Why does a female common cuckoo lay only one egg in each host nest?

The female common cuckoo (Cuculus canorus) practices brood parasitism, a strategy where she relies on other bird species to rear her young. She places only one egg in each host nest to ensure the survival of her offspring, as the young cuckoo typically engages in "nest-mate eviction" shortly after hatching.1

By removing the host's own eggs or nestlings, the young cuckoo eliminates competition for food and ensures it receives the full attention of its foster parents. Because the cuckoo must find a new nest for each of the 15 to 20 eggs she produces in a season, this behavior is essential to her reproductive success. Furthermore, because many hosts will reject foreign eggs that do not match their own, the female cuckoo—which belongs to a specific clan or gente—must carefully select a host species whose eggs resemble her own to minimize the risk of rejection.12