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Based on North American Breeding Bird Survey (BBS) data since 1966, Ohio has traditionally hosted 1 of the highest breeding season densities of red-winged blackbirds (Agelaius phoeniceus) of any U.S. state or Canadian province. However, from 1966 through 1996, breeding populations of red-winged blackbirds in Ohio showed a marked decline (x̅ % change/yr in birds per route = -3.9), with breeding population indices decreasing by over 53%. Because the red-winged blackbird successfully adapted to habitats created by agricultural expansion over the last century and became a recognized pest of crops such as corn (Zea mays), understanding the decline of this species in Ohio is important from both ecological and damage control perspectives. We examined 35 crop and climatic factors relative to their relationship with the observed breeding population trend for the red-winged blackbird in Ohio 1966 to 1996. Each year, we found that the area of non-alfalfa (Medicago sativa) hay harvested, the combined area of corn and soybeans (Glycine max) harvested, the area of non-alfalfa hay cut by 30 May of the index year (1966-1996), and the area of hay (all types) cut by 30 May of the year prior to the index best explained the variance in the breeding population trend of the red-winged blackbird in Ohio. Given our findings, we suggest that a long-term population trend for this abundant bird in Ohio is negatively associated with the efficiency and expansion of modern agriculture.