Quantitative genetics of eggshell colouration

Exploring the evolutionary architecture of female sexual traits and their potential evolvability is important to understand their possible role as post-mating sexual signals. Egg colouration has been proposed to be one of these post-mating sexual signals, honestly advertising female quality in birds, especially in blue-green laying species. In this study, we used an animal model in a Bayesian framework to estimate the evolvability of multiple descriptors of blue-green egg colouration and egg size in a wild long-term monitored population of spotless starlings (Sturnus unicolor). Our results show low to moderate heritability (h2 = 0.31 – 0.44) for three egg colour descriptors (blue-green chroma, chroma and lightness) and egg size. Using the coefficient of additive genetic variance (CVA) and the evolvability (IA) as proxies of evolutionary potential of all components of this trait, we found low values of CVA for all these variables, suggesting a small evolutionary potential of these phenotypic traits, contrasting to previous results reported in another blue-green egg laying species. Our results indicate a modest raw genetic material of this trait on which sexual selection can act upon and, therefore, a small probability for these traits to respond easily to selection.

Methods
We monitored the breeding of a population of spotless starlings (Sturnus unicolor) from 2012 to 2019 nesting in 250 nest boxes in an open woodland in Soto del Real, central Spain. At the beginning of every season, from early March until about a week before the first eggs were found, we captured the pairs roosting in the nests before sunrise. 

Bird identities of previously captured individuals were confirmed by both the ring number and the unique code of a passive integrative transponder tag (PIT-tags:  Trovan Ltd., Douglas, UK). Previously unmarked individuals were ringed and PIT-tagged by inserting the transponder under the skin of the upper third of the back. Later in the season, fledglings were similarly marked to ease their future identification as recruits.

Once pairs were established and the eggs laid, mothers were identified at night during incubation through a medium-distance PIT-tag reader (GR-250, Trovan Ltd., Douglas, UK) allowing us to unambiguously assign each clutch to a female.