I took 2 female and 2 male cowbirds out of the mix so far this year.
If other's aren't aware, cowbirds are parasitic breeders & lay their eggs in other nests. Most often the nest parents can't discern which is their own young and end up feeding the cowbirds, in which they grow larger and usually kill the other chicks in the nest by either attacking them or being aggressive in feeding and causing the others to become too weak to survive.
While it is distressing to watch a little wren trying to feed a baby cowbird three times her size, at the expense of her own brood, they are also part of nature’s plan. Man’s intervention, regardless of how well-intentioned, rarely produces positive outcomes.
The results are positive on the birds in my area, personal feelings may vary, but I don't see how nature has a plan.... these birds act more like leaches on the system that works around them.