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.