Your tools disappear when you stop eating: phenotypic variation in gizzard mass of eiders

Animals show phenotypic flexibility in their digestive system in response to seasonal changes in diet, activity, metabolic rate and reproduction. Many birds are capital breeders that alternate between periods of extensive feeding used for storage and periods of fasting during reproduction. Here, we analyzed the mass of the gizzard (gizzard mass without content) in 885 male and 348 female adult common eiders Somateria mollissima shot by Danish hunters during winter and spring in relation to alternating periods of foraging and fasting during reproduction. Gizzard mass of adult female eiders varied annually from 31 g ( N =  25) during reproduction to 119 g during pre‐breeding ( N =  314), or almost a four‐fold difference in mass. During winter, both male and female eiders with large gizzard mass had eaten a larger number of preferred blue mussels Mytilus edulis that constitute the main food. Adult female eiders with large gizzard mass had eaten larger but not more mussels. Individual adult females with large gizzard mass that had larger numbers of mussels in their gizzards were in superior body condition. These findings of phenotypic flexibility in gizzard mass show that individuals with larger gizzards consuming larger food items had an advantage in terms of superior body condition, potentially allowing for elevated reproductive success.