Abstract
Simple SummaryThe earth’s magnetic field, celestial cues, and retention of geographical cues en route provide birds with compass knowledge during migration. The magnetic compass works on the direction of the magnetic field, specifically, the course of the field lines. We tested Red-headed Buntings in orientation cages in the evening during spring migration. Simulated overcast testing resulted in a northerly mean direction, while in clear skies, birds oriented in an NNW (north–northwest) direction. Buntings were exposed to 120° anticlockwise shifted magnetic fields under simulated overcast skies and responded by shifting their orientation accordingly. The results showed that this Palaearctic night migrant possesses a magnetic compass, as well as the fact that magnetic cues act as primary directional messengers. When birds were exposed to different environmental conditions at 22 °C and 38 °C temperatures under simulated overcast conditions, they showed a delay in Zugunruhe (migratory restlessness) at 22 °C, while an advance migratory restlessness was observed under 38 °C conditions. Hot and cold weather clearly influenced the timing of migrations in Red-headed Buntings, but not the direction.Red-headed Buntings (Emberiza bruniceps) perform long-distance migrations within their southerly overwintering grounds and breeding areas in the northern hemisphere. Long-distance migration demands essential orientation mechanisms. The earth’s magnetic field, celestial cues, and memorization of geographical cues en route provide birds with compass knowledge during migration. Birds were tested during spring migration for orientation under natural clear skies, simulated overcast skies at natural day length and temperature, simulated overcast at 22 °C and 38 °C temperatures, and in the deflected (−120°) magnetic field. Under clear skies, the Red-headed Buntings were oriented NNW (north–northwest); simulated overcast testing resulted in a northerly mean direction at local temperatures as well as at 22 °C and 38 °C. The Buntings reacted strongly in favor of the rotated magnetic field under the simulated overcast sky, demonstrating the use of a magnetic compass for migrating in a specific direction.
Highlights
Billions of songbirds migrate from their wintering quarters to breeding quarters each year with their extraordinary navigation capabilities, covering several thousand kilometers following their endogenous circannual clock
The same birds were tested under the simulated overcast sky in rooms with no availability of celestial cues in the natural magnetic field oriented highly significantly towards the north (358◦ ± 18◦ at a confidence interval of 95%, r = 0.752, p < 0.0001, n = 22; Figure 1b)
Red-headed Buntings tested under simulated overcast conditions displayed an intense migratory activity similar to birds tested under clear skies and showed a significant northerly mean orientation, illustrating that they do not have any trouble in orienting themselves under simulated overcast skies
Summary
Billions of songbirds migrate from their wintering quarters to breeding quarters each year with their extraordinary navigation capabilities, covering several thousand kilometers following their endogenous circannual clock. Birds can utilize the earth’s magnetic field for obtaining directional information during migration Their magnetic inclination compass responds to the direction of the field lines and ignores the polarity of the geomagnetic field [5]. Translocated Eurasian Reed Warbler, Acrocephalus scirpaceus, a migratory songbird, could re-orient as a navigational response towards their migratory destinations using all the earth’s natural magnetic cues (magnetic field intensity, magnetic inclination, and magnetic declination) of unknown magnitude, but not when just one cue, i.e., magnetic declination, changed [26]
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