Shining Cuckoo

Forest and Bird, Volume 20, Issue 3, 1 August 1989, Page 34

Shining Cuckoo

By

Brian Gill

é n November 1978 I experienced a stroke of luck the like of which comes rarely during fieldwork. It was hot and sunny, and as I walked through the dry kanuka forest in my study area at Kowhai Bush near Kaikoura, the air hummed with the stridulations of cicadas. Above the din I heard the distinctive whistling call of a shining cuckoo quite close at hand, and I went in pursuit. There is something about this cuckoo's call that makes its source very hard for the human ear to locate. It starts softly and gradually rises, so that its distance is hard to judge. Coupled with that the birds are remarkably cryptic when they perch in a tree, despite having such gaudy green colouring and boldly striped underparts. As Eileen Duggan put it in her 1929 poem The Pipiwharauroa (Shining Cuckoo): "And I burnt my eyes with gazing. Still I see the poplars shiver, Still I hear the little runnels down the folded gully falling, But I never saw the bird!" Well I persevered and was eventually rewarded with close views of two cuckoos. It's a privilege to watch such secretive and little-known birds, but to cap it off I thought I glimpsed a colour band on one bird's leg. Something to write home about? It is when you know how few shining cuckoos have ever been colour banded. I lost the cuckoos that day, but 10 days later I found them again in the same area. Cuckoos have short legs which are tucked well in when they perch. Also the feathering extends much further along the leg than in song-birds, so it is very difficult to see the legs clearly to check for bands. I persisted for half an hour. Luck was with me and I eventually approached the birds to within 5 metres — closer than the binoculars would focus! There was no doubt — one of the birds had a metal band on its left leg and one red plastic band on its right. This was B-40201, banded as a nestling by me exactly two years before at a nest a kilometre away. Presumably this cuckoo had departed New Zealand on its winter migration at least once, and was now back in its natal area attempting to breed. In my three years of studying shining cuckoos at Kowhai Bush I banded only 11 nestlings and

no adults — so seeing a bird two years after banding was lucky indeed.

Glossy Cuckoos

The shining cuckoo (Chrysococcyx lucidus) has four breeding populations — in the southwest corner of Western Australia, in southeast Australia including Tasmania, in New Zealand, and in the New Caledonia-Vanuatu area. In the first three areas the cuckoos are absent or very rare in winter because they migrate north to the tropics. During the southern winter they are found from the Lesser Sunda Islands in Indonesia, east through New Guinea to the Solomon Islands. In the New Caledonia area the population is non-migratory. There are about 11 another species of cuckoo in the genus Chrysococcyx spread through Australia, south-east Asia and sub-Saharan Africa. They are the smallest of all cuckoos — shining cuckoos are sparrowsized at about 23g. With one exception they all have bright iridescent plumage and so they are known collectively as the glossy cuckoos. Shining cuckoos are remarkable birds on three counts — their migration, their breeding and their foods.

Spectacular Migration

Shining cuckoos from New Zealand tend to have slightly wider bills than their conspecifics from Australia. This is very convenient because the specimens that have been collected from New Britain, New Ireland and the Solomon Islands (present there during the New Zealand winter months) also tend to have wider bills than those collected to the west in New Guinea and the Lesser Sunda Islands. Thus the deduction that New Zealand shining cuckoos migrate to the New Britain-Solomons archipelago, and no further west. Note that this is deduction and not "hard fact’ as might be obtained from the recovery of banded birds. Very few shining cuckoos have ever been banded, and chances of recovery are slim over huge, sparsely-populated areas like Melanesia and Australasia. Consider that thousands of European cuckoos have been banded in Europe, yet only one or two of these have ever been recovered south of the Sahara in proof of their wintering ground there. What chance do

we have in this part of the world? Perhaps in the future tiny transmitters will allow shining cuckoos to be tracked by satellite. Many authorities have assumed that shining cuckoos migrate directly between New Zealand and the Solomons with the only possible land-falls en route at Norfolk and Lord Howe islands. If correct, this return journey of 6,000km is the most spectacular transoceanic migration by any land bird. I have measured the bill widths of more than 400 shining cuckoo specimens in museums around the world. All birds from Victoria and Tasmania were narrow-billed, but I discovered unexpectedly large numbers of wide-billed cuckoos among the narrow-billed birds from New South Wales and Queensland. The wide-billed birds had been collected only during the months of migration. Thus it seems that at least a portion of the wide-billed New Zealand population migrates via eastern Australia. This extends the return journey to about 12,000km but involves shorter oceanic stretches (just over 2,000km between New Zealand and Australia and just under 2,000km between Australia and the Solomons) and allows ample opportunity for "rest and recreation" on the Australian mainland. The colour-banded cuckoo that I saw at Kowhai Bush had returned to its natal site (assuming it hadn't been hiding in the bush all along). Banding has shown that European cuckoos tend to return to their natal areas. It makes sense that the homing instinct should in part comprise a return to the area in which the bird was hatched. Feats of migration give us much to marvel at. As Eileen Duggan put it: "Are the seas to you as homely as our fields of curling clover? What old memory sends you blindly over hill and over hollow? Do you never doubt the way?"

Parasitic Breeders

All birds in the cuckoo subfamily are broodparasites — they do not build a nest of their own, but seek out the nests of a host species in which to deposit their eggs. The young cuckoo usually hatches before its foster siblings because cuckoos have evolved very short incubation periods for this purpose. By

hatching first the cuckoo gets a head start — and cuckoos are usually much bigger than their hosts anyway — so it is soon strong enough to kill its foster siblings by trampling them or evicting them from the nest. The foster parents feed and care for the cuckoo as they would their own. After laying eggs the adult cuckoos have nothing further to do with their progeny. Shining cuckoos parasitise the grey warbler on the New Zealand mainland, and the Chatham Island warbler on the Chatham Islands. The grey warbler is a tiny bird weighing about 6.5g. This is the same as a rifleman, but the latter — usually labelled New Zealand's smallest bird — is shorter because it has a reduced tail. The act of laying by the female European cuckoo has been well studied and even filmed for television. The cuckoo closely watches the hosts over a wide area to find nests at just the correct stage for parasitism. When the time is right she flies to the host's nest, takes a host egg in her bill, moves over the nest to deposit her own egg, and flies off without so much as a backward glance. It all takes a few seconds. She carries off the host's egg and eats it. One November day I was walking quietly along a track at Kowhai Bush when | saw a shining cuckoo perched on a branch ahead. I stopped to watch, and was surprised when the cuckoo suddenly stiffened, sleeked its feathers and adopted a strange frozen posture. I was puzzled until I noticed a pair of warblers foraging nearby. The cuckoo very slightly rotated its head as if to keep the warblers in view as they passed along. Later I found the warbler nest 30m away and in due course it was parasitised. It was a circumstantial observation, but it suggests that close observation of the hosts may be important for our cuckoo as well. Laying by the shining cuckoo has never been seen but it is probably as quickly completed as in the European cuckoo. The only extra complication is that the shining cuckoo's major hosts in Australia, New Caledonia and New Zealand build pear-shaped enclosed nests with a small entrance hole at one side. The enclosed grey warbler nest probably presents no great problem to the female cuckoo in New Zealand. She weighs about 23g but the nests are capable of holding four nestling warblers with a combined

weight of up to 30g. I think it most likely that she enters the nest, turns around, lays and departs just as the female warbier does. I examined 6 nests within a few hours of laying by a cuckoo and none was damaged. Shining cuckoos lay one egg per nest and they remove a host egg just as the European cuckoo does — at least, nests with four warbler eggs (the typical clutch-size) hold three warbler eggs plus one cuckoo egg after parasitism. It makes sense to fool the hosts by keeping their clutch-size constant. Many species of cuckoo lay eggs that precisely mimic the colour and pattern of their host's egg, but the shining cuckoo's egg is totally different from the warbler’s. This suggests that the warblers are undiscriminating. The shining cuckoo’s egg hatches in 14-17 days whereas the average incubation period of grey warbler clutches is 19.5 days. Cuckoos usually hatch before any warblers, but they are not strong enough to evict the nest contents until 3-7 days old, by which time some or all of the grey warbler eggs have usually hatched. The young shining cuckoo develops a broad, flat back — nestling song-birds tend to have a ridge along their back — and the cuckoo holds out its little fleshy wings and is able to grip the nest liming at a very early age. These adaptations enable it to ease under an egg or nestling and push backwards until the object falls out the entrance hole. The grey warbler eggs or nestlings fall to the ground, or get caught in intervening foliage, and are ignored by the parent warblers. Birds have very limited intelligence, and at this stage in the nesting cycle the parent warblers are "programmed" by instinct and by their hormones to respond parentally to nestlings in the nest and nothing else. The young cuckoo grows quickly once it has the nest to itself, and fledges after about 19 days. It follows the foster-parents about and is fed by them for another month. It is comical to see the tiny warblers approaching the insatiable cuckoo with their offerings of food. The cuckoo begs noisily by gaping widely and fluttering its wing's vigorously, and even snaps at the warblers after they have delivered their insect prey. A begging cuckoo is for small birds what animal behaviourists call a "superstimulus’. Parent birds at the stage of feeding fledglings can hardly resist. Other species — such as fantails — have

been seen feeding fledgling cuckoos. People have assumed that the fantails reared the cuckoo but it is more likely they were simply passing with food for their own young when they were "superstimulated" and fed the cuckoo instead! Everyone feels sorry for the apparently overworked warblers, but it may be a relief for them to rear a cuckoo! When I studied the visits of warblers to the nest I found that they made fewer visits with food to a cuckoo — and had fewer faecal sacs to remove — than was the case with a brood of three or four of their own young. As for the effect of parasitism on the grey warbler’s numbers, it is negligible. Warblers start laying early (September) when the cuckoos are only just arriving in New Zealand, thus only their late clutches (November) are parasitised. In my study at Kowhai Bush 55 percent of late nests were parasitised. | was able to calculate that only 17 percent of late warbler eggs (and of course none of the early eggs) failed on account of parasitism. Warblers lose far more eggs or nestlings to introduced mammalian predators like rodents and mustelids.

Favoured Foods

Cuckoos in general are well known for their tendency to eat large numbers of insects like ladybirds and hairy caterpillars that are warningly coloured and distateful to most other birds. I have examined the stomach contents of more than 40 shining cuckoos — museum specimens that died from natural causes. About 46 percent of the food items were caterpillars and 27 percent beetles. Most of the beetles were ladybirds — of 5 species, all introduced to New Zealand. When alarmed, ladybirds are supposed to discharge a fluid toxic to vertebrates. About a third of the caterpillars were larvae of the magpie moth which have a dense covering of barbed spines. The soft, fleshy stomach-lining of cuckoos that had been feeding on these caterpillars was pierced all over by clusters of spines — looking much like the way bristles are inserted in a toothbrush. European cuckoos also eat hairy caterpillars and it has been established that they get rid of the spines by sloughing off patches of mucous membrane from the stomach wall and regurgitating it. Shining cuckoos probably do the same. Three times I have found egg-shell fragments in the stomachs of shining cuckoos — all females collected in November, the peak period of laying by cuckoos. Twice these have been speckled eggs, probably grey warbler eggs removed at the time of parasitism. But in a third instance the stomach contained unspotted blue egg-shell fragments and a baby bird — probably a fully developed embryo from the ingested egg. If the nest in question held eggs with full-term embryos then it was the wrong time for parasitism, so perhaps this is proof that shining cuckoos are general nest-robbers as many of the larger cuckoos are. The blue egg may have belonged to a silvereye or hedge sparrow. Thus we learn a little about the secret lives of these fascinating birds. 4

From 1976 to 1979 Brian Gill studied the breeding of grey warblers and shining cuckoos for his Ph.D thesis at Canterbury University. He is now Curator of Birds at the Auckland Institute and Museum.