In contrast to the thoroughly studied incubation and nestling periods, the postfledging period of the Common Cuckoo (Cuculus canorus) remains virtually unknown. Here, we report detailed observations of a cuckoo fledgling attended by a male Chaffinch (Fringilla coelebs). Molecular data (nuclear DNA) showed the fledgling was a male belonging to the Cuculus c. canorus/C. saturatus clade while mitochondrial DNA data confirmed that it did not belong to blue egg gens that parasitizes Redstarts (Phoenicurus phoenicurus), which is the most common local host and the only regular Common Cuckoo host. During one week of observations, feeding rates did not change, body mass decreased (by 10%), and wing length increased (by 16%). Video recordings showed that the provided diet consisted mostly of larvae and that the fledgling also self-fed on lichens. A radio transmitter fitted on the fledgling revealed that daily movement distances ranged from 0 to 650 m and significantly increased with age. We suggest that future studies should focus on the postfledging period in brood parasite young because this stage currently represents a major gap in our understanding of parasite–host arms races.

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