The Secretive Singer

Forest and Bird, Issue 310, 1 November 2003, Page 12

The Secretive Singer

BRIAN GILL

BRIAN GILL

It's ‘drab but lyrical’ says

who has studied our tiniest

songster, the grey warbler. Pictures by GEOFF MOON.

might rarely be noticed were it not for their tuneful singing. Yet, with the fantail and silvereye, they are one of our commonest native garden and bush birds, found almost throughout the country wherever there are trees and shrubs. The grey warbler or riroriro Gerygone igata, at around 6.5 grams, is just as small in body-weight as the rifleman — only one third the weight of a house mouse. Yet we think of the rifleman as New Zealand's smallest bird because its truncated tail makes it look so small. Like riflemen, grey warblers are restless birds, almost never still as they flit about the foliage in search of food. The grey warbler’s body plumage may be dull grey, but close inspection reveals outer tail feathers boldly patterned with black and white bars, and an eye that is ruby-red (in adults). There are about 20 species of Gerygone, mostly in Australia and New Guinea, with a few extending west to south-east Asia, one to the Solomon Islands (also Vanuatu and New Caledonia), and two to New Zealand. The second New Zealand species is the Chatham Island warbler G. albofrontata, which, at and dull-plumaged, grey warblers

about 9.5 grams, is the giant of the genus. The grey warbler’s song is a plaintive wavering trill that rises and falls. It is given only by males. They sing throughout the year but most vigorously in spring when they nest. Such a New Zealand sound! The warbler’s energetic singing was never more eloquently described than by our great natural history writer, Herbert GuthrieSmith. In his 1910 book Birds of the Water, Wood and Waste Guthrie-Smith described how: ‘Presently, from some manuka thicket, a sombre plumaged little bird will emerge, light on some topmost twig, and pour forth to three-quarters of the globe — for in his ecstasy he nearly sings a circle — this faint sweet trill that heralds fuller spring. There is a similar quality to the songs of many of the Australian species. Writing of the western gergygone (G. fusca) Alan Bell wrote very poetically: ‘I have never heard sounds so plaintively microscopic, so clear and yet scarcely perceptible. The ghost of a kitten’s mew — the echo of dwarf violins played in the moon — these were the bird’s notes.’ The generic name Gerygone (‘ger-ig-onee’ or ‘jer-ig-onee’) is compounded from

ancient Greek words that can be interpreted as meaning something like ‘child of sound’ The specific name igata was bestowed in 1830 by French zoologists, and was said to be the grey warbler’s Maori name as used in Tasman Bay during the voyage of the French ship Astrolabe. Grey warblers eat insects and other arthropods. Their light weight enables them to hover momentarily in mid-air and snatch prey at the tips of delicate twigs that they and other birds are too heavy to perch upon. Our other small insectivorous birds either cannot hover, or rarely do so, and in this way grey warblers have their own specialisation that gives them a competitive edge in the search for food. Grey warblers are unique among mainland New Zealand song birds in building an enclosed, pear-shaped nest with a small entrance at one side. The nest begins as a small wad of material attached around a twig and this is enlarged and extended downwards before the cavity is formed and the nest is expanded to its final shape. The nest has a framework of rootlets and grass stems and is filled out with fine material, such as dried moss, that is bound in place

with silk from spiders’ webs. The chamber is lined with soft material like feathers, downy seeds and tree-fern scales. Some nests hang completely free, apart from their point of attachment at the top, while others are attached at the back or sides as well. Only the female builds the nest. She alone incubates the eggs and broods the nestlings, but the male helps to feed both nestlings and fledglings. The warbler’s tiny egg, about 17 millimetres long and 12 millimetres wide, is white with fine, reddish-brown speckles. It weighs about 1.5 grams which, however, is very large (23 percent) relative to the adult weight. No wonder, then, that the female lays the eggs of a clutch 48 hours apart, rather than 24 hours apart which is typical of song-birds. The usual clutch-size is four, so in seven days the female lays a clutch equal to 92 percent of her own body weight! The globular nest may play a critical role in conserving heat for such a small bird whose first clutches may be laid at the end of winter (late August). The incubation period (17-21 days) is quite long. The nestlings have fluffy white natal down that camouflages them against the feathered nest-lining when they are at rest. They spend 15-19 days in the nest.

Though the warbler’s nesting cycle is relatively long, the breeding season is also long, and they are able to raise two successful broods per year. The enclosed nest, the two-day laying interval, and other aspects of the warbler’s breeding are shared by other members of the distinctly Australasian family (the Acanthizidae) to which, with groups like the thornbills, the grey warbler belongs. The grey warbler is the host of the shining cuckoo on mainland New Zealand. This brood-parasite slips an egg into the warbler’s nest, and the warblers rear the baby cuckoo alone after it has evicted the warblers’ own eggs and young (see Forest ¢& Bird August 1989). Most birds are not particularly intelligent, driven instead by instinct and the hormones that govern reproduction. When feeding nestlings, adult warblers are ‘programmed’ to deliver food to the nest, and just do not respond to their own chicks once the cuckoo has pushed them out of the nest. The warbler chicks soon die. Being larger than the warbler, the cuckoo’s chick fills the nest by the time it is ready to fledge. Neither parasitism by the cuckoo nor depredation by rats and other introduced mammals seem to threaten grey warblers in the long term. Though so tiny, they can live

for more than five years once they reach adulthood, which gives them plenty of opportunity to replace their numbers. Many New Zealand birds have an innate tameness, not seen to the same degree in typical birds of the northern hemisphere. During my study of grey warblers, one female at a low nest allowed me to remove small nestlings from beneath her (for weighing) while she continued to brood. Once an entire nest with feathered young came away in my hand and I wired it to a new position 30-40 centimetres away. The parents were confused at first and continued to visit the point where the nest had been, but guided by the chicks’ cheeping they soon resumed feeding them. Drab and insignificant to look at to be sure, but grey warblers have a fascinating life history as well as a beautiful song. We are lucky indeed to number them among our common native birds.

Curator of Birds at Auckland War

Memorial Museum, studied grey warblers in forest near Kaikoura. He has written several books, including New Zealand’s Unique Birds (Reed, 1999), with GEOFF MOON who took the photographs for this story.

Grey warblers are unique among mainland New Zealand song birds in building an enclosed, pear-shaped nest with a small entrance at one side. The nest begins as a small wad of material attached around a twig and this is enlarged and extended downwards before the cavity is formed and the nest is expanded to its final shape. The nest has a framework of rootlets and grass stems and is filled out with fine material, such as dried moss, that is bound in place with silk from spiders' webs. The chamber is lined with soft material like feathers, downy seeds and tree-fern scales. Some nests hang completely free, apart from their point of attachment at the top, while others are attached at the back or sides as well. The author, Brian Gill, sketched these variants during his research.