Module 4

Sun Compass & Time-Compensation

Gustav Kramer’s 1950 orientation-cage experiments with starlings, later extended to pigeons, established the sun compass: birds set a course relative to the sun’s azimuth while compensating for its ~15° per hour westward motion using an internal circadian clock. Clock-shift experiments cleanly dissociate the two components.

1. Kramer’s Experiment

Kramer placed migratory-restless starlings in a circular cage with a visible sun and recorded the direction of their hopping. Birds hopped in the appropriate migratory direction only when the sun was visible. Rotating a mirror to deflect the apparent sun by 90° caused a corresponding 90° shift in orientation — the clean behavioural readout.

\[ \phi_{home} = \phi_{sun}(t) - \theta_{internal}(t),\qquad \theta_{internal} \sim 15^\circ/\text{hr} \]

2. Clock-Shift & Time Compensation

Hoffmann 1954 kept pigeons under a 6-hour-advanced or retarded light cycle for a week, then released them. Released at solar noon with their internal clock set to 6 p.m., they treated the sun as if it were 6 hours westward — producing a ~90° error in homing bearing. This demonstrated independent, modular time compensation separable from the sun’s immediate azimuth.

Simulation: Clock-Shift Error

Python

script.py28 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# Clock-shift experiment: 6 h advance shifts orientation ~90 deg
hour = np.linspace(0, 24, 200)
# Sun azimuth (simplified cosine)
sun_az = (hour - 12) * 15   # deg per hour
correct_az = sun_az - 90    # 90 deg right of sun for homing
# Clock-shift +6 h: bird uses pre-shift circadian time
shifted_az = (hour - 18) * 15 - 90

fig, ax = plt.subplots(figsize=(11, 6), facecolor='#0a0a1a')
ax.set_facecolor('#111827'); ax.tick_params(colors='#cbd5e1')
for s in ax.spines.values(): s.set_color('#334155')
ax.plot(hour, correct_az, color='#fbbf24', lw=2.5, label='Correct homing direction')
ax.plot(hour, shifted_az, color='#dc2626', lw=2.5, label='Clock-shifted (+6h) bird')
ax.axhline(0, color='#cbd5e1', ls=':')
ax.set_xlabel('Local hour', color='#cbd5e1')
ax.set_ylabel('Heading angle (deg)', color='#cbd5e1')
ax.set_title('Kramer clock-shift experiment: 90 deg orientation error',
             color='#c7d2fe', fontweight='bold')
ax.grid(True, color='#334155', alpha=0.3)
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')
plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Kramer 1950: clock-shifted birds orient by their shifted internal time')
print('Sun moves ~15 deg/hour; 6h shift = 90 deg error')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. Polarized-Skylight Fallback

On overcast days when the sun disc is obscured, pigeons can read the E-vector of polarised skylight to recover sun azimuth. Waterman 1989 and Horváth 2014 review the polarisation-pattern use; sensitivity is mediated by UV-cone double cones in the dorsal retinal fovea. The polarisation pattern is ~90° from the solar meridian, so a 90° rotation relates sky polarisation to sun position.

4. Circadian Pacemaker

The avian circadian pacemaker is distributed across the pineal, retina, and hypothalamus suprachiasmatic nucleus (SCN). Pineal melatonin acts as the darkness-signalling hormone. Pinealectomy or SCN lesions disrupt the sun compass but leave magnetic orientation intact, consistent with independent compass systems integrated downstream (M6).

Key References

• Kramer, G. (1950). “Weitere Analyse der Faktoren, welche die Zugaktivität des gekäfigten Vogels orientieren.” Naturwissenschaften, 37, 377–378.

• Hoffmann, K. (1954). “Versuche zu der im Richtungsfinden der Vögel enthaltenen Zeitschätzung.” Z. Tierpsychol., 11, 453–475.

• Able, K. P. (1993). “Skylight polarization patterns and the orientation of migratory birds.” J. Exp. Biol., 141, 241–256.

• Horváth, G. (2014). Polarized Light and Polarization Vision in Animal Sciences. Springer.

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