The effects of high and low protein food on molting in penned, wild-strain Mallards (Anus plutyrhynchos) from southwestern Sweden were studied in 1979, 1980, and 1985. Body condition, as indicated primarily by lipid deposits, declined during the flightless period. This decline was probably more the result of a strategy to recover flight capability rapidly than the result of stress. By metabolizing stored lipids, birds may be able to seek refuge in habitats offering protection from predators, even if food in such habitats is scarce. During the flightless period, food quality did not affect the growth rate of the primary feathers. But when exposed to low protein food, even after recovering flight capability, birds, especially males, developed shorter wing feathers; this feather shortening may reduce their condition capacity, i.e., their capacity to increase body condition without reducing their flight capacity in late autumn. A lowered condition capacity could lead to decreased survival of the ducks when exposed to severe winter conditions. Female Mallards demonstrated a higher condition capacity than males, which may contribute to the more rapid recovery of female primaries. Since the final length of the primaries is affected by food quality during molting, wing length is probably not a variable suitable for use in a condition index.

No 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.

The duckling production of the Oldsquaw (Clangula hyemalis) was studied between 1969 and 1974 in Torne Lapmark in northern Sweden. The degrees of relationship among various indices of reproductive output, climatic factors, and fluctuations in small rodents were estimated. Total brood and duckling production were calculated. A correlation between first observations of broods and time of arrival of spring indicated that the Oldsquaw breeds as early as possible, an adaptation to the utilization of ephemeral lakes. Peaks in duckling production correlated well with peaks in the abundance of small rodents but not as well with climatic variables. Twice as many ducklings were produced during the rodent peak years as during the intervening years, and the densities of the Oldsquaw population were highest the year after these peaks in brood production. The positive correlation between duckling production and population size in small rodents seems to be analogous to grouse–rodent cycles, and both are best explained by the predator-shift or the alternative prey hypothesis.

During a six-year period, changes in the composition of dominant plant species of importance to foraging birds in a salt marsh on the Swedish west coast were followed inside and outside exclosures to document effects of grazing on herbage quality and seed production. Since marshes provide an important habitat for foraging geese and ducks, it was of interest to determine how cattle grazing would affect herbage production in Agrostis stolonifera and Puccinellia maritima and seed and root-tuber production in Scirpus maritimus. Measurements of cover and height in permanent plots revealed that a wetter weather type favoured Agrostis, probably through reduced salinity, at the expense of Puccinellia, which was the most favoured food of both cattle and birds. Agrostis out-competed Puccinellia when grazing pressure was low. Seed production in Scirpus maritimus was reduced by cattle grazing, particularly when Phragmites australis formed part of the vegetation. In the absence of cattle grazing, both herbage-and seed producing plants were gradually reduced, and Phragmites increased.Since high herbage consumption and high seed production are mutually exclusive, grazing rotation in combination with mowing is suggested as a management strategy.