Abstract

Previous studies have found strong relationships between calcium availability and eggshell pigmentation in the Great Tit (Parus major). According to the "structural function hypothesis", protoporphyrins, the pigments responsible for reddish spots on speckled eggs, are deposited in those areas of the shell where calcium deposition is less intense. In the study reported here, which was carried out in three Blue Tit (Cyanistes caeruleus) populations in central Spain, we provide partial experimental support for the association between protoporphyrin eggshell pigmentation and shell thinning. Contrary to our expectations, we did not observe a decrease in the size and intensity of pigment spots for those eggs from calcium-supplemented nests. However, we did find that the provision of calcium-rich material during the egg-laying period led to a more wide distribution of pigment spots and reduced the proportion of eggs with defective shells (deviant pigmentation, dull and rough surface). When only the supplemented nests were considered in the analysis, within the same clutch we also detected differences in the spotting distribution between those eggs laid by female Blue Tits observed to have consumed calcium consumption on the day prior to laying and those observed not to have consumed calcium on the day prior to laying. Clutch size was not affected by the calcium supplementation. Female Blue Tits experimentally supplied with calcium-rich material had a shorter incubation period than control females, and they laid eggs with thicker shells. Eggshell thickness markedly affects the probability of hatching and could explain the lower proportion of unhatched late eggs found in supplemented nests in comparison with control ones. This study highlights the role of calcium in eggshell maculation and its effects on breeding performance of small passerines. We found the spotting distribution to be a good predictor of calcium deficiency. However, our results provide only mixed support for the "structural function hypothesis": in our study populations, the thickness of the eggshell was intimately associated with calcium availability, but the relationship between calcium and protoporphyrin deposition remains far from clear.