A number of wildlife species are potentially at greater risk of elevated mercury exposures, and development of a monitoring network for mercury in wildlife must take into account numerous variables that can affect exposures (and potentially effects). Because they are generally at the receiving end of the mercury cycle (following releases of inorganic mercury, atmospheric and aquatic cycling and bioaccumulation), numerous factors upstream can affect the amount of mercury available for uptake. As is the case with aquatic biota, methylmercury is of particular concern due to its ability to accumulate to greater extents in wildlife. A number of factors can affect methylmercury uptake in wildlife, including diet (including seasonal or inter-annual variations) and functional niche, location (including consideration of exposure differences for migratory species), age, sex, reproductive status, nutritive status, and disease incidence. In identifying potentially good indicator species for mercury exposure, desirable characteristics include a well-described life history, relatively common and widespread distribution, capacity to accumulate mercury in a predictable fashion (including sensitive to changes in mercury levels, and ideally occurring across a gradient of contaminant levels), easily sampled and adequate population size, and having data on natural physiological variability. Sample collection for mercury analysis must consider methodological factors such as live (e.g., feathers, hair/fur, blood) vs. dead (e.g. internal organs) specimens, time of exposure in relation to tissue sampled (e.g. more recent exposures in blood or eggs vs. longer-term exposures in kidney, fur, or feathers), site of the collection within tissue, potential for and extent of detoxification/depuration, differences within clutches, feathers, or hair locations in birds, and potential for exogenous contamination. In addition to consideration of mercury exposures in developing a monitoring network, effects of mercury could also be considered, including assessments across several levels of biological organization. While several endpoints of mercury toxicity have been identified in wildlife (including growth, reproduction, and neurological), solid biomarkers of mercury effect meeting desirable criteria have to date not been identified. Based on research to date on numerous wildlife species and consideration of indicator criteria identified here, candidate wildlife species for bioindicators of mercury exposure, by habitat type, include the following:

terrestrial — Bicknell’s thrush and raccoon; lake — common loon; freshwater wetland — tree swallow; lake/coastal — herring gull, bald eagle and common tern; riverine — mink; estuarine — saltmarsh sharp-tailed and seaside sparrows; nearshore marine — harbor porpoise; offshore marine — Leach’s storm petrel; comparison across aquatic habitats — belted kingfisher. It is recommended that monitoring be done annually, considering time after arrival at breeding site for migratory species. Several medium-to long-term monitoring efforts have been conducted for mercury in wildlife (including for egrets and herring gulls). However, clear consideration of the numerous factors affecting mercury uptake and mobilization within individuals, intra-and interspecies variability, and resulting statistical issues must be taken into account in designing a monitoring network that can adequately address questions on spatial and temporal trends of mercury exposure (and potentially effects) in wildlife.