Many songbirds migrate twice a year to exploit seasonally available resources. These journeys are energetically demanding and the energy reserves of these small birds are limited. Accordingly, most of the time migrating is spent during stopovers that serve to rest, replenish resources and recover. While external influences, like prevailing weather conditions and resource availability, are well studied with regard to stopover behavior and departure decisions, studies on how birds’ individual physiological conditions and stopover decisions may be linked are scarce.

We used a large-scale radio-telemetry network covering the German Bight (SE North Sea) to study how birds’ immunological constitution at stopover may correlate with departure and flight behavior in five species of short- to medium-distance migratory songbirds. We measured markers of the innate (bacterial killing activity, lysozyme concentration, natural antibodies, and complement titers) and acquired immune function (immunoglobulin Y) in the birds’ plasma. After sampling, we tracked the birds’ behavior in terms of stopover duration as well as flight routes and flight distances within the telemetry network after departure.

We found that stopover durations were positively related to natural antibodies and immunoglobulin Y across species and to the bacterial killing ability solely in song thrushes in spring, while no relations became apparent in fall. Flight distances were linked positively to immunoglobulin Y concentrations in both spring and fall, though not in all of the investigated species. Coastal and offshore-oriented routes were taken independent of the birds’ immune status.

Our study for the first time shows that the migratory behavior of songbirds in the wild is correlated with their immune status, not only during but also beyond stopover. Further, birds with increased complement titers and Immunoglobulin Y concentrations, either due to recent infection or greater investment in their immune function, took additional time at their stopover sites, probably to reach their breeding grounds in good condition. During the less time-constrained fall season, stopovers were generally prolonged, independent from the birds’ immune status, and any delays taken to improve immune status are most likely not detrimental in terms of fitness.

Birds of five species of short- to medium-distance migrating songbirds were caught at the coast of Schleswig-Holstein, Germany during three consecutive autumn seasons, blood sampled and released with radio-telemetry tags. From blood samples, four parameters of the constitutive innate immune system (bacterial killing activity against Escherichia coli, lysozyme concentration, natural antibodies and complement titers and one component of the constitutive acquired immune system (total immunoglobulin Y concentration) were measured. From radio-telemetry data filtered for false positive detections (see Brust et al. 2019 for details), we assessed the duration of stopover at the coast, taken route, and flown distance within the radio-telemetry network.

Data are provided as R dataframe.