In seasonally fluctuating environments, timing of reproduction is a crucial determinant of fitness. Studies of birds show that late breeding attempts generally result in offspring of lower reproductive value, with lower recruit-ment and long-term survival prospects. Several proximate mechanisms, including a seasonal decline of immune sys-tem functioning, may lead to a seasonal decline of offspring fitness. We investigated seasonal variation in offspring quality by subjecting first-and second-brood chicks of a sexually size dimorphic species, the European starling Sturnus vulgaris, to an immune challenge with a bacterial endotoxin (LPS), and evaluated their growth and physiolog-ical response in terms of total plasma antioxidant capacity (TAC), concentration of reactive oxygen metabolites and he-matocrit. LPS challenge did not affect chick growth or oxida-tive status. However, hematocrit of second-brood chicks was higher in LPS chicks compared to controls. Body mass half-way through the rearing period (days 8–9 post-hatching), TAC and hematocrit were lower among second-vs. first-brood chicks. Interestingly, sexual dimorphism in body mass at days 8–9 post-hatching markedly differed between broods, first-brood males being 4.7% and second-brood males 22.7% heavier than their sisters, respectively. Pre-fledging mortality occurred among second-brood chicks only and was strongly female-biased. Our findings suggest that starling chicks, even if in poor conditions, are little affected by a bacterial challenge, at least in the short-term. Moreover, our study indicates that sex differences in body size, possibly mediated by sex-specific maternal investment in egg size, may heavily impact on pre-fledging survival in a different way in the course of the breeding season, resulting in sex-specific seasonal decline of offspring fitness. Finally, we suggest that levels of circulating antioxidants should be regarded among the proximate causes of the association between timing of fledging and long-term survival in avian species.