Birds as bioindicators of river pollution and beyond: specific and general lessons from an apex predator

Birds can be impacted by pollution but are seldom used as bioindicators. One exception involves the Dippers Cinclus spp., a genus of five passerines adapted uniquely to swim and dive in rivers on five continents to feed on aquatic invertebrates and small fishes. Here, we review the effectiveness of Dippers as pollution indicators while identifying further opportunities, caveats and uncertainties that are transferable to other indicator organisms. Dippers have been used as biodindicators i) through relationships linking their distribution, breeding performance and behaviour to river pollution through effects on prey quality and quantity; ii) where contaminants occur in their eggs, tissues, faeces or regurgitates, notably metals (Hg, Se), persistent pollutants (e.g. PCBs, PBDEs, DDE, HEOD) and microplastics. Most data are from C. cinclus in Europe and C. mexicanus in North America. While some pollution effects on Dipper distribution or fitness are well-evidenced, particularly acidification, the resulting impairments are not sufficient to diagnose the source of impact without additional data on water quality or prey abundance. Dippers in these cases provide a general rather than definitive indication of pollution. For contaminants, Dippers have revealed the distribution of specific pollutants at scales ranging from point-sources and regions to different continents. Influences of land use, trophic pathways, diet-shifts, contaminant transport, intergenerational transfer and trends through time have all been identified and supported by detailed knowledge of prey use, territoriality, dispersal, migration, life history, isotopic signatures and energetics. We suggest opportunities to expand the role of Dippers as bioindicators into other locations (Asia and South America), other influences on water quality (e.g. agriculture, wastewater), other contaminants (e.g. PFAs, pharmaceuticals) and through developments in modern biology such as 'omics. Initial data also show that Dippers could integrate the effects on rivers of habitat modification, flow modification and climate change by indicating effects both directly and through interactions with other multiple stressors. This group of birds illustrates how fundamental ecological information aids the development of bioindicators but reveals the importance of using complementary environmental data when diagnosing bioindicator response. We suggest these are important lessons for ecological indicators more generally.