Abstract
Daily food requirements scale with body mass and activity in animals. While small species of birds have higher mass-specific field metabolic rates than larger species, larger species have higher absolute energy costs. Under energy balance, we thus expect the small species to have a higher energy value diet. Also the weight and time constraints for flighted and diurnal foragers should set a maximum to the amount of prey items taken in one meal and to the daily number of meals, respectively. Further, avoidance of competition causes the species to reduce the amount of shared prey in their diet. Some diet segregation is therefore to be expected between species. We tested these hypotheses and investigated the role of body mass in the diet composition of 12 sea duck species (Somateria mollissima, Somateria spectabilis, Somateria fischeri, Polysticta stelleri, Bucephala clangula, Bucephala islandica, Bucephala albeola, Melanitta nigra, Melanitta perspicillata, Melanitta deglandi, Histrionicus histrionicus and Clangula hyemalis) wintering in North America. This study was based on a literature survey with special emphasis given to the diet data from the former US Bureau of Biological Survey. The data supported our hypothesis that the energy value of winter diet of sea ducks scales negatively with body mass. Diet diversity also scaled negatively with body mass. Our results suggest the existence of a minimum for the energy value of avian diets.
Highlights
An animal must balance energy intake with energy expenditures to maintain body condition and survive
We investigated the diet of sea duck species wintering in North American coastal environments
The order of increasing body mass is consistent across the two taxonomic levels with the exception that Steller’s eider, which belongs to the monospecific genus Polysticta, switches from well below median at the inter specific level to above median at the inter genus level
Summary
An animal must balance energy intake with energy expenditures to maintain body condition and survive. For a foraging animal, obtaining food can be broken down into a series of activities such as searching for a prey patch, capturing and handling that prey, and digestion while defending against competitors and avoiding predators Each step in this array of processes takes time and energy and can potentially contribute to regulate the rate of assimilation in a consumer. Prey preference changes according to ambient temperature, body condition, prey availability, competition pressure and predation risk [12], [13], [14], [15], [16], [17], [18] This set of constraints is exacerbated when the daily time available for foraging and processing prey is limited, for instance during winter in temperate and boreal regions [8], [19], [20], [21]
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