Well, with the cow bird living off the insects picked off of herds of bison, there wasn’t time to fight for a territory, build a nest, incubate and raise chicks before the food source sought greener prairies... so just dropping eggs in a couple local bird’s nests along the way did the trick. The likelihood of the chick’s survival was higher with string of hosts, as there are some species of birds which can recognize an egg that’s not theirs and push it out, while others just aren’t that bright. They practice the adage “Don’t put all your eggs in one basket.”
They seem to eat a variety of bugs; but they do stay out of forests and in the grasslands...