Like coyotes, cowbirds have spread beyond their original range and, like coyotes, they aren't always welcome. Cowbirds used to follow bison, consuming insects the herds stirred up in their grazing. Now they follow cows.
Like cuckoos, the females don't make their own nests; instead they find a nest that already contains eggs and lay their eggs (as many as seven) there. That's the end of motherhood for Ms. Cowbird -- she has a herd to follow. But she will do this again and again and may produce more that three dozen eggs in a summer.
The original nest builder does the incubating. The cowbird eggs usually have a shorter incubation time and hatch first. The hatchlings sometimes roll the other eggs out of the nest and snag all the food the poor host parents can provide.
Fascinating creatures -- there's more about them HERE.