The effects of experimental reservoir creation on the bioaccumulation of methylmercury and reproductive success of tree swallows (Tachycineta bicolor)

Reservoir creation results in decomposition of flooded organic matter and i ncreased rates of mercury methylation. Methylmercury (MeHg), the most toxic form of mercury, bioaccumulates through aquatic food webs. Our objective w as to quantify the transfer of MeHg from aquatic food webs into terrestrial organisms. We examined rates of MeHg bioaccumulation in an insectivorous s ongbird, the tree swallow, breeding near an experimentally created reservoi r. We also determined the impact of flooding and MeHg bioaccumulation on th e reproductive success of these birds. Mean MeHg burdens in nestling swallo ws from near the experimental reservoir increased from 1210 +/- 150 ng befo re flooding to 2200 +/- 102 ng after flooding. Postflood MeHg concentration s in both the body and feathers of the birds were significantly greater tha n preflood MeHg concentrations. Although MeHg burdens in swallows were elev ated in postflood years, we found no overt toxicological affects. An increa se in dipteran productivity (the primary food source of tree swallows) afte r reservoir creation resulted in earlier nest initiation, larger eggs, and faster growth rates of wing and bill length in nestlings raised during post flood years.