Long-term decline and short-term crash of the once abundant Rusty Blackbird

Abstract

The Rusty Blackbird (Euphagus carolinus), a formerly common breeding species of boreal wetlands, has exhibited the most marked decline of any North American landbird. North American Breeding Bird Survey (BBS) trends in abundance are estimated to be -12.5% / yr over the last 40 years, which is tantamount to a >95% cumulative decline. Trends in abundance calculated from Christmas Bird Counts (CBC) for a similar period indicate a range-wide decline of -5.6% / yr. Qualitative analyses of ornithological accounts suggest the species has been declining for over a century before the period covered by the estimated declines. Several studies document range retraction in the southern boreal forest, whereas limited data suggest that abundance may be more stable in more northerly areas. This pattern is both supported and contradicted by winter declines based in CBC data. The lower estimates of decline in the CBC data compared to BBS is consistent with the idea that the coverage of BBS is biased towards the southern boreal whereas CBC covers the entire winter range. However, the CBC declines are similar between the South Atlantic coast (with populations derived from the southeastern boreal) and the Mississippi Valley (populations from the northwest boreal). The major hypotheses for the decline include degradation of boreal habitats from logging and agricultural agricultural development, mercury contamination, and wetland desiccation resulting from global warming. Other likely reasons for decline include loss or degradation of wooded wetlands of the southeastern U.S and mortality associated with abatement efforts targeting nuisance blackbirds. We present a matrix of hypotheses and predictions that test them based on the geography of decline and more detailed indicators of population health which should form the strategic basis of future research.