Cuckoo-host evolutionary dynamics

A perfect example of selection pressures: Some host species are very good at recognizing cuckoo eggs. In other species, however, this is not the case. Why? Because there, the cuckoos actually check if their egg has been kicked out, and destroy all the other eggs if the hosts recognized the cuckoo eggs. So you have a selection pressure against being able to discriminate cuckoo eggs. (The second article even describes other species and their tactics, all of this via reddit)