This file contains data on moulting status, moult progression and baseline innate immune function (bacterial-killing ability, lysis, agglutination, haptoglobin concentration) in adult and juvenile blackbirds captured in Lund, southern Sweden, in 2019–2020 (Aug–Oct).

Moult, the regular replacement of feathers, is a resource-intensive avian annual cycle stage. Resource allocation to support feather regrowth may require downregulation of other physiological processes, such as immune function, which is critical for preventing disease-related mortality. At the same time, the emergence of new feathers may increase the risk of skin injury and infection, potentially requiring sustained or enhanced immunocompetence during this period. While previous studies have compared immune function between moulting and non-moulting individuals, it remains unclear how immune function changes throughout moult. Moreover, little is known about how these dynamics manifest in age-specific moulting strategies, such as the partial juvenile or complete adult moult typical of most passerines. Here, we investigated how immune function relates to moult progression in Eurasian Blackbirds (Turdus merula), focusing on post-juvenile body moult in first-year birds and on primary wing feather moult in adults. We also assessed immune function prior to moult to determine whether immunity is down- or upregulated during moult. Four functionally distinct parameters of baseline innate immune function were quantified: bacterial-killing ability, complement activity, natural antibody titres, and haptoglobin concentration. We did not detect differences in any immune parameter between moulting and not-yet-moulting adults, and immune function remained stable throughout moult progression in both age groups (i.e. for both age- specific moulting strategies). Our results, therefore, do not provide evidence for a trade-off between immunity and feather regrowth. Instead, maintaining immune function may be needed to offset increased infection risk during moult, but further research – ideally incorporating measures of individual condition or experimental approaches – is warranted. Our study provides new insights into immune regulation during a critical, yet understudied, annual-cycle stage, and indicates directions for future research.