Recently, while setting up mist nets in a marsh, I found this old reed warbler's nest and took it home.
Reed warblers are small European migrants, who spend the winter in Africa, south of the Sahara, and return to Europe to breed in the spring.
Reed warblers are one of the species often parasitized by European cuckoos. Not all cuckoos do, but European cuckoos reproduce by way of brood parasitism, that is, they have a different species raising their young. Female cuckoos within the same population specialize in parasitizing different species, learning to mimic the eggs of the host species. Host species do watch out and if they spot a cuckoo near their nest, or the intruding egg, they will abandon the nest. Often, however, they incubate the cuckoo's egg.
As soon as it hatches, the young Cuckoo instinctively ejects any solid object it finds in the nest, using a small depression on its back.
From now on, it will receive all the care from its foster parents. And it's really necessary, as the cuckoo quickly grows bigger then its parents, who have to work extremely hard to raise it!