Intra-clutch variation in avian eggshell pigmentation covaries with female quality

Abstract

Among the most eye-catching traits of avian eggs are their background coloration and pigmentation, consisting in many passerine birds of dark protoporphyrin spots. Although variation in protoporphyrin pigmentation among clutches has been shown to reflect female quality, within-clutch variation in egg pigmentation remains less well understood. Here, we hypothesize that female quality may also be reflected in within-clutch variation in egg pigmentation as a result of energetic constraints and/or increased susceptibility to oxidative stress, and test this hypothesis in a free-living population of Great Tits (Parus major). Within clutches, both pigment ‘darkness’ and ‘spread’ (reflecting intensity, distribution and size of pigment) increased with laying order. For pigment ‘darkness’, this was most strongly so in larger females and in females showing lysis (as a measure of constitutive innate immunity), suggesting that intra-clutch variation in pigment ‘darkness’ positively relates to both structural as well as condition-dependent female traits. In contrast, for pigment ‘spread’, no relationships were detected with body size, body condition, age, and two components of constitutive innate immunity. Among clutches, ‘darkness’ and ‘spread’ of pigments also varied. However, this variation was not related to any of the female characteristics we measured. To the best of our knowledge, this study is the first one to relate intra-clutch variation in protoporphyrin egg pigmentation to structural and condition-dependent traits of laying females. Further experimental study is, however, required to better understand the underlying causal mechanisms.