set. The hosts of common cuckoos almost never reject cuckoo chicks—even though they look nothing like host young. Even a few weeks after hatching, when a cuckoo chick is more developed, the host birds are strangely incapable of identifying the little monsters that have hijacked their nest, and will care for “Rosemary’s baby” until it fledges. 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
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