A factoid

Blackbirds, I have discovered, will not feed cuckoo chicks. Researchers discovered this by putting cuckoo chicks in blackbird nests. The chick turfed out the blackbird eggs, but the parents just would not feed it. Song thrushes would, but cuckoo chicks could not remove eggs from song thrush nests as they are too deep. Neither species is parasitised by cuckoos. But the researchers don't know why blackbirds respond in the way they do. Maybe they are former hosts who learned to their cost the pitfalls of parental generosity.

In Australia, where cuckoos have evolved with their hosts for far longer than here, cuckoo chicks look like the chicks of the host species. That is not the case with our cuckoos, but the hosts still feed them.

In Japan, a certain magpie, the azure-winged magpie, is becoming a new host and in some places all the nests had cuckoo chicks in them. The magpies will evolve to notice the different eggs and the cuckoos will evolve to mimic them.

Magpies, for their part, will happily eat other birds' eggs... and chicks too. The small birds might rate them as a worse threat than the cuckoos!