A New Subspecies of Xenicus
[Read before the Dunedin Science Congress, May, 1935; received by the Editor, May 1, 1936; issued separately, December, 1936.]
On some of the islands to the south-west of Stewart Island there is to be found a wren allied to the bush wren, Xenicus longipes longipes, of the mainland, but distinct in many respects. It was first brought to the notice of ornithologists by H. Guthrie-Smith, who described it under the name of bush wren (X. longipes) in “Birds of Land and Shore.” In 1931 I spent five weeks on these islands and saw that the bird was not the same as X. longipes, and I now propose to describe it as a new subspecies under the name of Xenicus longipes variabilis.
In point of size it is slightly smaller than X. longipes, but on comparing it with a series in the Natural History Museum, South Kensington, I found that the measurements overlapped. The tarsus in twelve specimens of the new subspecies varies from 24 to 25 mm., while that of the mainland form runs from 25 to 26 cm. The wing varies from 52 to 56 mm., with an average over twelve specimens of 54 mm., compared with wing-lengths from 52 to 59 mm. for the mainland bird, with an average for nine specimens of 56.5 mm.
In its colouring X.I. variabilis differs from X.l. longipes in having the back much less suffused with green; the brown of the head runs farther down the neck; the white eyebrow is much less distinct, while the chin is not so white. I have taken as the type a specimen in my collection as near as possible to X.l. longipes; but this new form varies considerably in colouration, more especially in that of the upper surface. In some cases the brown of the head extends down the back, including the scapulars and upper wing-coverts.
Description of Type.
General colour of upper surface brownish-green, including the wings and back; greener on the rump. Head and nape brown with a scaly appearance, a grey-white superciliary streak runs from the bill to the nape, where it fades into the brown. There is a dark spot immediately in front of the eye. Ear-coverts brown, slightly lighter than that of the crown; primaries light brown, outer web green; wing-coverts greenish-brown. Chin, light grey; breast, abdomen and vent slate-grey with faint brown tinge; sides of body green, slightly lighter than back; under aspect of primaries brown, lighter on inner edge; under wing-coverts green with yellowish tinge; under tail-coverts light green.
In some individuals the whole of the upper surface is brown with only a faint tinge of green at most, and in this form the white superciliary line is absent, the appearance at a few yards being that of a self-coloured brown bird. All gradations between this and the greenest form are to be met with, the variation being independent of sex or age, the birds of the various colours pairing indiscriminately.
Besides the differences in size and colouration, there is a considerable difference in habit between X.l. longipes and X.l. variabilis. The former is chiefly arboreal in habit and nests in trees; the latter is largely terrestrial and often nests in holes in the ground, though sometimes in prostrate logs or in clumps of fern.
Perhaps the most remarkable difference between the two birds is in the size of their eggs, those of the larger, X.l. longipes, being 18 × 13.5, while those of X.l. variabilis are 21.5 × 15.5 and 21 × 15—very nearly half as bulky again.
X.l. variabilis is fairly abundant on at least three of the islands to the south-west of Stewart Island and possibly on two others.