How do host birds defend their nests against cuckoo eggs?

Host birds have evolved several behavioral defenses to protect their nests against brood parasites like cuckoos. When a host bird detects a foreign egg in its nest, it may respond in one of the following ways:

• Desertion: The host may abandon the nest entirely to avoid raising the parasite's young.1
• Ejection: Some species have developed the ability to discriminate between their own eggs and foreign ones, allowing them to remove the parasite's egg from the nest.12
• Covering: Hosts may build a new nest layer directly on top of the parasitized one, effectively burying the foreign egg.13

These defensive behaviors exert evolutionary pressure on cuckoos, driving the development of egg mimicry, where the parasite produces eggs that closely resemble those of the host to avoid detection. In some cases, hosts that have not been recently targeted by cuckoos may lose these defensive traits over time, only to re-evolve them if they become targets again. Additionally, some hosts nest in dark holes, which can make it more difficult to distinguish between eggs, rendering the parasite's mimicry less critical.12