Salinity as a constraint affecting food and habitat choice of mussel‐feeding diving ducks

Abstract

Observations of freshwater drinking in Eiders feeding primarily on mussels led us to hypothesize that the highly saline sea water enclosed in mussels could cause salt‐related dehydration problems in the ducks, since they consume entire mussels. The proportion of sea water increases with increasing mussel size. Smaller duck species are more sensitive to the higher salt content of larger mussels than are larger ducks; however, salt stress may be avoided by feeding in habitats with lower salinity, by feeding on less salty food items, by utilizing smaller mussels, by drinking fresh water, or by employing low energy foraging techniques. A possible evolutionary strategy for solving the salt problem might be to increase body mass, enabling ducks to utilize larger mussels without passing an upper salt consumption limit. At the same time, foraging on larger mussels is more economical. Although large size facilitates the utilization of brackish and marine environments, it may be selected against in ducks breeding in fresh water, where fish competition may reduce optimal food item size. In conclusion, salinity is an important habitat barrier in both breeding and overwintering diving ducks, but there are various ways of crossing this barrier. To understand better how ducks utilize their habitats, however, it is necessary to measure habitat salinity levels and the size of both ducks and their preferred and less‐preferred food types.

Understanding the energetic costs of living in saline environments: effects of salinity on basal metabolic rate, body mass and daily energy consumption of a long-distance migratory shorebird

“…These results support empirically that the processes of developing and maintaining an active osmoregulatory machinery are energetically expensive (McArthur and Gorman, 1978;Burger and Gochfeld, 1984;Nyström and Pehrsson, 1988;Klaassen and Ens, 1990;Dosch, 1997). If the BMR is the sum of the metabolic rates of all the organs and tissues (Hulbert and Else, 2000;Turner et al, 2004), then the significant increase in BMR could be due to increased size and metabolic intensity of the organs and tissues involved in salt excretion.…”

“…Specifically, we predicted that when dunlins are relying on saltwater habitats they have higher BMRs than when they are relying on freshwater habitats. It has been suggested that the processes of developing and maintaining active salt glands are energetically expensive (McArthur and Gorman, 1978;Burger and Gochfeld, 1984;Nyström and Pehrsson, 1988;Klaassen and Ens, 1990;Dosch, 1997). On this basis, we also predicted that the dunlins' BMR in a saltwater regime would remain high until the birds were moved back to a freshwater regime.…”

“…The increase in BMR and food intake in saline environments may play a significant role in the individual's energy budget, and could help explain diet and/or habitat selection patterns along the flyway (Nyström and Pehrsson, 1988;Adair et al, 1996;Woodin et al, 2008). Routine movements of several species of diving ducks (Aythya spp.)…”

“…This group of aquatic birds is considered to be 'osmotic generalists' (Blakey et al, 2006). To cope with saline environments, they have evolved paired orbital salt glands that secrete excess ingested salt via the nostrils (Fänge et al, 1958;Schmidt-Nielsen et al, 1958;Schmidt-Nielsen, 1960;Staaland, 1967), and they also present some behavioural adaptations (Mahoney and Jehl, 1985a;Mahoney and Jehl, 1985b;Nyström and Pehrsson, 1988). We here describe a laboratory and field study in which we investigated the effect of different saltwater regimes on the BMR, daily energy consumption and body mass of one of these long-distance migratory shorebirds, the dunlin (Calidris alpina).…”

Complementary use of natural and artificial wetlands by waterbirds wintering in Doñana, south‐west Spain

“…Apart from the differences in hydroperiod, salinity is a major determinant of which plant and invertebrate food is available in each site (Espinar et al, 2002;Frisch et al, 2006b). In addition, consumption of prey originating from brackish water increases costs of osmoregulation (Nystrom and Pehrsson, 1988), and this may contribute to the preferential use of DNP by waterbirds.…”