Egg-size variation in Algerian populations of the Blue Tit (Parus caeruleus ultramarinus): Effects of altitude and habitat

Egg-size, as a component of life history, is expected to be traded-off with other components and, consequently, to display a pattern of variation between different habitats and altitudes. Relevant data being very scarcely available for just a few bird species caused us to study interhabitat and interaltitude variation in egg dimensions and mass in the Blue Tit Parus caeruleus ultramarinus, taking the advantage of the occurrence of this species in a variety of habitats and at different altitudes in North Africa (north-eastern Algeria). In 1991-1993 we selected 6 study sites representing two types of habitat (rich deciduous forests dominated by Alnus glutinosa and Quercus faginea vs poor evergreen forests of Quercus suber) at three elevations (30 m asl, 500 m als and 900-1 000 m asl). We analysed both multiple response variables characterizing egg dimensions and individual egg traits. Eggs turned out to differ between altitudes (the lowest elevation against the two higher ones) but did not differ between habitats. They also display relatively high within-clutch repeatabilities, 0.6 on average for all traits. Egg traits were not significantly correlated with clutch-size within study stations, but the analysis of a bivariate response variable composed of egg volume and clutch-size suggested that these components of life history were negatively linked. Moreover, this bivariate response variable differed both between habitats and altitudes.