The Trade-Off Between Offspring Number and Quality in the Great Tit Parus major

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Abstract
(1) Trade-offs between brood size and offspring growth and survival were studied by manipulating the size of 221 great tit Parus major L. broods between 1983 and 1987. (2) Nestling mass, wing length and tarsus length were inversely related to brood size. Hatching date also affected nestling growth, but its effect differed between years. Intrabrood variability in nestling size increased with brood size. (3) Nestling survival was inversely related to brood size. Small nestlings suffered higher mortality than large ones. The difference in nestling survival rate between experimental categories was too small to equalize their productivity. (4) Fledgling survival to the autumn and to the following breeding season was also inversely related to brood size; fledglings from reduced broods survived better than those from control broods, which in turn survived better than fledglings from enlarged broods. This resulted in the mose productive category being the control broods. The survival of fledglings increased with their size as nestlings and decreased with their hatching date. (5) The sex-ratio among independent fledglings was affected by brood size manipulation; proportionally more males survived in enlarged than in reduced broods. (6) Dispersal distance of juveniles was not affected by brood size, size as nestling or hatching date, whereas it was affected by sex. The effects of starvation and social competition on fledgling survival and dispersal are discussed.