Here is a link to SORA where one paper was published from this study area looking at cowbirds in New Mexico. It is interesting because it looks at communting distance of cowbirds from feeding areas to parasitizing areas. The one paper I was thinking of where they discuss parastism rates across species was published in the Journal of Wildlife Management and isn't accessible to general public (that I know of).

http://elibrary.unm.edu/sora/Auk/v117n03/p00795-p00799.pdf

Jeff Kozma

Yakima