Migration of the long-tailed cuckoo

Brian Gill, Auckland Museum’s Curator of Land Vertebrates, is reassembling data on the long-tailed cuckoo’s migration, using data from specimens from museums throughout the world and literature records.

Long-tailed cuckoos (Eudynamys taitensis) breed only in New Zealand, parasitising three song-birds (whitehead, yellowhead and brown creeper). After the most remarkable overwater migration of any land bird, they winter in a vast arc of Pacific islands extending 10,000 km from Palau in the west to Henderson Island (Pitcairn group) in the east. 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.

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 at the start of the breeding season (October-December) 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 were produced in the current summer and have never left the country. A pale juvenile plumage, not previously noted, has also been identified.

Museum study-skins of long-tailed cuckoos showing the white underparts and thick streaks of adults (top) and the buff underparts with thinner streaks of immatures (bottom)

A typical immature cuckoo (top) and a bird in the newly-identified pale juvenile plumage (bottom). The birds presumably fledge in the juvenile plumage and moult to the immature plumage before migrating to the Pacific