Constitutive immune function variation across the non-breeding season in an extreme long-distance migrant

Migratory animals often display remarkable adaptations in order to successfully complete their journeys. While there is substantial evidence on immunomodulation during breeding and at stopover sites en route, the immune status of migratory birds upon reaching their non-breeding grounds and throughout this stationary season remains poorly understood. Here we used the Hudsonian godwit Limosa haemastica—an extreme long-distance migrant that breeds in the Arctic and spends the non-breeding season in Patagonia—to investigate constitutive immune function (CIF) in adult individuals under contrasting conditions: on birds moulting primary feathers when they had recently arrived at non-breeding grounds (post-arrival birds), and four months later on birds moulting into breeding plumage, a couple of weeks before their departure to breeding areas (pre-migratory birds). We found comparable CIF between post-arrival and pre-migratory birds. When each group was analysed separately, agglutination titres and bacterial killing ability increased as birds completed their breeding plumage moult, although the association with bacterial killing ability appeared to be driven primarily by males. Bacterial killing ability was also higher in pre-migratory females than in males. None of the evaluated CIF parameters were affected by primary feather moult in post-arrival birds. Our results suggest important immunomodulation occurring at the end of the non-breeding season in pre-migratory godwits, probably due to the profound physiological and metabolic changes required in preparation for endurance migration. Our work contributes to the little-known topic of immune dynamics in extreme long-distance migratory birds across the non-breeding season.