Magnetic orientation in migratory birds
Sammanfattning: My thesis focuses on magnetic orientation in migratory birds, i.e. how they can perceive information from the Earth’s magnetic field and use it for orientation. To examine magnetoreception and the function of the light-dependent magnetic compass in birds, I performed behavioural orientation experiments with European robins, Erithacus rubecula, under different spectra and intensities of light. The birds were well-oriented under low-intensity 560.5 nm green light, but completely disoriented under 567.5 nm green-yellow light. Under low-intensity red light (617.0 nm) the birds shifted their preferred direction. This indicates that birds might possess at least two antagonistically interacting, magnetically sensitive spectral mechanisms, a short-wavelength mechanism in the blue-green and a long-wavelength mechanism in the red part of the spectrum. Five papers in this thesis are based on experiments carried out during an expedition to northern Canada. By displacing juvenile and adult birds across the Canadian tundra to areas beyond their normal experience and repeatedly testing their orientation we could (1) study the orientation and navigation abilities of inexperienced and experienced birds, (2) test the sensitivity of the magnetic compass at steep angles of inclination and (3) examine the use of magnetic and celestial compasses at geographic latitudes where the properties of the geomagnetic field change rapidly across longitudes and where the midnight sun makes star compass orientation impossible. White-crowned sparrows, Zonotrichia leucophrys, and Savannah sparrows, Passerculus sandwichensis, oriented towards the seasonally expected migratory directions with access to magnetic compass cues only. White-crowned sparrows could select a magnetic compass course in magnetic fields with an inclination deviating by only 1.4° from the vertical, but were disoriented at the magnetic North Pole. The change in orientation shown by both juvenile and adult white-crowned sparrows as a reaction to changing declination along the eastward displacement shows that these birds were aware of their position relative to home. In cue-conflict experiments, the birds shifted their preferred direction according to an artificial deflection of the magnetic field and recalibrated their celestial compasses, thus prioritized magnetic compass information in favour of celestial cues. By extrapolating potential migration routes from the empirical data collected during the expedition we could on a theoretical basis study the feasibility of using different compasses to reach the expected goals. Finally, I investigated the interrelationship and calibration between magnetic and celestial compass cues in a review of the existing literature and described how the different compasses might interact.
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