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The effect of natural brood size variation on offspring quality was studied in a blue tit (Parus caeruleus) population on the island of Gotland in the Baltic Sea. Offspring quality, measured as nestling body mass at day 13 post-hatch, declined significantly with increasing brood size, as did offspring structural body size (tarsus length). A quantitative genetic analysis revealed a high heritability of tarsus length, but also that the shorter tarsi of young from larger broods represented a negative environmental deviation from the genotypic values of their parents. Similarly, positive environmental deviations in tarsus length were found in small broods. Nestling mortality increased with increasing brood size, and smaller and lighter nestlings suffered higher mortality between day 13 and 20 post-hatch. These findings, together with those of previous studies showing that the survival prospects of malnutritioned passerine young are greatly reduced, provide evidence for a trade-off between the quantity and quality of young under non-manipulative conditions.