Diving Duck Distribution, Abundance, and Food Habits in Chesapeake Bay
MATTHEW C. PERRY, PETER C. OSENTON, and EDWARD J. R. LOHNES
USGS Patuxent Wildlife Research Center
11410 American Holly Drive
Laurel, MD 20708, USA
Diving ducks wintering in Chesapeake Bay during the last 50 years have accounted for 23% of Atlantic Flyway and 9% of North American populations based on aerial surveys. Continental and local factors have affected these population changes. Loss of submerged aquatic vegetation (SAV) due to degradation of water quality, has been a contributing factor, although, many other factors related to human population increases have been implicated in the changes in the distribution and abundance of diving ducks.
The locations of diving ducks in Chesapeake Bay during the winter was determined by aerial surveys conducted by USFWS and state wildlife agencies. Surveys are conducted during mid-winter and ducks are identified and population size is estimated. Scaup (lesser and greater), and scoter (black, white-winged, and surf) are not separated by species during aerial surveys.
Present food habits were determined by analyses of ducks obtained from hunters or from law enforcement personnel. The gullet (esophagus and proventriculous) and gizzard were removed and maintained separately for analyses. Both organs were used in interpreting food choices from a diversity perspective, but because of the problem of bias associated with gizzard samples, the gullet samples received more attention in interpreting food choices from a quantitative perspective.
Organisms were separated by species, and dry weights and volumes were determined for each sample. Frequencies of occurrence, average weight, and average volume of the food items were determined for each duck species. Grit content was determined and calculated separately from food items. Historic food habits data for sea ducks on the Chesapeake Bay were reexamined and compared to present food habits.
The Chesapeake Bay has wintered approximately one million birds on average during the last 50 years that surveys have been conducted. Diving ducks have had a significant decline during this period and now the Bay winters approximately 200,000 diving ducks. Results of mid-winter surveys show a decline in all pochard species (canvasback, redheads, and scaup), ruddy duck, and goldeneye. Although there have been major changes during some periods the long term trends of seaducks (long-tailed duck and scoters) show that numbers in the 1990s are similar to the numbers of these species during the 1950s. Diving ducks that have shown increases in populations based on mid-winter survey data include the bufflehead and mergansers.
New food habits research is being conducted to further explain the changes in distribution of diving ducks in Chesapeake Bay that are related to habitat conditions. Some species of diving ducks have shown major long-term changes in the food eaten in Chesapeake Bay, whereas other species show no change in diet. The percentage of invertebrates in the diet of most pochards and ruddy ducks has increased as SAV declined. The Baltic clam (Macoma balthica) constitutes over 90% of the food of canvasbacks. Both species of scaup feed on hooked mussels (Ischadium recurvum), but also rely on clams and barnacles. The redhead did not alter food preference for SAV, and populations of this species have declined from over 100,000 to less than 1000.
The food habits of goldeneye show that hooked mussels and the Baltic clam are important foods. Bufflehead also feed on the Baltic clam, but also feed heavily on the dwarf surf clam (Mulinia lateralis). Seaducks rely heavily on mollusks especially the hooked mussel and several species of clams. The long-tailed duck appears to select the gem clam (Gemma gemma) at rates higher than other divers, but food habits of this species show a diverse diet of other mollusks and crustaceans.
Factors affecting diving duck populations include direct and indirect causes, including excessive development of Bay tributaries, increased year-round boat traffic, and increased levels of contamination. Food habits indicate major changes in some species of diving ducks especially the pochards, since SAV has declined. Other species of diving ducks, show little changes in food habits and still are feeding on mollusks species that have traditionally formed the bulk of their diets. Species, like the pochards, that feed in shallow water are probably more affected by environmental changes in the Bay than species that feed in deeper water.
Figure 2: Chesapeake Bay Food Habits Diving Ducks.