PATTERNS OF EGG AND CLUTCH SIZE VARIATION IN THE MONTAGU HARRIER

We describe the phenotypic plasticity of egg and clutch size in the Mo ntagu's Harrier (Circus pygargus) based on measurements of 1292 eggs f i om 403 nests and clutch sizes recorded at 579 nests in four study ar eas in western France and central Spain. Variability of egg size, clut ch size and clutch volume was high (coefficients of variation 10, 24, and 28%, respectively). Egg volume and shape (length/width) were posit ively correlated and bigger eggs tended to be relatively longer. Shape was not normally distributed. with elongated eggs being more common t han rounded ones. Montagu's Harriers appeared, therefore, to be less c onstrained to increase egg length than egg width. Egg volume was posit ively correlated Kith clutch size. The first and last eggs in each clu tch were on average smaller than the intermediate ones, but the differ ences were not significant. High interannual variation existed in clut ch size, but not in egg size. No regional variation was found in egg o r clutch size, once interannual differences were taken into account. O verall, clutch size appeared to be a more plastic trait than egg size, although the large variability in the latter suggested that it had th e potential, at least, to vary according to environmental or individua l conditions.