Piecing together the epic transoceanic migration of the Long-tailed Cuckoo (Eudynamys taitensis): An analysis of museum and sighting records

Long-tailed Cuckoos (Eudynamys taitensis) breed in New Zealand and winter in an arc of small Pacific islands extending 11000km from west to east. To understand this migration better, we compiled records of Cuckoos in time (month) and space (latitude, longitude), using museum voucher specimens and literature records. At the start of the breeding season (October-December) virtually all birds in New Zealand are in adult plumage; non-adult Cuckoos are almost entirely absent until late summer and autumn. In the northward post-breeding migration, adults move first and non-adults show a lag of up to 3 months in the timing of their departure. Non-adults are scarce after June in the Pacific wintering islands, and transitionals (with plumage between that of adults and non-adults) are very scarce after September, suggesting that they quickly moult into adult plumage. From June to October the average longitude of Cuckoos wintering north of New Zealand decreases progressively, consistent with a movement westward. This suggests that a portion of the Cuckoo population may undertake an anticlockwise loop-migration. This may also partly explain the unusual distribution of the species fanning out from the breeding to the wintering grounds.