Cowbirds adopted a strategy of laying their eggs in the nests of other birds so they could be free to follow wandering buffalo herds. As a result, they became referred to as nomadic parasites. Of course, they followed the buffalo, because the large bison herds stirred up lots of insects for these birds to feed on. So eventually, the cowbird worked out a strategy of laying its eggs for other birds to raise, enabling it to follow the bison and not go hungry.
Nature has endowed the cowbird with wondrous characteristics. Because its eggs require a short incubation period, they hatch in only 11 or 12 days, which is a day or two sooner than most bird eggs. But the trick is, the cowbird has to add her eggs to the nest at the same time the would be foster parent is laying her eggs. So female cowbirds watch other birds from high perches, to determine if its egg laying time. Then, the cowbirds flies in and deposits her egg into the nest.
Because the cowbirds egg is general in appearance, it matches the eggs of many host birds, like warblers, sparrows, finches, cardinals, and others. And although the cowbird is larger than most host birds it uses, her eggs are relatively small, and this also is a factor in why the cowbird is successful.
Cowbird chicks grow quickly because they gobble up most of the food the foster parents bring, resulting in the parents own young to perish, either from starvation, or being pushed from the nest.
When the Pilgrims arrived in Plymouth Rock, they also brought with them some of their cattle. The cowbirds found the cattle a great substitute for the bison, and as a result, the range of cowbirds expanded. This move provided the cowbird with many more hosts. Hosts that were poor at detecting cowbird eggs in their nests.
The eastern bluebird often raises cowbirds, at the expense of their own young, while the western and mountain bluebird, because of their long association with cowbirds, had more time to evolve anti-cowbird tactics.
Fragmented forestation, because of growing cities and towns, has increased the forest's edge, where cowbirds thrive. Its easy for cowbirds to detect nests in that kind of situation, and that has been devastating to some bird populations, such as wood thrushes, but a boon to cowbird populations. So the expansion of their range, plus the cowbirds finding even more suckers to raise its young, has been the result. Since only large forested areas provide refuge against cowbirds, their numbers are going to increase even more, unless something is done.