There’s a theoretical model suggesting why this is so. Bird parents “imprint” on the chicks that hatch from the eggs in their very first clutch, and after that, reject any chick that’s different. Host parents that are unlucky enough to be parasitized in their first clutch will imprint on the cuckoo chick and rebuff their own young forever after. The high cost of mistakenly imprinting on a cuckoo nestling should prevent host birds from evolving the ability to reject chicks.
