Farmer and pheasant-hunt host David Gillen opines in the Pierre Capital Journal that the Conservation Reserve Program is bad for pheasants and South Dakota wealth:

Full 160-acre tracts of CRP are not that desirable for young chicks. After a couple years of CRP that is not cut for hay, the amount of plant growth present is too thick for the young chicks to move through. If we get a cold, wet rain or heavy dew when the chicks are small, they get too cold from all the wet plant material around them. The hen will try to move them to an open area for sunshine, but in the large CRP fields, sometimes it is too far to an open area to get warmed up from the sun.

...Pheasant hunting and profitable agriculture creates new wealth in this state for all of us to prosper. CRP destroys wealth by taking tax dollars from people that generate income and pays landowners to idle land that could create new wealth through agriculture [David Gillen, "What You Should Know About CRP: A Farmer's Perspective," Pierre Capital Journal, 2013.12.23].

Gillen serves as a manager at Prairie Ethanol, and he's been president of the South Dakota Corn Growers Association and a board member of the American Coalition for Ethanol. Gillen defends an industry that has done enormous harm to the prairie with its voracious push for ditch-to-ditch, stream-to-stream cultivation of every acre a tractor can reach.

But let's counter Gillen's industry-boosting anecdote with scientists and research. CRP means more pheasants:

More than 800,000 ha of Iowa farmland were enrolled in the Conservation Reserve Program (CRP) from 1986–1991. I evaluated the relationship between CRP enrollment and ring-necked pheasants (Phasianus colchicus) in Iowa and how cropland and weather affected that relationship. Six percent of the land area in Iowa was enrolled in the CRP between 1986 and 1991. Pheasant numbers increased 30% during the first 5 years of the CRP compared to a similar period before the program began (P = 0.026). Numbers increased 34% (P = 0.018) in counties with >70% cropland and 26% (P = 0.12) in counties with 50–70% cropland. I did not detect increases in counties with <50% cropland (P = 0.71). Pheasant numbers were positively related to the CRP, but this function was also influenced by percent cropland and cumulative snowfall [Terry Z. Riley, "Association of the Conservation Reserve Program with Ring-Necked Pheasant Survey Counts in Iowa," Wildlife Society Bulletin, Vol. 23, No. 3 (Autumn, 1995), pp. 386-390].

CRP means more than  just pheasants for Gillen's customers to pay to shoot at:

"Goodness, there's thousands of species that live in grasslands, including several hundred species of higher plants," says Carter Johnson, an ecologist at South Dakota State University in Brookings, S.D. Plus, permanent grass cover keeps soil from washing away.

"With those deeps roots that grasses have, and thick thatch, the water has a hard time getting a hold of the soil," says Johnson.

So more land in CRP means cleaner streams, less fertilizer runoff and more carbon stored in the soil [Dan Charles, "Grasslands Get Squeezed as Another 1.6 Million Acres Go into Crops," NPR, 2013.12.22].

Additionally:

In the central USA, more than 90 bird species were documented using CRP plantings during summer breeding periods, with direct evidence of nesting by more than 40 species. Direct comparisons of crop and CRP fields revealed no differences in total number of species occurring in the two habitat types. But CRP fields supported from 1.4 to 10.5 times the number of individual birds as did crop fields. Nest abundance was from 8.8 to 27 times higher in CRP fields than in crop fields. Nest success of songbirds was slightly higher in CRP fields (40%) than in crop fields (36%). Overall, CRP fields produced about 14 times as many songbirds as did rowcrop fields. Several assessments suggest nongame bird populations increased because of the CRP. Duck nesting in cropfields is uncommon. Duck nest densities in CRP fields were similar to those occurring in habitats managed for waterfowl. Duck nest success in CRP fields was equal to or higher than that on lands managed by public agencies for waterfowl. Ring-necked pheasant numbers were three to five times higher after CRP plantings were established. The success of pheasants nesting in CRP fields was greater than that necessary for population growth [Mark R. Ryan, Loren W. Burger and Eric W. Kurzejeski, "The Impact of CRP on Avian Wildlife: A Review," Journal of Production Agriculture, 1998. 11:61–66.].

Gillen's anecdote sounds plausible—wide tracts with dense ground cover may not be ideal for pheasants in wet conditions—but the research seems pretty clear that CRP acres increase bird numbers.