As their eggs become ready to lay, female buffalo birds scout around for a place to lay them in already feathered nests containing newly laid eggs — always of another species, because buffalo birds don't make nests. The bird people call such birds brood parasites. Buffalo bird nestlings need to eat most of their weight every day. The logistics of depending on beater buffalo for food makes rearing a brood a chancy business. So traveling with the herd and being a brood parasite go together nicely. Somewhere else in all that DNA are instructions for finding bison and using them as beaters.