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Birds sometimes skip a day along the sequence of egg laying, which may vary the mass or the composition of delayed eggs compared with those that were laid consecutively. Our literature review shows that this has been interpreted as a short-term adaptation that enables females to overcome energetic constraints during the laying period, but other hypotheses implying the influence of weather, pollution, or hormonal cycles have also been proposed. We collected freshly laid Grey Partridge Perdix perdix eggs to determine the effects of laying gaps on egg characteristics. Egg shape, as well as egg components (beta-carotene, avidin and lysozyme concentrations) did not vary in relation to skipped days. Eggs were slightly heavier when one or two days were skipped (0.72% and 0.45%, respectively). However, when examining the hatching rate, we found a significant decrease in relation to skipped days, hence eggs following laying gaps showed a lower hatching rate than other eggs. The pattern observed could indicate the presence of some physiological stress that caused females to skip one or two days and to lay eggs that hatched less. [ABSTRACT FROM AUTHOR]

Immunocompetence is considered a reliable indicator of general body condition and ultimately of fitness. It has been suggested that, as a parameter subjected to intense directional selection, the level of additive genetic variance expressed should be reduced; on the other hand, theoretical models of host–parasite coevolution assume that variation in parasite resistance has a genetic basis. Contradictory results have been reported in birds, since the heritability of immune responses varies from nil to high. In this study of Grey Partridge (Perdix perdix (L., 1758)), we examined the heritability of immune condition (PHA reaction, an index of T-cell-mediated immunocompetence) and of some parameters traditionally considered important for chick survival, such as egg mass and chick growth. Two statistical approaches were used: parent–offspring regression and full-sibling intra-clutch repeatability. The repeatability of other parameters that reflect egg quality (egg proteins, lipids, and carotenoids) and of substances that confer innate immunity (lysozyme and avidin) was also investigated. In agreement with previous studies, we found nonsignificant heritability for cell-mediated immunocompetence. In contrast, there were significant repeatabilities of chick mass and several egg characteristics (mass, size, total proteins), while lipid and carotenoid concentrations were not repeatable. For the first time in birds, we found significant repeatability of two molecules, lysozyme and avidin, that confer innate immunity to the developing embryo. [ABSTRACT FROM AUTHOR]