Subtle stressors-Strong responses. Consistent negative effects of avian blood parasites on phenotypic and demographic traits across songbirds.

Stressors that subtly yet persistently deplete energetic resources-such as heat, pollutants or parasites-are well studied in laboratory and clinical settings, where their physiological effects are often well understood, yet their influence on phenotypic and demographic traits in free-living populations remains critically understudied. A prominent example is pathogens and parasites that cause sublethal infections, often considered as relatively benign, particularly in species adapted to their presence. However, parasite-induced effects on phenotypic and demographic traits are often inconsistent, leaving researchers uncertain about their impact and whether they have meaningful fitness consequences. Here, we present a meta-analysis evaluating the effects of avian blood parasites (Plasmodium, Haemoproteus and Leucocytozoon), a widespread and cosmopolitan stressor, on songbird species. Through a systematic review of 2473 publications, we identified 35 studies spanning 51 species and extracted 172 relevant effect sizes assessing host condition, phenology, reproduction and survival. Our findings reveal consistent negative impacts: reductions in body condition, reproductive success and survival, along with delays in phenological events such as breeding. Furthermore, our findings revealed a critical research gap: despite the widespread study of avian blood parasites, only a limited number provide suitable effect sizes for assessing parasite impacts on demographic traits-let alone behavioural traits. This scarcity of data highlights the urgent need to understand pathogen-induced effects on animal behaviour and demography, especially in the face of accelerating global change. We advocate for an integrative approach, combining behavioural, phenotypic, and demographic traits, to uncover the cascading consequences of parasitic infections on wild populations.