Early-life conditions impact fitness, but whether the combined effect of extrinsic stressors is additive or synergistic is not well known. This is a major knowledge gap, because exposure to multiple stressors is likely to be frequent. Telomere dynamics may be instrumental when testing whether combined stressor effects are additive or synergistic, because many factors affect telomere shortening, and telomere shortening predicts survival. We evaluated the effects of manipulated brood size and natural infestation by the carnid fly Carnus hemapterus on nestling growth and telomere shortening of wild jackdaws (Corvus monedula). Telomere length, measured in blood using TRF, shortened on average by 264 bp. In enlarged broods, nestlings’ telomeres shortened more when parasitized, while in reduced broods there was no effect of infestation on telomere shortening. We conclude that there is a synergistic effect of number of siblings and Carnus infestation on telomere shortening rate: blood-sucking parasites may negatively impact telomeres by increasing cell proliferation and/or physiological stress, and coping with infestation may be less successful in enlarged broods with increased sibling competition and lower per capita feeding rate. Larger nestlings had shorter telomeres independent of age, brood manipulation or infestation. Growth was independent of infestation but in enlarged broods, nestlings were lighter at fledging. Our findings indicate that (i) evaluating consequences of early-life environmental conditions in isolation may not yield a full picture due to synergistic effects, and (ii) effects of environmental conditions may be cryptic, e.g. on telomeres, with fitness consequences expressed beyond the temporal framework of the study.