Answer :

(a) Parasitic bird lays resembling eggs in the nest of host bird Cuckoo (Koel) lay eggs in the nest of crow.

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Eggs of cuckoo (Koel) have evolved in time to resemble the eggs of the crow, koel lays eggs in the nest of the crow and lets them be hatched there, cuckoo is the parasitic bird here exhibiting brood parasitism.
(b) When evolution of one species is tightly linked with the evolution of other species.
Plant pollinator interaction / fig species and wasp / any other relevant example