Jail Birds

So I am in the process of going through all the photos I took in Texas and I almost forgot about this. We visited the Kerr Wildlife Management Area to see the Black-capped Vireos. While there we noticed these trailer-cages. Unfortunately, you cannot see the 'inmates' in this photo, but they are all Brown-headed Cowbirds. We suspect that since the Black-capped Vireos are a threatened species, these brood parasites are captured here during the breeding season so as not to interfere with the vireos. I wonder what they feed them and if they simply release them at the end of the vireo's breeding cycle.