Is UV and blue-green egg colouration a signal in cavity-nesting birds?

Abstract

We analysed the eggs of the European starling Sturnus vulgaris for variability of UV and blue-green colouration. We noted the calcium deposition in eggshells, egg measurements, light conditions in nests and starling eye properties. Egg colouration shows high within-clutch similarity. Intensity of the blue-green egg colour neither depends on Ca deposition in the eggshell nor correlates with egg measurements, clutch size and laying date. This suggests that the intensity of the blue-green colour of starling eggs does not reflect female condition, which is reported to affect egg and clutch parameters. Low light intensity in cavities enables only mesopic vision, which is characterised by degraded visual acuity and colour sense, as well as Purkinje shift. As a consequence, the probability of precise assessment of the differences in eggs colour by starling males, crucial for the signalling hypothesis (Moreno & Osorno2003), is not high. On the other hand, irrespective of its intensity, blue-green is the last colour visible under decreased illumination. Thus such coloured eggs may attract higher parental attention in dark cavities. We also demonstrated a strong negative correlation between intensity of blue-green and UV colouration of starling eggs, as well as constrained sensitivity of the starling eye to the range of eggs' maximum UV reflectance. Our study indicates that it is blue-green, not the UV component of the egg's spectral characteristics, which seems to increase the perception of eggs by adults in dark cavities. We suggest that it might have been the most important pressure towards evolution of blue-green colouration of eggs in cavity-nesting birds.