and Wiltschenko, 2009) showed that when the magnetic field or constellation maps were shifted, birds altered their flight direction to accommodate this change in orientation.

Monitoring avian brain activity (Heyers et al., 2007) has shown that a certain area of the frontal lobe of the brain, called Cluster N, becomes active in a magnetic field and that birds with lesions in this frontal lobe were no longer able to take magnetic directional clues. What is even more fascinating is that activity in Cluster N stops when the birds' eyes are covered, suggesting that there may be some sort of visual aspect to how birds perceive magnetic fields. 

As I write this article, a research paper has just been published by scientists from Baylor College of Medicine in Texas which shows further evidence of animals being sensitive to magnetic fields (magnetosensation). Their study discovered a microscopic structure at the end of a neuron in earthworms that is sensitive to magnetic fields. They knew that worms, when hungry, tend to move down in the soil and so they brought worms to the lab from different global locations and found that each worm moved at the precise angle which would have been downwards back home. For example, a worm from Australia moved up in the soil! This showed that each worm has a finely tuned sense of its local magnetic field. It is believed that other animals including birds have the same magnetic sensor neurons.

We know that the Earth's magnetic field inclines progressively towards the poles. Research (Beason, 1992) suggested that birds are not only able to detect magnetic north, but may also have a sense of that inclination, thus giving them a clue about their latitudinal position. Another magnetic indicator may come from the fact that the strength of Earth's magnetic field is not uniform across the globe. So, these variations may be detectable by migrating birds and – along with topographical features and visual astral clues – form part of their memory maps acquired whilst on migration. These references are then used to refine their navigational technique as they progress from simple ‘hardwired' juveniles to experienced migrating adults. The previously mentioned Thorup experiment on migrating white crowned sparrows appears to confirm this ‘refining' process.

There is still a huge amount to learn about bird migration. This task is particularly difficult with a secretive species such as woodcock, which has such a huge geographical range and employs a wide range of migration strategies. Despite what we have already learned so far from our use of tracking equipment and a significant increase in ringing, there is still a lot that we don't know about the enigmatic woodcock.

woodcock_migration_by_owen_williams2I often reflect on how fortunate I am as a non-scientist to be able to contribute in a modest way to this research. Little did I know in 2008 when we set up the Woodcock Network that our dedicated team of over 30 woodcock ringers would have increased the number ringed from rarely more than 50 to over 1,300 each year. This is providing both valuable biometric data and field observations that are contributing to future research.

Last year I launched my first bronze of a woodcock and the proceeds of the sales of this have funded the purchase of three geo-locators that I have personally fitted to woodcock on my ringing site in Wales. It has been wonderful to have the purchasers of the bronze join me to fit their tags to the birds. One very generous purchaser decided to fund his own satellite tag; and it was a special moment to share when this passionate woodcock shooter joined me to fit his tag in the field.

We can feel very proud that the shooting community has funded virtually all of this exciting work being done by Andrew Hoodless of the GWCT, and the Woodcock Network. There are new technologies just around the corner that will enable us to refine our understanding, not only of woodcock migration, but also the population movements and weather related behaviour of both migrant and resident woodcock. Further support through funding is essential if this exciting research is to continue. 

You can follow the journeys of the satellite tagged woodcock on the Woodcock Watch website. Data from recovered rings is also important, so if you have a ring from a shot woodcock please report it with the BTO.

www.bto.org

www.woodcockwatch.com

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