The rusty blackbird (Euphagus carolinus) has received much attention during the last decade, in part due to drastic population declines. We analyzed data from 15 y of fall migration monitoring at the Observatoire d'oiseaux de Tadoussac (OOT), located at the mouth of the Saguenay River on the north shore of the St. Lawrence River estuary in Quebec, Canada. The trend observed suggest an ongoing decline. Numbers of rusty blackbirds varied considerably between years, with peak movements occurring at 5-y intervals and possibly reflecting high reproductive success for those years. The numbers of adult boreal owls caught and banded at the OOT, and the proportion of juveniles, were negatively and positively correlated with rusty blackbird numbers, respectively. Peaks in rusty blackbird abundance occurred when red-backed voles (Clethrionomys gapperi) were abundant in the eastern boreal forest. Also, rusty blackbird numbers were positively correlated to the annual and winter North Atlantic Oscillation (NAO) indices and negatively to the combined precipitation for June, July, and August, suggesting that environmental factors may have contributed both directly and indirectly (through food web processes) to the cyclic variations in abundance observed. Current declines may be exacerbated by NAO fluctuation patterns, that is, more frequent negative indices may negatively affect reproductive success and possibly winter survival.
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