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Uncovering the mysteries of the long-tailed cuckoo

by Associate Emeritus Dr Brian Gill

Little is known about the native long-tailed cuckoo (koekoea, Eudynamys taitensis), a migratory bird which breeds in New Zealand in summertime. To help uncover some of the mysteries around this bird, Associate Emeritus Dr Brian Gill has investigated its diet. Dr Gill's work was done in Auckland Museum’s Land Vertebrates Department and is the first detailed study of this cuckoo's diet. Dr Gill tells of his discoveries here. 

Two species of cuckoos migrate to breed in New Zealand during our summer.  They spend the winter in tropical Pacific islands to the north. There is not much known about the birds as they inhabit New Zealand’s tall, dense forest so they are hard to see and find and can be difficult to study. 

Cuckoos have several remarkable habits. One is brood-parasitic breeding, which means the cuckoos do not build a nest or rear their own young. Instead, the females lay their eggs individually in the nests of other species, who become the foster-parents, and take no further part in the process. Long-distance migration is another spectacular attribute of many cuckoos. In my recent book (The Unburnt Egg – More Stories of a Museum Curator) I give more details of migration and breeding in a chapter devoted to the long-tailed cuckoo. I describe how I found a whole egg in the oviduct (the tube through which the egg passes from the ovary) of one dead cuckoo. Eggs of the long-tailed cuckoo are almost as rare in the museums of the world as hens’ teeth. 

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Image credit (right): These two immature long-tailed cuckoos were found dead by members of the public. In this image they are yet to be dissected. 

Long-tailed cuckoos breed in New Zealand in the summer and spend winter in a huge arc of South Pacific islands. Image credit: Milvia Romici

A third intriguing feature of cuckoos is their unusual diet. Museums typically rely on members of the public to bring in dead birds that have crashed into windows, been struck by cars or killed by cats. During 32 years as Auckland Museum’s curator of land vertebrates, I received 50–100 dead birds every year. I made a special study of long-tailed cuckoos whose average rate of receipt was just over two birds per year. I took measurements, made notes on the plumage, sexed the birds by dissection and preserved their stomachs in alcohol. Just recently I have had the time to open the stomachs and identify and count the insects that the cuckoos ate. This recent research offers a small contribution to understanding our native animals and how they live.

In total, I examined 62 gizzards containing food and identified 888 food items, 94% of which were insects. It turns out that long-tailed cuckoos eat a narrow range of insects however many of their insects of choice are large. The main foods were cicadas and shield-bugs (48% of items), stick-insects (19%) and katydids and wetas (13%).  Cicadas, stick-insects and praying mantids are large insects that reach peak abundance in late summer. These items made up 57% of the diet of immature cuckoos but only 13% of the adult diet. The immature bird leaves New Zealand on migration several months after adults and this delay during late summer and autumn gives them an opportunity to exploit a seasonal abundance of large insects immediately before departure. 

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Image credit (left): Head of a weta, which would have been about 32 mm long, from the stomach of a long-tailed cuckoo.

Adult long-tailed cuckoos (upper pair) have different plumage (feathers) from immature birds (lower pair).

Long-tailed cuckoos ate a few lizards and birds’ eggs and nestlings and I found these in 13% of stomachs, making up 1% of total food items. These cuckoos are big enough, about 125 g, similar to the weight of a common myna, and with a strong enough beak, about 25 mm long, that they can prey on the contents of the nests of any small birds that they may come across. DNA from small chewed up nestlings in the cuckoos’ stomachs was isolated and identified by a team at Auckland University. This new information added three species to the previous list of 10 species of birds, both native and introduced, that long-tailed cuckoos are known to eat through nest predation.

Cuckoos are hugely fascinating and it has been a pleasure and privilege for me to use Auckland Museum's collection to uncover some of the cuckoos’ secrets.

Image credit (right): A low-magnification stereoscopic dissecting microscope was used to identify and count chewed insects from the cuckoo stomachs.

Brian Gill has two books of museum stories (The Owl that Fell from the Sky and The Unburnt Egg, Awa Press). They focus on natural history specimens he came to know during his curatorial career and bring in biology, background history and details of people and personalities. The books are available in Auckland Museum's store or from www.awapress.com (free postage).