Stable immune function during moult regardless of age-specific moulting strategy in a European passerine

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.