Friday, November 16, 2012

About a Speckled Egg — A Modest Speculation

Two nests, one nestling, and an egg: those are the fruit that fell from our garden’s totara tree this spring. 

The egg, the latest find, is incomplete. The yolk left to congeal in the shell leads me to think that the chick inside didn’t grow very much, let alone make its own way out.

I’ll admit a certain eagerness for this to be, in urban backyard terms, something interesting and unusual (
from a native bird). But I’m also crossing my fingers that only an unhatched house sparrow, officially exotic yet actually fairly humdrum, came to grief as a result of taking the plunge. That’s not scientific, dispassionate, or kind: why should a sparrow matter less? (And according to the Bible, “not one falls to the ground” without God’s knowing.)

Anyway, to learn about what I’d discovered, I turned first to Which New Zealand Bird?, whose back pages match species to life-sized images of variously coloured eggs. ‘My’ egg seemed most likely to come from a grey warbler or riroriro. These birds are
very common (and plainer than sparrows in appearance); at this time of year I often hear, and occasionally see, a male of the species singing in the crabapple trees outside my window. Weighing just 6.5 g, the warbler is so tiny that its entire body moves to project its powerful, memorable melody.

Despite the efforts of the bird book’s author and publisher (aptly, Crowe and Penguin), it’s hard to match a picture on a page with a real egg in all its three-dimensional glory. But a paper published nearly 30 years ago in the ornithological society journal, Notornis, described grey warbler eggs as having reddish-brown speckling that varied:

from almost none to intensive, and the blotches ranged from minute to about 1.5 mm wide.... Speckles were usually concentrated into a dense band at the egg’s larger end...

This sounded just like the specimen I’d collected.

In the paper, one B. J. Gill was reporting on three seasons’ breeding by riroriro across a 30 ha bush area at Kaikoura, detailing not only numerous observations but also the methods and equipment  used. The latter included tape recordings and hair nets to lure and catch the incubating females, of which it was said, “None deserted.” The implication? They were sufficiently unperturbed to see out their commitment to the next generation.

Who was B. J. Gill, and what of his or her subsequent work, I wondered? The end of the paper linked the writer with Canterbury University’s Zoology Department but noted a present (1983) address at Auckland Institute and Museum.

He turned out to be Brian Gill, wh
o recently explained the pied blackbird I encountered at the botanic gardens. That explanation is, to my mind, a major contribution to science, but for anyone who is yet to be convinced, Dr Gill maintains dominion over land vertebrates at Auckland Museum. His new book, The Owl that Fell from the Sky, tells “stories of a museum curator”, and he is credited as the main source of riroriro information in the comprehensive Field Guide to the Birds of New Zealand.

With the patience of a saint scientist, Dr Gill examined the photos I emailed and announced that he couldn’t be sure if my egg came from a warbler. It was probable, he said, but several other species (including the fantail, fernbird and — yes — the ever-present house sparrow) also lay white eggs with reddish speckles.

I’m voting for the riroriro. I hope that the other three eggs in its clutch are safe, and that the pipiwharauroa or shining cuckoo I heard a while back hasn’t commandeered its nest — though that could explain the egg I found. It would also render my garden drama much more gripping.

The pipiwharauroa is, like the riroriro, more often heard than seen. The “brood parasitism” for which it is famous involves chucking an egg out of the riroriro
’s nest and laying one of its own there instead, although nobody seems to have witnessed this.

Bird enthusiasts marvel at this achievement, because the cuckoo is more than three times the warbler’s weight, the latter’s dome-shaped nest has an entrance of just 3 cm, and the structure seems unaffected by all this unscheduled activity. After the cuckoo chick hatches, it evicts the remaining warbler eggs or chicks and gets a set of surrogate parents all to itself.

I’ve been castigating myself for my sentimental, unscientific observation of garden life. But if nobody knows exactly how a cuckoo infiltrates the grey warbler’s domain, perhaps it’s not a crime for me to speculate about a single speckled egg.

Pictured left: Dr Gill’s drawings of grey warbler nests he observed, as published in Notornis (Gill 1983), but shown here rather smaller. Reproduced with permission from the author and publisher.

Dr Gill lists the warbler’s nesting materials as: moss; cobwebs; spider’s egg cases; lichen; sheep’s wool; hair of horses, cows, deer and humans; feathers; leaves, pine needles and leaf skeletons; scales and fibres from ferns; twigs; bark; rootlets; thistledown and willow catkins; decayed wood; scraps of paper; fine creepers.

Pictured below: riroriro / grey warbler. Reproduced with the permission of Steve Attwood,


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