Introduction
Colouration is ubiquitous in nature and plays a significant role in communication between different species (e.g., in the context of pollination, competition for resources and predator-prey interactions) or between individuals of the same species (e.g., in the context of mating and parental care) (Cuthill et al. 2017; Endler & Mappes 2017; Postema et al. 2022). The most colourful taxa in the animal kingdom are coral fishes, tropical frogs, certain groups of insects, and birds, and those species have historically received most attention in studies about the role of colouration in animal communication.
To understand the great variation in colour expression between individuals of the same species, evolutionary biologists have mainly focused on sexual selection processes. Colourful traits are here seen as handicaps that seem not to increase longevity or fecundity, so they are potentially not favoured by natural selection. Conspicuous colourful patches - displayed mainly by males - would rather allow to attract mates or to bully potential competitors (Andersson 1994). Hence, colourful ornaments are thought to function as signals of quality to reliably inform conspecifics about, for example, condition (Hill 2011), immune status (Rodríguez-Ruiz et al. 2020) or parasitic burden (Megía-Palma et al. 2016). Honesty in signalling traits is achieved through associated costs to produce and maintain them (Andersson 1994). Taken together, a significant number of studies have proven the role of colouration in a sexual selection context, which implies that these studies were performed with sexually mature individuals.
However, there are conspicuous colourful traits that are also expressed in sexually non-mature or even in newborn individuals. In these cases, colouration is displayed in a non-sexual selection context (West-Eberhard 1983). These traits could have evolved as by-products of selection acting on colouration in adults, at least when both offspring and adults display the same traits (similar to the evolution of female ornaments that can be explained through correlated selection on these traits in males; Amundsen 2000). However, offspring colouration may have important signalling functions in itself like, for example, in intra-family interactions (Lyon et al. 1994; Parker et al. 2002). The expression of structural ornaments such as plumage colouration requires a substantial investment of resources such as carotenoids, and thus they can inform parents and other family members (like siblings and breeding helpers) about individual quality (Caro et al. 2016). Like in a sexual selection context, honesty can be achieved if the offspring pay a cost for displaying or maintaining such signalling traits, which prevents cheating (handicap principle). Therefore, nestling colouration has the potential to evolve as a condition-dependent signal to which other family members respond (honest signalling models, Godfray 1991, 1995, Laidre & Johnstone 2013; Fromhage & Henshaw 2021).
A good model system to study whether adult-like colouration shows similar patterns of condition-dependence in the offspring is the carotenoid-based colouration of the yellow breast plumage of blue tits (Cyanistes caeruleus ). Blue tit adults exhibit both UV/blue crown feathers and yellow breast feathers. In adults, it is well established that UV/blue colouration functions as a sexual signal that reflects condition (Delhey et al. 2006) and shapes the parental investment of mates (Limbourg et al. 20013a; 2013b). Similarly, yellow breast feathers reliably reflect aspects of individual quality like parasite burden (del Cerro et al. 2010), parental capacity (García-Navas et al. 2012) and laying performance (Midamegbe et al. 2013). Furthermore, the UV chroma of adult breast plumage functions as a signal in parental interactions during offspring care (García-Campa et al. 2022). Here, partners of UV-reduced individuals (either males or females) increased their parental investment during offspring provisioning, perhaps to compensate for the apparent lower condition of their mates (García-Campa et al. 2022). Blue tit nestlings do not exhibit the UV/blue crown colouration, but there is some evidence that two colour parameters of the yellow breast plumage, carotenoid chroma (Johnsen et al. 2003) and UV chroma (Jacot & Kempenaers 2006; Morales & Velando 2018), co-vary with nestling body mass. Moreover, family members rely on nestling UV chroma to adjust their decision rules over parental investment. Concretely, nestlings with experimentally blocked UV colour beg more during feeding rates and in sib-sib competitive interactions (Morales & Velando 2018). In addition, when resources are limited, parents favour chicks with higher UV chroma, thus, presumably those of high quality (Morales & Velando 2018; García-Campa et al. 2021). It is possible that the different components of colouration reveal different aspects of individual quality (Candolin 2004), as they involve different dimensions of avian colour perception (Jacot & Kempenaers 2006): reflectance in the ultraviolet region of the spectrum (UV chroma), carotenoid-based reflectance (carotenoid chroma) and total reflectance (brightness). Hence, in order to understand the signalling function of yellow breast plumage colouration in blue tit nestlings, it is valuable to investigate the different colour components as well as their relationships.
In this study, we first explored the associations between UV-chroma, carotenoid chroma and total brightness of blue tit nestling yellow breast feathers. Then, we investigated the relationship of each of the three colour components with body mass in three consecutive breeding seasons. We hypothesised that only individuals in good condition (i.e., nestlings with higher body mass) would be able to achieve, in particular, a higher reflectance in the ultraviolet region of the spectrum, as this has been experimentally demonstrated previously (Morales & Velando 2018). Furthermore, as multiple chicks per nest were measured, we tested whether any effect of body mass on colouration was due to an among-nest or a within-nest effect, which, to our knowledge, has not been explored to date. The within-nest effect allows testing whether chick colouration varies according to within brood differences in body mass, reflecting condition-dependence at the nest level. The among-nest effect in turn would show whether the correlation of body mass and colouration is due to, for instance, genetic effects, parental quality effects, or other (common) environmental effects at the nest level. If nestling yellow plumage functions as a signal in intra-family interactions, we expect a within-nest effect of body mass on colouration, since this would allow other family members to assess individual quality relative to other siblings in the nest. The interaction between the within and the among-nest effect then again would allow testing whether the strength of condition-dependence is influenced by brood identity. Decomposing how genetic and environmental effects contribute to condition dependence is of significant relevance for our understanding of the signalling function of colouration (Hooper & Bonduriansky 2022).