Heeb P. (1994) Intraclutch egg-mass variation and hatching asynchrony in the Jackdaw Corvus monedula. ARDEA 82 (2): 287-297

The patterns of intraclutch egg-mass variation and their associated hatching intervals were studied over three breeding seasons in a Jackdaw Corvus monedula, population. The data were used to test alternative predictions from hypotheses proposed as explanations for intraclutch egg-size variation. Although clutch size did not differ between years, significant differences in laying date and mean egg-mass were found. The mean egg-mass of the clutch decreased seasonally but did not vary with clutch size. The intraclutch patterns of egg-mass variation were clear: in 45 egg-clutches, egg-mass increased from the first to the second egg and then decreased with laying order. In nearly all clutches the final egg was the lightest and its relative mass decreased with clutch size in two years. In the year when birds laid the lightest eggs, the relative mass of the final egg did not decrease in relation to clutch size. In 5-egg clutches, the relative mass of the final egg was negatively correlated with the mean egg-mass of the clutch. Hatching intervals did not differ between years and increased with clutch size. In 4-egg clutches, the mass difference between the Second and the final egg was negatively correlated with greater hatching intervals, however this effect was absent after accounting for year and mean egg mass. In this study, there were high rates of brood reduction and the final egg(s) in 4-5 egg-clutches produced practically no fledglings. Fledging success was lowest in the year when the birds laid the lightest eggs and decreased seasonally in two years out of three. The level of brood reduction was greater in large clutches and the number of young fledged was not correlated with clutch size. Overall, the patterns of intraclutch egg-mass variation and hatching asynchrony described in this study are consistent with Lack's brood reduction hypothesis. Female Jackdaws appear to allocate greater resources in the eggs of their clutch which have the highest probability of producing fledglings. Alternative hypotheses of intraclutch egg-size variation based on proximate constraints on the female during egg laying cannot be totally excluded. However, most of their predictions were not supported by the results.