Perez Badas, E.; Bauch, C.; Boonekamp, J. J.; Mulder, E.; Verhulst, S.

Ectoparasite infections during development are known to impact fitness-related traits, but their physiological costs in relation to sex or growth trade-offs are understudied. In birds, previous studies at the brood level have found costs of infection by blood-sucking ectoparasites in molecular biomarkers (i.e., increased telomere shortening). However, infection probability also varies with age, sex, or within broods. Whether such costs are age or sex-dependent is not well known, and disentangling these effects is of interest, because telomeres potentially reveal the long-term impact of infection. Here, we used 6-year-data from jackdaw nestlings (Coloeus monedula) to investigate individual level associations of infection by the fly (Carnus hemapterus) at two intermediate time points during development, with telomere dynamics over the nestling period. We evaluated (i), Carnus presence in relation to sex, brood size manipulation, body mass, and size, and (ii), how telomere shortening varied in relation to parasite presence and achieved growth. Early in development (day 10), flies were more prevalent on heavier nestlings, while later in development (day 20), they were more prevalent on smaller and lighter nestlings, suggesting different ectoparasite preference. Telomere loss was higher in female nestlings when exposed to parasites only on day 20, and in males, when they were heavy earlier in development. In conclusion, infection is a dynamic process, and ectoparasite effects may depend on the timing of infection and reveal sex-specific growth vs. self-maintenance trade-offs. Our study indicates a need to sample at multiple developmental stages when assessing host-parasite interactions and the physiological costs of infection.