Within-Clutch Repeatability of Egg Size Traits in Blue Tits Cyanistes caeruleus in a Long-Term View

Abstract. Female birds are exceptionally consistent in laying rather uniform and individually specific eggs, so that variation in eggs of individual females is low, while variation among females is high. Repeatability is a population-level parameter used in quantitative genetics to quantify the level of intraindividual consistency in relation to interindividual variation in phenotypic traits, in this case, the egg traits. The statistical measure of repeatability is intraclass correlation of particular egg traits clustered in clutches. This study is based on a long-term dataset on repeatabilities of the length, breadth, volume and shape of eggs of Blue Tits Cyanistes caeruleus investigated in two ecologically varying sites (an urban parkland versus a mature deciduous forest) around the city of Łódź, central Poland in 1999–2013. Coefficients of repeatability of egg length, breadth, volume and shape in the study populations of Blue Tits showed some variation between years and study sites, but were generally very high, on average 0.7 or more, and did not significantly differ between the traits within years. The observed differences between repeatabilities do not seem to be strongly associated with year-specific weather indicators for the first half of April. Only coefficients of variation in mean daily temperature influenced repeatabilities of egg length and breadth, whereas effects of mean temperature were non-significant. Repeatability of egg length in the forest and breadth in both sites increased with increasing variation in daily temperature, whereas repeatability of egg length in the urban park site showed a reverse relation with variation in temperature. Year-site-specific mean laying dates, clutch sizes and egg volumes, considered as indicators of breeding conditions, tended to negatively affect the repeatabilities. However, in general, repeatabilities of different egg traits were not consistent in their variation between years and sites, which suggests an important role of stochastic factors in shaping this variation. External factors that are able to force females to diversify traits of eggs in a particular laying sequence do not seem to directly result from the environmental conditions prevailing during a spring period critical for laying eggs in a population in a given year (the first half of April), but seem to be linked to more subtle aspects of variation in the conditions for egg laying, as shown by links with the mean values of basic breeding parameters.