Nutrient acquisition by female Harlequin Ducks prior to spring migration and reproduction: evidence for body mass optimization

Abstract

We analysed variation in body mass of adult female Harlequin Ducks ( Histrionicus histrionicus (L., 1758)) on coastal wintering sites in southern British Columbia, Canada, to investigate nutrient acquisition prior to migration and reproduction. On average, female mass increased by 7% from late winter to premigration; however, the chronology of mass gain varied depending on prey type. Females feeding on superabundant roe from spawning Pacific herring ( Clupea pallasii Valenciennes, 1847) became considerably heavier than they had been before the herring spawning period (March) and appeared to be heavier than females eating marine invertebrates such as crabs, limpets, and snails during the herring spawning period. By mid-April, prior to migration, females at all sites had similar body masses, with birds at sites without spawn gaining mass and those at sites with spawn maintaining their earlier mass gain. Stable isotope analyses confirmed that birds at these different sites consumed very different diets. These results suggest that female Harlequin Ducks target an optimal premigratory body mass, regardless of whether they have access to a superabundant food source; this body mass is presumably shaped by the costs and benefits of nutrient storage for migration and reproduction.