Magnetic and sunset orientation in migratory redwings, Turdus iliacus

Abstract

The migratory orientation behaviour of redwings (Turdus iliacus) was studied by orientation cage and release experimentation, performed in eastern Finland after sunset periods during the three autumn migration seasons of 1999–2001. The experiments were conducted outdoors under clear and overcast skies in control and two magnetic conditions: (1) in a deflected horizontal magnetic field (mN shifted 90° CCW towards geographic west); and (2) in a reversed vertical magnetic field (to test for inclination compass). Redwings showed a mean orientation towards the SW in control clear sky tests, which is an appropriate autumn migratory direction for the season. Under overcast skies, control birds' orientation was scattered. In the horizontal (1) magnetic tests, redwings showed orientation towards the direction of the setting sun under clear skies, but were likely to choose random directions under overcast skies. In the vertical (2) magnetic tests, redwings were oriented again towards the west under clear skies, but chose northern directions under overcast skies. This suggests different integration of directional information between clear and overcast conditions. With these results, one can demonstrate that redwings use a magnetic compass during the autumn migratory period, and that their magnetic compass may function as an inclination compass. In the release tests, redwings showed consistency with cage tests in both magnetic test conditions, which indicate that they had recalibrated their celestial compasses on the basis of the manipulated magnetic field they had just experienced in orientation cages. The results indicate a compromise orientation between the position of the setting sun and the expected magnetic direction, rather than a pure phototactic response.