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.