| Summary: | Breeding has a high energetic cost and, in central place foragers, also restricts the ability of parents for self-foraging, with afecting on their body and immunological conditions. In migratory species, breeding overlaps the period of recovery of body conditions before the following outbound migration. Those factors may reduce the breeding success and even lead parents to abandon their ofspring if parental body condition cannot guarantee the success of the next migration. We studied the body and immunological conditions of a low breeding success population of Brown Skuas ( Stercorarius antarcticus lonnbergi ) throughout their breeding season. We evaluated changes in their body mass, blood parameters and heterophil:lymphocyte ratios (H:L), at three stages of their breeding period: Incubation, Early rearing and Late rearing. The body mass of female Brown skuas decreased during the initial stages of the breeding period and recovered towards the end. The changes in metabolites suggest a use of protein body reserves but not of lipid reserves. The H:L ratio did not indicate changes in the immune condition of the birds. Our results suggest that this low breeding success population begins its breeding period with a poor body condition that it continues to decline throughout the season. Those poor body conditions added to the need for recovery for the next migration may be the cause of the abandonment of almost totally feathered chicks. Our evidence highlights the importance of food availability, in either the breeding or wintering sites, or both, on the health of the populations of migratory top predators. Facultad de Ciencias Naturales y Museo
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