<b>Spectral and visual differentiation of blue-green eggs in passerine birds from contrasting nest light environments</b>

Natural tree cavities are relatively scarce because of deforestation and expansion of land used for animal husbandry. Artificial nest boxes were therefore installed at both sites to alleviate nest-site limitation. Nest boxes at the two sites had identical dimensions, with entrance holes 6 cm in diameter and standardized entrance orientations to minimize potential effects of nest-box structure and orientation on the internal light environment (Fig. 2C, Liu <i>et al</i>. 2023).</p><p><br></p><p dir="ltr"><b>Egg color and UV reflectance measurement</b></p><p dir="ltr">Eggshell reflectance spectra over 300–700 nm were measured using an AvaSpec-ULS2048×64 USB2 fibre-optic spectrometer (Avantes, Inc.), an AvaLight-DHc compact deuterium–halogen light source, and an FCR-7UV200-2-ME reflection probe.

During measurement, the probe was maintained perpendicular (90°) to the eggshell surface at a distance of approximately 1 mm, with an integration time of 200 ms.</p><p dir="ltr">Before each measurement, a dark-reference calibration was performed with the light source switched off. After the light source was switched on, a WS-2 polytetrafluoroethylene diffuse white reflectance standard, providing >99% reflectance across the measured wavelength range, was used for white-reference calibration (Nahid et al. 2024).

Each egg was divided into three regions along its longitudinal axis from the pointed to the blunt end. One measurement point was randomly selected within each region, producing three reflectance spectra per egg. The mean of the three spectra was used as the representative reflectance spectrum for that egg.</p><p dir="ltr">Whether species with similar egg coloration but contrasting nest types exhibit distinct spectral characteristics remains unclear.

We compared the 300–700 nm eggshell reflectance spectra of the open-nesting grey bush chat (<i>Saxicola ferreus</i>) and two cavity-nesting species, the white-shouldered starling (<i>Sturnia sinensis</i>) and crested myna (<i>Acridotheres cristatellus</i>), and evaluated spectral and perceptual differences using spectral colour metrics, principal component analysis (PCA), and avian visual models. Grey bush chat eggs showed lower overall brightness and relative blue reflectance but higher UV reflectance and a greater relative UV contribution than eggs of the two cavity-nesting species.

In contrast, eggs of the two cavity nesters exhibited higher overall brightness and relative blue reflectance. Avian visual modelling further showed that grey bush chat eggs occupied a broader region of colour space, and that visual differences between grey bush chat and the two cavity-nesting species were dominated by achromatic rather than chromatic contrasts, indicating that perceptible interspecific differentiation was concentrated mainly along the luminance dimension.

These findings suggest that nest light environments may contribute to egg-colour differentiation through different optical and visual dimensions, providing new evidence for understanding the spectral differentiation and potential ecological functions of blue-green avian eggs in contrasting nest environments.</p>