SELECTION ON FLEDGING MASS IN THE COLLARED FLYCATCHER AND THE GREAT TIT

The selection on residual mass (relative to body size) at fledging was investigated in the Collared Flycatcher Ficedula albicollis and the Great Tit Parus major. Directional selection for high fledging mass was evident in 6 of 7 yr in Collared Flycatchers but only in 1 of 5 yr in Great Tits. Three different methods of demonstrating and testing stabilizing selection are compared. Stabilizing selection on relative mass occurred in both species, but the frequency with which this was established depended on which method was used. None of the methods seemed to be universally reliable, and the problem of defining and testing stabilizing selection is discussed. In Collared Flycatchers mass seemed to be most important for juvenile survival. Also in Great Tits mass was important, but in addition other traits (e.g., hatching date) strongly influenced juvenile survival in this species. This possible difference between the two species is suggested to relate to differences in their territory establishment.