Module 2

Vision & Pentachromat Eye

Pigeons see in five colour channels — four single cones + double cone — including UV. They carry two foveae per eye (one lateral, one frontal) and stabilise retinal image during walking via the famous head-bob. This module resolves the anatomy and the ecological function.

1. Five Cone Classes + Oil Droplets

Pigeons are pentachromats: UV (peak ~355 nm), S (460 nm), M (510 nm), L (570 nm), plus a double-cone photoreceptor for motion detection. Each cone carries a coloured oil droplet that sharpens spectral tuning by absorbing shorter wavelengths. Toomey 2016 identified the astaxanthin/zeaxanthin pigment repertoire; Bowmaker 1997 measured the full spectral sensitivities. UV vision reveals flower nectar-guides and feather iridescence invisible to humans.

2. Two Foveae

The lateral fovea points outward through the sclera and is specialised for scanning at medium distance (ground predators, flock-mates). The frontal (temporal) fovea is used in binocular convergence for feeding and obstacle-avoidance during flight. The bifoveal architecture is typical of raptors and granivores that need both panoramic vigilance and close manipulation.

Simulation: Spectral Sensitivity & Head-Bob

Python

script.py49 lines

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

# Pigeon cone classes (5 including UV), with oil droplet filters
wl = np.linspace(300, 700, 400)
def gauss(c, s): return np.exp(-((wl - c) / s) ** 2)

cones = {
    'UV (S)':   (355, 25, '#a78bfa'),
    'S (blue)': (460, 35, '#38bdf8'),
    'M (green)':(510, 40, '#34d399'),
    'L (red)':  (570, 45, '#fbbf24'),
    'Double':   (560, 60, '#f87171'),
}

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(14, 5), facecolor='#0a0a1a')
for ax in (ax1, ax2):
    ax.set_facecolor('#111827'); ax.tick_params(colors='#cbd5e1')
    for s in ax.spines.values(): s.set_color('#334155')
    ax.grid(True, color='#334155', alpha=0.3)

for n, (c, s, col) in cones.items():
    ax1.plot(wl, gauss(c, s), color=col, lw=2.4, label=n)
ax1.axvspan(300, 400, alpha=0.1, color='#a78bfa', label='UV band')
ax1.set_xlabel('Wavelength (nm)', color='#cbd5e1')
ax1.set_ylabel('Relative sensitivity', color='#cbd5e1')
ax1.set_title('Pigeon 5-cone visual system (+UV)',
              color='#c7d2fe', fontweight='bold')
ax1.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1', fontsize=9)

# Head-bob optic-flow pattern
t = np.linspace(0, 2, 400)
head_x = 1.5 * t   # walking body
head_y = 0.0 * t
# Head-bob: stationary pulses + thrust
for kpulse in range(4):
    mask = (t > kpulse*0.4) & (t < kpulse*0.4 + 0.2)
    head_x[mask] = head_x[mask][0]
ax2.plot(t, head_x - 1.5*t, color='#818cf8', lw=2.5)
ax2.set_xlabel('Time (s)', color='#cbd5e1')
ax2.set_ylabel('Head position relative to body', color='#cbd5e1')
ax2.set_title('Head-bob: 15 Hz pulse for optic-flow sampling',
              color='#c7d2fe', fontweight='bold')

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Pigeons are pentachromats: 5 cone classes including UV (Toomey 2016)')
print('Head-bob stabilises retinal image during stance phase at ~15 Hz')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. Head-Bob & Optic Flow

The head-bob is not rhythmic walking artifact — it is an optic-flow stabilisation strategy. During the “hold” phase (~100 ms), the head stays fixed while the body moves underneath; during the “thrust” phase, the head jumps forward to the next fixation point. Frost 1978 and Friedman 1975 showed that restraining head motion impairs visual tracking, confirming the functional role.

4. Homing Visual Cues

Pigeon homing uses both hemisphere-scale compasses (M3, M4, M5) and short-range visual landmarks. Guilford 2013 GPS-tracking showed homing pigeons follow linear landscape features (roads, rivers) during the last 10–30 km, implying a learned cognitive map anchored on visual familiarity.

Key References

• Toomey, M. B. et al. (2016). “High-density array of wavelength-selective photoreceptors in diurnal bird retina.” Proc. Natl. Acad. Sci., 113, E4761–E4768.

• Bowmaker, J. K. et al. (1997). “Visual pigments and oil droplets in the pigeon.” Vision Res., 37, 2183–2194.

• Frost, B. J. (1978). “The optokinetic basis of head-bobbing in the pigeon.” J. Exp. Biol., 74, 187–195.

• Guilford, T. & Biro, D. (2013). “Route following and the pigeon’s familiar area map.” J. Exp. Biol., 216, 169–179.

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