By Jonas Waldenström
At this time of the year the air is full of migrating birds. Some, as cranes or geese with their conspicuous formations are easily spotted with the naked eye, while other birds, including most smaller songbirds, fly at altitudes where you need a scope to see them. But you can often hear them; each species has its own tune, and an experienced ear can tell them apart on call alone.
The question “how do they find their way” is as old as the field of ornithology itself. Generally, migration wouldn’t be possible without some sort of compass; a way of telling the bird in which direction to move. It has been shown that birds may use the sun, the stars, and the earth’s magnetic field for assessing their heading. And in some species also visible cues, a sort of map sense from previous travels, or even olfactory cues (a posh word for smelling where home is). As the vast majority of birds migrate without the guidance of their parents (which seems reserved to some flock-living species), a juvenile bird must be born with not only the tools to assess where it is, but also a sense of where it should go.
One of the pioneering fathers of ornithology was the Dutch professor Albert Christiaan Perdeck. He made one of the first real tests on how birds can sense where they are going, and how they can adjust the course if they get out of track. In order to test this he wanted to do a displacement study, where birds should be experimentally transported to a novel site, far from the catching site. As this study was conducted in the 1950s, in the pre-gadget era of ornithology, he needed a species that he could catch in large quantities, and where ring recovery data could be collected. His choice of study animal was the European Starling Sturnus vulgaris, a common farmland bird in most of Northern Europe. Starlings in autumn can aggregate in huge flocks, sometimes consisting of several thousand individuals, and was thus a good target species for Perdeck.
With a remarkable enthusiasm, the team caught and ringed thousands of starlings. Some were released at the ringing site in the Hague, while the other half were transported with airplanes to Switzerland and released. After some time the ring recoveries started to come in, and the results were extremely interesting. It seemed as the young starlings had a vector compass, as the birds that were transported south stayed on the same heading as they had when they were caught. But instead of ending up in Holland, the young starlings ended up way south, sometimes even on the Iberian peninsula. I wrote ‘young’ deliberately, as there was a clear age effect. Where the juvenile birds continued on the same vector, the adult starlings compensated for the displacement, changed course and headed to the original winter quarters. Adult birds are more experienced, and in the starling case they were able to adjust to the circumstances and get back on the right track. A quite remarkable feat – some of my colleagues cant find their way to the university canteen without a helper…
Spurred by the old studies (classics, you could say) and the advancement of new tracking tools we conducted a similar experiment with Mallards. The study was a collaborative effort with scientists from Sweden, Germany, the UK and Denmark (with the lead from Professor Martin Wikelskii at the Max Plank Institute for Ornithology, in Constance, Germany). Today’s gadgets can do stuff Perdeck could only dream about. During two autumn seasons, we caught juvenile Mallard females at Ottenby – our beloved duck field site – and equipped a total of 76 birds with satellite GPS transmitters. Half of the ducks were released at Ottenby, and the other half were transported in a private airplane to Lake Constance in southern Germany and released there. The tags had solar panels and, in the best of circumstances, had the potential to send data for at least two years; providing highly accurate GPS fixes at several times a day.
However, the best of circumstances is not often met in nature. The tags on the birds in Ottenby had problems with the lack of sunshine during Swedish late autumn and winter, and many of them just went offline. But a fair number of tags delivered data on movements both in autumn/winter and in spring, when birds headed to their breeding grounds. Contrary to the Perdeck’s starlings, our displaced Mallards did not continue migration in autumn; they stayed in the Lake Constance region. Of the Mallards released at Ottenby, some continued migration to the general wintering area of our study population, that is Denmark and Germany, south to The Netherlands.
After the winter: “most of the translocated ducks headed straight north-north-east, as if heading towards Ottenby, with one duck going as far as northern Sweden. Three of the transported ducks, however, first headed in a more easterly direction and turned northwards when reaching the longitudes of the area the control birds migrated to. It is unclear how these birds decided when to turn north, but the movement trajectories could be interpreted as if individuals had noticed that they were in the wrong place and then corrected for the southward translocation. Based on the observation that this second group of transported ducks ended up in their potential natural breeding grounds, and the first group had a more northerly heading than the control group, we conclude that mallards, just like the starlings from Perdeck’s original experiment, can correct for translocation during the spring season following the experiment.”
Thus, there was quite large differences between individuals in the translocated group, from those that seemed to take the shortest route north to Ottenby in spring, to those that followed a eastern direction (and then going north), more in the direction of what they should have had if the stayed in the normal wintering grounds: a flexibility in continental navigation and migration.
The article is open access and can be found here.