Wildlife managers and farm program administrators need information on how much habitat grassland birds need to support or expand their populations. We quantified the relationships between the amount of Conservation Reserve Program (CRP) habitat in 15 agricultural landscapes and relative abundance of ring-necked pheasants (Phasianus colchicus), gray partridge (Perdix perdix), and meadowlarks (Sturnella spp.) in south-central Minnesota, USA, over a 10-year CRP enrollment cycle. For each 10% increase of grass in the landscape, pheasant survey counts increased by an average of 12.4 birds per route in spring and by 32.9 birds per route in summer. Pheasant indices also varied by year, and the magnitude of year effects were equivalent to a change in grass abundance of 26–36%. Regardless of the amount of grass habitat available, partridge indices in our study declined dramatically from a peak in 1990 to a low in 1994–1995. Meadowlark indices increased by an average of 11.7 birds per route in summer for each 10% increase of grass in the landscape, while indices simultaneously declined from 1990 to 1998. Our results indicate that conversion of cropland to CRP grassland in intensively cultivated landscapes is associated with higher population indices of pheasants and meadowlarks, but not partridge. Managers should assess the success of habitat programs over periods of ≥5 years because population indices may fluctuate dramatically over time with little apparent change in habitat abundance.