“Little Owl (Ruruwekau), Strix parvissima. A very scarce bird, not larger than a starling. The head is very large. I have never obtained a specimen, but have seen it among the forests. It is an exceedingly shy bird.” [1] 

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“In this division there is a most remarkable little owl, the smallest in the world. It is known to the natives as the ruru wekau; it has an unusually large head, flies by day, is exceedingly shy, and is about half the size of the common ruru. It inhabits dense forests.” [2]

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“No. 6. – Strix parvissima, Ellman. (Zool., 1861)
Little Owl

Amongst the desiderata of our collections the Little Owl has for some time held a place; many doubt its existence, few have seen it, still fewer have preserved any note or observation concerning it. From the information that has been gleaned about this rare bird, it would appear that its habitat must be the bushes about the Rangitata River [Canterbury Region, South Island].
One correspondent saw it on the bank of a creek at no great distance from Mount Peel Forest, it was between the roots of a large tree; observation was drawn to it by the proceedings of several tuis, who were persecuting it to the best of their ability; it was whilst its attention was engaged by these noisy assailants that the bird was secured. It was about the size of a kingfisher, and its captor felt quite certain of its being an adult specimen; it was carried home to be shown as a curiosity, and was afterwards liberated. Unlike the more-pork, when captured it was exceedingly gentle.
Another specimen was procured by a gentleman in one of the bushes far above the Rangitata Gorge; on being observed on a branch of a tree, it was knocked down and caught during its fall; there was fur on its beak, as though it had not long before devoured a mouse; this bird was also set at liberty.
Two other instances of its occurrence have been communicated, but without further information. It may be mentioned that one of these was again on the Rangitata.
At Shepherd Bush Station, on the Rangitata, opposite Peel Forest, a specimen was observed in the house, greatly resembling A. Novae Zelandiae [ruru (Ninox novaeseelandiae)], except in size, which was about that of a kingfisher; it was most gentle in its habits, remaining quiet during the daytime and sallied forth in the evening, regaining its perch by entering through a broken window. This pretty little visitor thus frequented the house for about a fortnight; it should be added that the house stands close to a small bush composed chiefly of Leptospermum, Griselinia, etc., of which there are many aged specimens.
From these notices it may be safely inferred that the Little Owl is arboreal in its habits, and possibly not so strictly nocturnal as its better known congeners; whether it is to be considered identical with either of the species referred to by Dr. Finsch is, of course, at present unknown; it is certain it is not a tufted species, or such a remarkable form would have been noticed.” [3]

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What can we make of this little owl that apparently once existed in New Zealand?

Are these accounts referring to an actual owl or rather to some other bird, maybe even to a last surviving population of the New Zealand Owlet-Nightjar (Aegotheles novaezealandiae (Scarlett))?

This nightjar species is known only from subfossil remains that date to 1200 AD and which usually are not found in association with Maori middens, it was also not necessarily a small bird and may have been quite the same size as the ruru and only slightly smaller than the larger whēkau (Ninox albifacies); furthermore it is thought to have been flightless or at least nearly so. 

There is, however, a slight chance that these eyewitness accounts indeed refer to a last surviving population of this now extinct creature, we will probably never know for sure.

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References:

[1] J. B. Ellman: Brief Notes on the Birds of New Zealand. The Zoologist 19: 7464-7473. 1861
[2] J. B. Ellman: Correspondence. The Press 3(136): 2. 1863
[3] T. H. Potts: On the birds of New Zealand (Part II.) Transactions and Proceedings of the New Zealand Institute 3: 59-109. 1870

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edited: 05.11.2021