Trace metals, including mercury (Hg) and lead (Pb), were reported to be immunotoxic, however, but relatively little is known about their effects and modes of action for avian species, especially waterfowl, which are frequently linked to the transmission of avian influenza infections. The objective of this thesis is to investigate whether the realistic environmental levels of Hg(II) and Pb(II) exposure could modulate the immune responses in waterfowl upon a viral-like immune challenge and to explore the underlying modes of action. A stepwise approach from field study with Arctic Barnacle goslings to controlled in vivo study with ducklings and then to in vitro studies with two chicken cell lines was applied. This approach enabled us to gain insights into both the environmental relevant effects and the modes of action at the cellular level. Our results suggested that trace metals modulated the immune responses mainly via inducing inflammation and impairing B-cell functions.