On the Effects of Small Scale Variation in Temperature and Food Availability on Laying Date and Egg Size in Great Tits (Parus Major)

Extensive long term studies on Great Tits (Parus major) identified several factors which affect their timing of reproduction and their reproductive output (see Perrins 1979 for a review). Variation in breeding parameters such as date of first egg laying or clutch size could be successfully correlated with variation in temperature, breeding density, food availability and other proximate factors (e. g. Kluyver 1952, Perrins 1965, van Balen 1973, Dhondt et al 1984, Schmidt 1984, Perrins and McCleery 1989). Although mechanisms which act on the individuals are envisaged, the relationships are based on mean values of large groups. Breeding parameters vary within a breeding population by about the same magnitude as between different years. Variation among individuals could be partly attributed to such factors as, for example, age (Perrins 1965 and Dhondt 1989 for laying date and clutch size), body size (Jones 1973 for laying date) or genetic differences among them (Perrins and Jones 1975 and van Noordwijk et al. 1981a for laying date, van Noordwijk et al. 1981b for clutch size, van Noordwijk et al. 1981c and Ojanen et al. 1979 for egg size). But a considerable part of the variation among individuals in a breeding population remains unexplained. Differences in local environmental conditions could help us to explain more of this remaining variation, as environmental conditions can explain some of the differences between years.