Migratory birds are able to navigate over great distances with remarkable accuracy. How birds determine their position during the navigational process is still not fully understood. Professor Richard Holland from the School of Environmental and Natural Sciences, Bangor University tells us more about a new study that tested whether Eurasian reed warblers could determine their position using two magnetic cues - the magnetic inclination and the magnetic declination.
Once they have made one round trip, Migratory songbirds are known to be able to correct for huge displacements from their migratory path, and head back towards their final goal that was established in that first migration. This suggests that birds have something akin to a map to tell them where they are in addition to a compass to tell them what direction to fly. However, despite having known about this ability for over 60 years, progress on understanding how they do this has been slow. One cue that has been proposed is the Earth’s magnetic field, which varies in its total intensity (strength); inclination (angle to the surface of the Earth’s surface); and declination (angle of the magnetic field relative to the geographic pole). Evidence that birds use this has been scarce, partly because of the challenges of testing responses to the magnetic field in the wild.
However, there is an alternative. The orientation of migratory birds can be measured during their migratory period by putting them in an orientation cage. The direction they hop in the cage is, on average, oriented in the direction they would be migrating. We have been using this to study how birds respond to changes in the magnetic field by testing them inside a Helmholtz coil, that allows us to change the main parameters of the magnetic field. Our previous results have shown that if we change all these parameters to correspond to a different location, birds reorient as if they have been displaced there. We call this virtual magnetic displacement.
In our latest study, we did something slightly different. We changed only the inclination and intensity of the magnetic field to the values at a different location but left total intensity unchanged. Our birds responded as if displaced, and either do not pay attention to total intensity, or ignored it in favour of the other parameters that made sense in combination.
This raises an interesting question around how the magnetic field is sensed and is something of a paradox. Currently evidence suggests that there are different sensory pathways when birds use the magnetic field as a compass and when they use it as a map. However, birds detect the inclination (rather than polarity) of the magnetic field to determine which direction to fly relative to it, and declination is calculated by comparing the magnetic field direction to a celestial compass. Our results therefore suggests that this dichotomy of different sensory systems for different tasks should be revisited. It is possible that although humans describe and measure these parameters in the same way, that if two different senses are detecting them depending on the task, perhaps the birds are not viewing as the same thing, but this remains to be determined. It raises the question, if one sense can detect these parameters, why do they need another?
About the Authors
Florian Packmor is currently a manager at the National park Administration in lower Saxony. He was the postdoctoral researcher at Bangor University responsible for carrying out the research on this study. He received his PhD from Oldenburg University, Germany having been based at the Institute of Avian Research in Wilhelmshaven, working on decision making in migratory songbirds.
Dmitry Kishkinev is a Lecturer in Animal Behaviour and Behavioural Neuroscience at Keele University. He gained his PhD from Oldenburg University, Germany where he was instrumental in developing the technique of virtual magnetic displacement in migratory birds.
Thomas Zechmeister is the director at the Biological Station Lake Neusiedl. He is responsible for managing the biodiversity of the Lake, including the migratory birds that move through this area. He received his PhD from Vienna University.
Henrik Mouritsen is a Professor at Oldenburg University and the leader of the working group on Neurosensorics and Animal Navigation. He studies the behavioural, neuro- and molecular biology of the magnetic sense in animals. He received his PhD from Odense University, Denmark.
Richard Holland is a Professor in Animal Behaviour at Bangor University. He received his DPhil from Oxford University working on navigation in homing pigeons and has gone on to work on the navigation behaviour of animals as diverse as insects, fish, bats and birds at multiple spatial scales from short distance spatial memory to global migration.
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Image credits:
1 & 2 – Richard Holland
