Why are birds’ eggs speckled?

Abstract

Birds are unique in laying eggs with pigmented shells, but for most species (e.g. most passerines, which lay white eggs speckled with reddish spots of protoporphyrin) the pigmentsÕ function is unknown. We studied a bird population at a geologically variable site, and considered a hitherto untested hypothesis: that protoporphyrin pigments might compensate for reduced eggshell-thickness (caused partly by calcium deficiency), which is known to reduce eggshell-strength and increase eggshell-permeability. We found that pigment spots specifically demarcated thinner areas of shell, with darker spots marking yet thinner shell than paler spots. Variation in pigmentation was thus associated with variation in shell thickness both within and between clutches, so accounting for the eggshell's characteristic spot patterns. Geological variability at this site has resulted in a great range of calcium availability and, as predicted by the hypothesis, variation in calcium availability was found to affect between-clutch variation in both eggshell-mass (+) and pigmentation characteristics ()). We suggest a physiological mechanism and some important implications of these findings.

Dark nests and egg colour in birds: a possible functional role of ultraviolet reflectance in egg detectability

“…Recently, it has been proposed that blue and green egg colours may function as a post-mating sexually selected signal of female phenotypic quality to their mates in order to induce a higher allocation of paternal care (Moreno & Osorno 2003). Finally, Gosler et al (2005) have proposed that egg speckling caused by protoporphyrin pigments might compensate for reduced eggshell-thickness due to calcium deficiency.…”

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

“…In additon, the Great Tit has no brood parasites, and birds do not eject distinct-looking eggs (Kempenaers et al 1995, R. Hargitai personal observation). In a previous study, we showed that the distribution of eggshell spots was related to eggshell thickness in our Great Tit population (Hargitai et al 2013), suggesting a structural function of shell pigmentation (Solomon 1997, Gosler et al 2005, but the intensity and size of spots were not related to eggshell thickness (Hargitai et al 2013). In this study, we first tested whether eggshell spotting pattern was related to female characteristics that may indicate condition and state of health (body condition, body size, plasma oxidative status, plasma leukocyte count, and clutch size).…”

“…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).…”

“…Others determined pigment concentration of the eggshell by chemical analysis (Maurer et al 2011b, Cassey et al 2012, Duval et al 2013, Brulez et al 2014, or analyzed digitized pictures of eggshells to determine the percentage of spotted eggshell surface (Martínez-de la Puente et al 2007, Sanz and García-Navas 2009, Bulla et al 2012, Holveck et al 2012, Brulez et al 2014), or to determine hue, saturation, and brightness derived from HSB and RGB (Martínez-Padilla et al 2010, Cassey et al 2012, Holveck et al 2012), or to determine other spotting pattern variables, such as pattern contrast and marking size (Stoddard et al 2012). Finally, many studies used visual scoring of photographs, and spotting pattern was expressed as spot intensity, spot size, and spot distribution (Gosler et al 2005, Sanz and García-Navas 2009, López de Hierro and De Neve 2010, Bulla et al 2012, Holveck et al 2012, De Coster et al 2013, Hargitai et al 2013, Brulez et al 2014.…”

“…Alternatively, based on the ''low quality'' hypothesis, females that lay eggs with more intense eggshell pigmentation may be in poorer condition and a worse state of health as they may be exposed to higher levels of oxidative stress due to higher levels of protoporphyrin; thus, they may deposit lower amounts of antioxidants into the egg yolk. Finally, we also aimed to test whether the qualitative visual scoring of shell spottiness proposed by Gosler et al (2005) and brown spot chroma measured by a spectrophotometer were reliable estimators of the concentration of protoporphyrin in the eggshells of Great Tits.…”