INTRACLUTCH EGG-MASS VARIATION AND HATCHING ASYNCHRONY IN THE JACKDAWCORVUS-MONEDULA

The patterns of intraclutch egg-mass variation and their associated ha tching intervals were studied over three breeding seasons in a Jackdaw Corvus monedula, population. The data were used to test alternative p redictions from hypotheses proposed as explanations for intraclutch eg g-size variation. Although clutch size did not differ between years, s ignificant differences in laying date and mean egg-mass were found. Th e mean egg-mass of the clutch decreased seasonally but did not vary wi th clutch size. The intraclutch patterns of egg-mass variation were cl ear: in 4-5 egg-clutches, egg-mass increased from the first to the sec ond egg and then decreased with laying order. In nearly all clutches t he final egg was the lightest and its relative mass decreased with clu tch 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 clu tch size. In 5-egg clutches, the relative mass of the final egg was ne gatively correlated with the mean egg-mass of the clutch. Hatching int ervals 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. I n this study, there were high rates of brood reduction and the final e gg(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 fledg ed was not correlated with clutch size. Overall, the patterns of intra clutch egg-mass variation and hatching asynchrony described in this st udy are consistent with Lack's brood reduction hypothesis. Female Jack daws appear to allocate greater resources in the eggs of their clutch which have the highest probability of producing fledglings. Alternativ e hypotheses of intraclutch egg-size variation based on proximate cons traints on the female during egg laying cannot be totally excluded. Ho wever, most of their predictions were not supported by the results.