Avian eggshell coloration: new perspectives on adaptive explanations

Abstract

Recent work suggests that the evolution of egg coloration may have been constrained in three important ways that have not yet been critically synthesized in any review. First, on account of birds being able to see in the ultraviolet spectrum, the interaction between the properties of avian vision and the light environment of nests imply different perceptions of egg coloration from those experienced by humans. Second, a new hypothesis to explain blue-green egg coloration interprets it as a sexually selected signal to males of the laying female's genetic quality. Third, evidence from taxa as divergent as sparrowhawks and great tits indicates that protoporphyrin pigments responsible for maculation (spotting patterns) have a structural function in compensating for eggshell thinning, as caused by calcium stress, and, more recently, dichlorodiphenyltrichloroethane. We consider this to be the most convincing explanation for the primary function of spotting, although an important secondary function might arise through the fact that individual patterns of maculation may allow birds to identify their own eggs, effectively serving as signatures in the face of inter-or intra-specific brood parasitism. These constraints or hypotheses are not mutually exclusive, and should not be taken to imply that one, but not other, agents of selection might apply to any one species. However, the sexually-selected eggshell coloration hypothesis is least plausible for hole-nesting birds because of the poor light quality available, although such species have been the focus of research in this area, and only a single experimental study has shown a link between egg coloration and male provisioning. Furthermore, the observed relationships between female phenotypic quality and egg traits do not necessarily imply that they have signalling functions.

Egg crypsis in a ground‐nesting shorebird influences nest survival

“…In general, the eggs of ground-nesting birds absorb, rather than reflect, ultraviolet light (Bakken et al 1978, Avilés et al 2006 as does the vegetation of the region (Ruhland et al 2013), and most substrate materials (Chávez et al 2003), so eggs and nest substrate will tend to be similar. Although our method of examining egg and substrate coloration does not take into account ultraviolet color, the trend we found of decreasing nest survival as the contrast between eggs and substrate increased would likely hold true even in the ultraviolet spectrum (Cherry and Gosler 2010). An analysis that included ultraviolet reflectance would likely show that eggs in nests where the surrounding nesting substrate is very uniform, with either predominant vegetation or rocks, would have lower survival than areas with variation in the nesting substrate as the eggs would tend to stand out (Spottiswoode and Stevens 2010), similar to our finding that nest survival increased with substrate color diversity.…”

Darker eggshell spotting indicates lower yolk antioxidant level and poorer female quality in the Eurasian Great Tit (Parus major)

“…Furthermore, Stoddard et al (2012) found that neither the feeding behavior of the male parent nor nestling growth were related to eggshell spotting patterns in British Great Tits. Therefore, it is possible that the correlation between shell pigment intensity and yolk antioxidant level is due purely to some physiological constraint (see above), and birds do not use it as a signal of embryonic quality, e.g., because of limitations in the detectability of the intensity of pigmentation within dimly lit cavities (Cassey 2009, Cherry and Gosler 2010, Holveck et al 2010.…”

“…Several hypotheses have been proposed to explain the pigmentation of bird eggshells (Kilner 2006, Reynolds et al 2009, Maurer et al 2011a); these hypotheses may not be mutually exclusive and could share similar predictions. Protoporphyrin-based eggshell pigmentation may promote camouflage (Stoddard et al 2011, Lovell et al 2013; it may enhance egg mimicry or permit egg recognition in species exposed to brood parasites (Stokke et al 1999, Soler et al 2000, Moskát et al 2008; it may help resist bacterial penetration into the egg (Ishikawa et al 2010, Fargallo et al 2014; or it may structurally strengthen eggshells (Gosler et al 2005, García-Navas et al 2010, Bulla et al 2012, Hargitai et al 2013, the latter of which is possibly the most plausible explanation for shell spotting patterns in cavity-nesting birds (Solomon 1997, Cherry andGosler 2010). Furthermore, Moreno and Osorno (2003) proposed that eggshell color could be a sexually selected signal, which may advertise the quality of the female and that of her offspring to the male parent in order that he contribute more to the breeding attempt (sexually selected eggshell coloration [SSEC] hypothesis).…”

Eggshell spotting does not predict male incubation but marks thinner areas of a shorebird's shells

“…the maculated eggshell area) demarcate thinner areas of the shell (reviewed by Cherry and Gosler 2010). Such localized eggshell thinning at pigment spots has been reported in a few species in the Passeriformes (Gosler et al 2005), Charadriiformes (Maurer et al 2011a, 2011c, Bulla et al 2012, and Falconiformes (Jagannath et al 2008).…”

“…In particular, we found evidence that the concentrations of 2 micronutrients (Ca and Mg) and 2 trace elements (Cu and Cd) were significantly higher in speckled eggshell regions (which were also significantly thicker) than in their unpigmented counterparts. Some earlier studies showed a strong association between the availability of dietary or environmental Ca and eggshell maculation, implying that Ca shortages resulted in localized eggshell thinning (see Cherry andGosler 2010, Hargitai et al 2016b), but no analyses of Ca concentrations in speckled eggshell regions have previously been undertaken. We found that the maculated eggshell regions were thicker, and they contained significantly more Ca than the unpigmented areas, which suggests parallel cellular transport of protoporphyrin at high Ca levels (but see Maurer et al 2011a).…”