How does brood parasitism work in cuckoos?

Brood parasitism in cuckoos is a specialized reproductive strategy where females lay their eggs in the nests of other bird species, relying on the hosts to incubate and rear their young. This behavior involves several sophisticated adaptations:12

• Egg Mimicry: To avoid rejection by hosts, female cuckoos often produce eggs that closely resemble those of their host species in color and pattern. This is facilitated by the existence of "gentes"—clans of females that specialize in parasitizing a specific host species and have evolved matching egg characteristics.12
• Strategic Placement: Females monitor host nests and typically deposit their eggs after the host has begun laying but before incubation starts. They often remove or destroy one of the host's eggs to maintain the clutch size and reduce competition.1
• Nest-mate Eviction: In many species, the newly hatched cuckoo possesses a specialized depression between its shoulder blades. Within hours of hatching, it instinctively heaves the host's eggs or nestlings out of the nest, ensuring it receives all the food and attention from the foster parents.1

This process is a dynamic example of coevolution, as hosts develop defenses like egg discrimination, forcing cuckoos to adapt or shift to new host species.2