Annual patterns of body, tissue, and organ mass variation in long-tailed ducks <i>Clangula hyemalis</i>

Temporal variation in resource availability, physiological demands, and other factors are associated with many phenotypic changes in organisms. For example, there are often predictable stages of atrophy and hypertrophy in animals’ organs to accommodate changes in diet. Timing of stages may differ by sex given differences in life histories (e.g., egg-laying versus male–male competition). In this context, we quantified changes over the annual cycle in 153 long-tailed duck ( Clangula hyemalis) carcasses. We also tested whether timing of changes differed by sex. Total body mass was lowest in February and highest in November, whereas livers, spleens, kidneys, and salt glands were lightest in the middle of breeding seasons. Reductions in kidney and salt gland masses coincided with switches to using freshwater from marine habitats. Generally, timing in patterns of body mass change did not differ by sex. This was unexpected, and could arise from the compressed breeding season. We were also interested in evaluating whether changes in masses of tissues or organs were more dramatic for a species that breeds at such high latitudes than for species that breed at lower latitudes, but were unable to glean this information from the literature. We present coefficients of variation to facilitate future comparisons.