Just like brown-headed cowbirds (Molothrus ater), common cuckoos (Cuculus canorus) in Eurasia are obligate brood parasites that never raise their young. Cuckoos lay their eggs in the nests of smaller birds that foster the cuckoo chicks as their own. A stark difference between cowbirds and cuckoos, though, is their size. Cuckoo chicks can be 10 times larger than their hosts!
The cowbird is slightly larger than its host but the cuckoo chick when it leaves the nest will be 10 times the reed warbler’s weight.
This situation can be even more bleak, as shown in a Twitter post by John Deakins.
So if you think cowbirds are bad, consider common cuckoos.
p.s. I asked folks to tell me the identity of the foster parent standing on the chick’s back. Janet Campagna suggests meadow pipit — which looks right to me. Meadow pipits are one of three species most often parasitized by common cuckoos.
(photos from Wikimedia Commons plus an embedded photo from Twitter; click on the captions/tweet to see the originals)