Sex-Dependent Carry-Over Effects Between Physiological State and Reproduction in a Passerine Species

Carry-over effects (COEs) occur when an event in a life stage of an individual is affected by the experience, physiological state or reproduction of a previous life stage. COEs are insufficiently explored with regard to the connections between physiological state and reproduction in animals with seasonal iteroparity and short lifespans. We investigated within-individual temporal changes in haematocrit (an indicator of oxygen-carrying capacity and energetic demand) and heterophil granulocyte-to-lymphocyte (H/L) ratio (indicator of stress levels and health state) within and between breeding seasons, and COEs between haematocrit, H/L ratio, and reproduction (breeding onset, clutch size, brood size and the number of prefledglings) in a short-lived passerine, the collared flycatcher (Ficedula albicollis). Haematocrit varied across years and showed moderate repeatability overall, with higher repeatability observed in females. In males, haematocrit declined between the courtship and the nestling-rearing period. Lower haematocrit during nestling rearing was associated with higher brood size in males, suggesting a trade-off between self-maintenance and parental effort. While H/L ratio did not fluctuate across years, in females, increased reproductive effort in the previous year correlated with higher H/L ratios in the subsequent year, indicating physiological costs of reproduction. Additionally, females with higher H/L ratio during nestling-rearing had lower fledging success in the following year, suggesting long-term fitness consequences of stress. Our findings highlight that COEs between physiology and reproduction in the short-lived collared flycatcher manifest differently in the two sexes, likely due to distinct reproductive roles and energetic constraints.