Data from: From parasitism to mutualism: unexpected interactions between a cuckoo and its host

Avian brood parasites lay eggs in the nests of other birds, which raise
the unrelated chicks and typically suffer partial or complete loss of
their own brood. However, carrion crows Corvus corone corone can benefit
from parasitism by the great spotted cuckoo Clamator glandarius.
Parasitized nests have lower rates of predation-induced failure due to
production of a repellent secretion by cuckoo chicks, but among nests that
are successful, those with cuckoo chicks fledge fewer crows. The outcome
of these counterbalancing effects fluctuates between parasitism and
mutualism each season, depending on the intensity of predation pressure.