No 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.

Abstract. Waterfowl employ a broad array of strategies for acquiring the energy and nutrients needed for egg formation, ranging from storage of endogenous reserves prior to arrival on breeding areas to complete reliance on exogenous food sources available at breeding sites. We used stable isotope analyses (δ13C and δ15N) to quantify the relative nutrient inputs to Harlequin Duck (Histrionicus histrionicus) eggs and, therefore, to identify the strategy of nutrient acquisition and allocation used by females to meet the demands of egg production. Marine-derived endogenous nutrients are isotopically more enriched than freshwater dietary nutrients for Harlequin Ducks that migrate between marine wintering grounds and terrestrial breeding grounds. There was little evidence that endogenous reserves stored on marine wintering areas were allocated to clutch formation. Therefore, Harlequin Ducks relied on food available in streams on breeding grounds for egg formation, and reserves stored on marine areas were likely used during other energetically and nutritionally demanding periods.

Quantifying sources of variation in demographic rates can provide insight into processes underlying population dynamics and subsequently direct wildlife conservation. In the context of avian life history, understanding patterns of variation in survival rates of breeding females is particularly relevant because this cohort often has a disproportionately large effect on population dynamics. We estimated survival probability for 144 adult female harlequin ducks (Histrionicus histrionicus) that we marked with radiotransmitters and tracked at 4 breeding areas in western North America. Model selection results indicated both regional and temporal variation in survival rates, with most mortality attributed to predation. Cumulative survival probability (6SE) during the 100-day study period was lower at 2 sites in the Rocky Mountains of Alberta, Canada (AB1 and AB2: 0.75 6 0.11) than in the Coast Mountains of British Columbia, Canada (BC: 0.88 6 0.08) or the Cascade Mountains of Oregon, USA (OR: 0.89 6 0.08). Survival also was lower during incubation than nest-initiation or brood-rearing stages at all 4 study areas. In comparison to other annual cycle stages and locations, harlequin duck mortality rates were highest on the breeding grounds, suggesting that management actions designed to reduce mortality during breeding would achieve meaningful population-level benefits. Bond et al. N Harlequin Duck Survival 965

Breeding propensity, the proportion of sexually mature females that initiate egg production, can be an important demographic trait when considering reproductive performance and, subsequently, population dynamics in birds. We measured egg production using yolk precursor (vitellogenin and very‐low‐density lipoprotein) analyses and we measured nesting using radiotelemetry to quantify breeding propensity of adult female harlequin ducks (Histrionicus histrionicus) in British Columbia, Canada, in 2003 and 2004. Using both methods combined, and accounting for error rates of each, we estimated that breeding propensity of adult females that migrated to breeding streams was 92%. These data suggest that, despite speculation that harlequin ducks have low breeding propensity, almost all adult females on our study site were not constrained in their ability to produce eggs and that influences on reproductive performance at later stages likely have much stronger effects on population dynamics.