Why are mismatched cuckoo eggs rejected more often than well-matched eggs?

Mismatched cuckoo eggs are rejected more frequently than well-matched eggs because most host songbirds have evolved the ability to discriminate between their own eggs and foreign ones. When a host perceives an egg as foreign—often due to differences in color or spotting—it will typically abandon the nest, build over the existing nest, or eject the strange egg to protect its own reproductive investment.12

This defensive behavior is a result of coevolution between the cuckoo and its hosts. As hosts develop the ability to identify and remove eggs that deviate from their own, cuckoos are under evolutionary pressure to produce eggs that mimic the appearance of their specific host's eggs. Research indicates that cuckoo eggs placed in the nests of non-matching hosts face significantly higher loss rates (24 percent) compared to those placed in the nests of matching hosts (8 percent), demonstrating that mimicry is a critical adaptation for the survival of the brood parasite's offspring.12