Résumé
Egg production may be influenced by environmental conditions such as local climate or food availability, which may impose physiological constraints on the acquisition and mobilization of egg constituents. We analyzed egg composition of free-ranging female Blue Tits ( Cyanistes caeruleus (L., 1758)) in both deciduous and evergreen oak habitats, which showed large differences in temperature and food availability. We found marked interhabitat differences in yolk mass, shell mass, protein content, and the abundance of linolenic (18:3) and palmitoleic (16:1) fatty acids. A weak but significant decline in total lipid content, as well as 14:0, 16:0, and 18:0 fatty acids, through the laying sequence was also detected. To our knowledge, this is the first evidence of between-population differences in nutrient allocation in eggs for a wild passerine. These differences in egg composition could be viewed as evidence of habitat-specific physiological and nutritional constraints, which in turn may contribute to the contrasting differences in timing of breeding and clutch size that we observed between both habitats. Our results point out the importance of habitat differences in our understanding of the causes and consequences of interhabitat phenotypic variation in breeding traits (timing of egg laying, clutch size) and variation in nestling traits such as growth and development.