Cowbird control is controversial. Scientists suggest there is little evidence that cowbirds have population-level impacts on hosts. Further, they point out that control has limited impact on reducing parasitism on a broad geographic scale. Traps must be located where cowbirds congregate, like in feeding areas, and trapping appears to reduce parasitism mostly near such trap locations. The farther hosts are from the traps, the less likely they will benefit from control.
Cowbird control is expensive.
Cowbird control programs have proceeded without a general framework, with little coordination between programs, or between the land management and scientific communities. There has been little attempt to regularly measure progress and financial costs since control programs began three decades ago. No research has tested the effectiveness of large-scale control on the wintering grounds, where cowbirds congregate in large roosts. In the Least Bell's Vireo, populations increased following cowbird control and efforts to improve breeding habitat. Scientists suggest that cowbird control is a short-term solution that ignores the real problem of habitat degradation as a result of agriculture, grazing and development.