Introduction to Methods

Each species i s subject to sorne environmental heterogeneity within its breeding range. Consequently, sorne of its life-history traits such as its breeding season and its clutch-size vary, allowing the species to better adapt to the different parts of its breeding range. In the Blue Tit (Parus caeruleus), which breeds from the Canary Islands to northem Europe, and from the British Isles to the Ural Mountains, this variability is particularly marked in the Mediterranean area (Snow, 1 956; Blondel et al., 1 987; lsenmann, 1 987). Its mean clutch size drops from about 1 1 eggs in the deciduous habitats of southem France to 3.5 eggs in the Canary Islands (Isenmann, 1 987). We report here further data collected by two of us (EA and OM) in an evergreen Holm Oak (Quercus ilex) habitat in Andalusia (southem Spain) during three breeding seasons. This Blue Tit population belonging to the caeruleus group has a small size (Vaurie, 1 959; Martin, 1 988), the earliest known breeding season in Europe and a moderately low clutch size