Weather and regional effects on winter counts of Rusty Blackbirds (Euphagus carolinus)

A long-term and severe population decline of Rusty Blackbirds (Euphagus carolinus) has motivated biologists to search for possible causes of the decline. Several hypotheses have been forwarded, one of which is that habitat destruction on the overwintering grounds is responsible. Climate change is another possible explanation. We evaluated the population trend of Rusty Blackbirds in Arkansas by modeling their abundance recorded during Christmas Bird Counts conducted between 1965 and 2020. We used generalized additive modeling to evaluate population trends and explored the influence of weather, effort, habitat, and region on those trends. We found that counts of Rusty Blackbirds have increased by about 40 birds in Arkansas between 1965 and 2020; most of the increase occurred after 1995. We also found that proportion of forest land in each count circle’s county was inversely related to counts of Rusty Blackbirds but that temperature was a more important variable. During warmer years, fewer Rusty Blackbirds were counted. Rusty Blackbird geographic distribution also changed by decade; that change accounted for about 15% of the deviance in counts of Rusty Blackbirds. Finally, we observed a relationship between temperature and distribution; Rusty Blackbirds tended to overwinter in the northern portions of the state during warm years and more southerly portions of the state during cold years. Our analytical approach will be useful to anyone evaluating geographic shifts in populations that might be associated with climate change.

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