Physiological strategies of moult-migrating Black-necked Grebes (Podiceps nigricollis) in a polluted staging site according to blood chemistry

After breeding, Black-necked Grebes (Podiceps nigricollis) perform a moult-migration to autumn hypersaline staging sites, where they moult the flight feathers and forage on superabundant brine shrimp (Artemia spp.) before leaving for wintering areas. During the stay in moulting sites, the grebes experience changes in organs and muscle size (atrophy, hypertrophy), and almost double their body mass, which has been suggested to act as an insurance against a collapse in prey availability in late autumn. During two years we collected blood samples from hundreds of individuals at one of the most important European moulting sites (the Odiel marshes, SW Spain), which is a highly polluted area due to mining drainage and chemical industry. We assessed the potential effect of moulting stage, day of the year and body condition on 16 blood biochemical parameters. Because of the changes in prey availability and body composition of grebes, we expected some physiological adjustments during moult. Elevated levels of cholesterol suggested that birds in active moult increased foraging effort to face the costs of moulting. There was increased amount of lactate dehydrogenase, corresponding to periods of breast muscle atrophy. Birds in active moult augmented protein ingestion, likely to account for the requirements of feather growth. We also show that the probability of fasting due to low prey availability increased late in the moulting season, as demonstrated by an increase in plasma β-hydroxy-butyrate.