Re-examination of the epic transoceanic migration of the long-tailed cuckoo Eudynamys taitensis (Aves: Cuculidae)

Long-tailed cuckoos (Eudynamys taitensis; 125 g) breed only in New Zealand, parasitising three species of Mohoua (Pachycephalidae). After performing perhaps the most remarkable overwater migration of any land bird, they winter in a vast arc of Pacific islands extending 10,000 km from Palau (134.5°E) to Henderson Island (Pitcairn group; 128.3°W). Such an epic migration by so small a bird was originally doubted. After systematic collecting of birds on south Pacific islands by the Whitney South Sea Expedition (1920-32), a 1937 paper by Bogert established the bare details of the migration. This study aims to reassemble data on the long-tailed cuckoo’s migration, using specimens and literature records. The sexes are alike, but immatures (spotted back, rufous underparts) are readily distinguishable from adults (barred back, white underparts), allowing new analysis of migration patterns in relation to age. Preliminary results show that many birds in the wintering grounds have intermediate plumage (are presumably moulting from immatures to adults). A juvenile plumage, not previously noted, has also been identified. At the start of the breeding season (OctoberDecember) practically all birds in New Zealand are adults, and immatures in museum collections are overwhelmingly restricted to late summer and autumn. This establishes that all immatures in New Zealand are young-of-the-year. One of the three hosts (yellowhead M. ochrocephala) is now critically endangered. This must mean that the populations of cuckoos adapted to parasitising yellowheads are endangered or extinct now, in proportion to the decline of their host. Certain Pacific islands to which the yellowhead-cuckoos migrated may now lack cuckoos.