Discussion
We found that fuel stores were not correlated with an integrative
measure of constitutive innate immune function (microbial killing
ability) nor with a measure of constitutive acquired immune function
(IgY levels) in any of the four species during spring migration. This is
contrary to what was found in wheatears during autumn migration where
there was a positive correlation between fuel stores and both these two
same parameters of immune function (Eikenaar et al 2020b). Interestingly
and similar to this current study, there was no correlation between
energetic condition and immunoglobulin levels in spring migrating
thrushes in the Americas either (Owen & Moore 2008). However, Owen &
Moore (2008) did observed a negative relationship between energetic
condition and total White blood cell count as well as one out of five
leukocyte types. They did not measure innate immune function. Taking
together those studies may suggest that while there is a correlation
between immune function and fuel stores during autumn migration, such a
relationship may at most be weak in spring migrating birds. A potential
explanation for the differences between autumn and spring migrating
birds could lie in seasonal differences in migration speed that impact
resource allocation between immune function and the demands of
migration. There is evidence that progress during spring migration is
much faster than during autumn migration as birds travel with fewer
stopovers in spring (Nilsson et al. 2013; Schmaljohann 2018). The reason
for a more hurried spring migration is that there is a fitness advantage
for arriving early at the breeding grounds; individuals that arrive
earlier at the breeding grounds are more successful in finding a
territory and a mate (Kokko 1999, Aebischer et al. 1996). Perhaps,
spring migrants tend to invest their resources (energy and nutrients for
fuel) more into timely arrival at the breeding site rather than in
immune function compared to autumn migrants. Fewer stopovers may also
mean that spring migrants have less opportunity to recover their immune
function during stopover after strenuous endurance flights (Eikenaar et
al. 2020a; 2023). Some support for a seasonal difference in immune
function comes from comparing values from the current and previous
studies. Eikenaar et al (2020b) found that in autumn migrating Wheatears
IgY levels ranged from 45-60 mOD/min in autumn, whereas in the current
spring study these typically ranged from 20-35 mOD/min (Figure 3).
Similarly, in comparison to an autumnal study by Hegemann et al. (2018),
the mean IgY levels for Dunnocks, Song thrushes and Chaffinches were
much lower in the current spring study. It is important to note though
that direct comparisons should be made with caution, as the assays in
the various studies were performed with different standards and
dilutions. Nonetheless, they are suggestive of a lower (investment in)
immune function in spring than autumn. which could help to understand
why in the current spring stopover study we did not observe
relationships between migrants’ fuel stores and parameters of immune
function.
Three other, not mutually exclusive hypotheses might further help
explain why there is no correlation between immune function and fuel
loads in spring migrating birds and why they may have lower immune
function than birds during autumn migration. All three relate to
pathogen pressure, i.e. the risk of encountering pathogens and becoming
sick. First, densities of birds is usually much lower in spring than
during autumn, especially when considering that our study location is
relatively close to the breeding locations, because most annual
mortality happens between autumn and spring (Sillett & Holms 2002,
Leyer et al. 2013, Klaaassen et al. 2013). Lower densities of other
birds will decrease contact rate and hence the risk of transmission of
infectious diseases. Second, prevalence of diseases is often higher
during autumn than during spring (Latorre-Margalef et al. 2014, van Dijk
et al. 2014), which also reduces the risk of infection. Third, during
autumn birds are on their way to southerly wintering grounds, which
supposedly harbor more pathogens than the northerly breeding grounds to
which birds are heading to during spring (Westerdahl et al., 2014;
O’Connor et al., 2020). All three hypotheses reduce the risk of disease
contact and hence the need for strong constitutive immune function
(Horrocks et al. 2011, 2015, Hegemann et al. 2012). Hence this may allow
birds to invest more in migration and less in immune function during
spring migration compared to autumn migration.
We did observe a relationship between immune function and Julian day in
some species; individuals that arrived on Helgoland early in the
migration season had higher bacterial killing ability in case of Song
thrushes and Wheatears and higher levels of IgY in case of Song
thrushes. We can only speculate on the reason(s) for these
relationships, but perhaps birds arriving at stopover earlier are of
superior quality than the ones arriving later. It is also possible that
individuals wintering further South passed by Helgoland later, and had
lower immune function because of their longer migratory flights. We
furthermore observed significant differences in microbial killing
ability as well as IgY levels among the species, which is in line with
previous studies (e.g. Tieleman
et al 2015; Hegemann et al. 2022; Eikenaar et al. 2023). Variation in
immune function between species can have multiple reasons including
differences in risk of infection, phylogenetic history, migration
strategy etc. It is beyond the scope of our study to discuss or even
disentangle possible reasons, but the data provided might prove valuable
for future studies or meta analyses.
In conclusion, our study showed that unlike during autumn migration,
during spring migration fuel stores are not related to immune function.
Moreover, the current and previous studies hint at differential seasonal
investment in immune function, which could be further investigated in
focused comparative studies.