Although the negative impact of human-induced environmental effects on bird populations has been widely demonstrated, the question of whether adaptive adjustments may potentially arise as a result of unforeseen challenges is still unclear. Despite their obvious pervasive effect, human-induced challenges may activate, under certain circumstances, physiological and behavioural compensatory mechanisms that allow organisms to cope better with an altered and distressful environment. In this viewpoint, we highlight that understanding such compensatory responses (or the lack of them) requires adopting an ontogenetic and transgenerational perspective, as well as a multidisciplinary approach that integrates physiology, ageing biology, molecular processes and behaviour. Given the outstanding capacity for plasticity during development, we focus on how early-life (human-induced) experiences potentially shape, even prenatally, specific physiological and molecular processes (i.e., protection against oxidative damage and telomere maintenance mechanisms), and lifelong reproductive strategies (i.e., maternal allocation into eggs), which may in turn activate physiological and behavioural adjustments across generations. To test whether such adjustments in the developmental trajectory allow individuals to make “the best of a bad situation” or even increase their performance or that of their offspring in human-altered environments, we call for studies using a lifelong approach and that explore transgenerational effects. We thus propose experimental designs that could help the advancement in the field.

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.