OFFSPRING NUMBER AND QUALITY IN THE BLUE TIT - A QUANTITATIVE GENETICAPPROACH

The effect of natural brood size variation on offspring quality was st udied in a blue tit (Parus caeruleus) population on the island of Gotl and in the Baltic Sea. Offspring quality, measured as nestling body ma ss at day 13 post-hatch, declined significantly with increasing brood size, as did offspring structural body size (tarsus length). A quantit ative genetic analysis revealed a high heritability of tarsus length, but also that the shorter tarsi of young from larger broods represente d a negative environmental deviation from the genotypic values of thei r parents. Similarly, positive environmental deviations in tarsus leng th were found in small broods. Nestling mortality increased with incre asing brood size, and smaller and lighter nestlings suffered higher mo rtality between day 13 and 20 post-hatch. These findings, together wit h those of previous studies showing that the survival prospects of mal nutritioned passerine young are greatly reduced, provide evidence for a trade-off between the quantity and quality of young under non-manipu lative conditions.