Lots of birds compete for nests, evicting any other birds they find (if they can).
Brown-headed cowbirds are different. They don't kick out nesting birds, they (actually just the female, you'll see why) sneak into the nest and lay one of their eggs right on top of the eggs that are already there. When it hatches, the cowbird chick will be fed by its unwitting foster parents. (It will also evict their eggs or chicks, to eliminate competition.)
You'll often see a tiny adult finch or sparrow feeding a much larger baby cowbird. (The cowbird will keep following the adult around, begging for food, for several days after it leaves the nest.)
Example:
https://www.birdforum.net/opus/Image:Brown-headedCowbirdDSCN9172.jpg