Introduction
Every year billions of birds migrate between their wintering and
breeding grounds. During these migrations, birds encounter several
challenges, which they have evolved to overcome (Maggini et al. 2022).
Research shows that birds can regulate physiological processes such as
immune function, metabolic rate, and endocrine and neuro-endocrine
systems during migration (reviewed by Hegemann et al. 2019). Activation
and maintenance of these processes along with long-distance flight is
costly and makes migration a highly resource demanding period. In order
to keep up with these resource demands, birds are required to stop
periodically to deposit fuel (Alerstam and Lindström 1990). A recent
review shows that physiological recovery, avoiding adverse weather
conditions and spatiotemporal adjustments are also important functions
of stopover besides refueling (Schmaljohann et al 2022).
Although stopovers are crucial for successful migration, they may
facilitate the spread of diseases and increase the risk of encountering
novel pathogens (Todd et al. 2018). Hence, a well-functioning immune
system during migration is important to minimize disease-related
mortality. However, it has been shown that endurance flight may
negatively impact constitutive immune parameters in European Starlings,Sturnus vulgaris (Nebel et al. 2012) as well as in Red knots,Calidris canutus (Buehler et al. 2010), suggesting lower
immuno-competence in birds after a migratory flight. To (partly)
compensate for this, migrants are able to boost immune function during
stopover (Owen & Moore, 2008), within days of arrival at a stopover
site (Eikenaar et al. 2020a, 2023).
If endurance flight during migration negatively affects immune function,
while making a stopover positively affects immune function, i.e. helps
to recover immune function, then a positive relationship between fuel
stores and constitutive immune function of migrating birds at a stopover
is expected. Yet, relatively few studies have investigated if there is
such a correlation between immune function and migrants’ energy
condition. A study by Eikenaar et al. (2020b) found that fuel stores are
positively correlated with one innate and one acquired parameter of
immune function in two sub-species of Wheatears, Oenanthe
oenanthe during autumn migration. A partly similar result was found
during spring migration in four species of thrushes in a study by Owen
& Moore (2008) where birds in poor energetic condition had lower
leukocyte counts, a measure of acquired immunity, but they did not
measure parameters of innate immune function. Hence, more research is
needed to determine if this pattern is common among other bird species
and if a positive relationships between innate immune function, the
important first line of defense, and fuel stores holds true for spring
migration, which is much more hurried than autumn migration (Nilsson et
al. 2013), mainly due to fewer stopovers in spring (Schmaljohann 2018).
In this study, we investigate if there is a relationship between fuel
stores and one parameter of constitutive innate immune function,
microbial killing ability, and one parameter of constitutive acquired
immune function, the level of immunoglobulins (IgY), in four migrating
bird species during spring stopover. These two parameters are relative
broad measures of innate and acquired immune function, respectively, and
thus reflect an integrative measure of the immune system. We hypothesize
that there will be a positive correlation between fuel stores and the
immune parameters. We also investigate if the immune parameters differ
between species and if there is an effect of time within the migration
season on the immune parameters.