Zebra mussels can be a valuable food source for waterfowl species (reviewed in Molloy et al. 1997). They are now the dominant food consumed by lesser scaup and Goldeneye in western Lake Erie and northern Lake St. Clair (Custer and Custer 1996).
Their consumption by migrating or overwintering birds is well-documented. Zebra mussels are the main food item for 310,000 overwintering diving ducks in Lake Ijsselmeer in the Netherlands (bij de Vaate 1991) and a frequent prey item for diving ducks in Lake Erie during their fall migration (Hamilton et al. 1994). Their importance for coot populations in Poland (Mikulski et al. 1975) and for tufted duck populations in Belgium (Draulans 1982) and the British Isles (Olney 1963) has also been documented.
Because of their importance as a prey item, dramatic increases in flock sizes can occur following zebra mussel colonization of a water body, as observed in Lake Erie (Wormington and Leach 1992), Germany (Ziegler 1987), and Switzerland (Leuzinger and Schuster 1970, Pedroli 1981b). Prior to the appearance of zebra mussels in Swiss lakes, for example, birds were feeding on aquatic macrophytes in the fall, and after plant dieback, these waterfowl migrated to the south. Now, however, thousands overwinter (Leuzinger and Schuster 1970).
The location of zebra mussel populations in Europe cannot only affect waterfowl distribution, but also the timing and routes of their migration. The geographical range of tufted duck in England has expanded due in part to the spread of zebra mussels (Olney 1963). Food abundance and availability, particularly of Dreissena, were viewed to be the main factors governing lake choice by overwintering diving ducks in Switzerland (Suter 1994). Soon after the arrival of zebra mussels in western Lake Constance in the late 1960’s, 45,000 tufted duck, pochard, and coot were observed overwintering; this represented a ten-to-fiftyfold increase over previous levels (Suter 1982c). Following the establishment of zebra mussels in western Lake Constance, goldeneye began to arrive at overwintering areas earlier than in previous years (Suter 1982c). Conversely, in areas where zebra mussel populations have declined yearly, diving birds show a tendency to leave overwintering areas sooner, likely due to less effective predation (Suter 1982b). The presence of zebra mussels in Lake Neuchâtel (Switzerland) has also modified the migratory phenology of tufted duck, pochard, and greater scaup (Pedroli 1981a).
Use of zebra mussels as prey items, however, can be disadvantageous. Narrow reliance on zebra mussels as a primary food source by pochard and tufted duck was a contributing factor leading to severe population crashes in the winter of 1986. With zebra mussel populations below average in some feeding grounds, one of the coldest European winters ever recorded led to the starvation deaths of thousands of these diving birds (Suter and van Eerden 1992). Because of their efficient filter feeding, zebra mussels can bioaccumulate pollutants, thus posing a potential threat to the health of their predators. In a study where ducks fed on zebra mussels contaminated with cadmium and organochlorine compounds, these toxic substances were carried over into their eggs with teratogenic effects (de Kock and Bowmer 1993).