Methods.

In response to letters published in the newspapers, offering a reward of one shilling per bird, 243 owls were received between June 23, 1938, and June 26, 1940. Most of them had been trapped or shot, and a few were young, apparently taken from the nest. Though some were considerably decomposed or attacked by maggots, it was possible in all cases but one to examine the stomach contents. The bird was first weighed and in some cases measured. The stomach was then removed and preserved whole in formalin for subsequent examination. In some cases the intestine was opened, washed out, and the contents examined under the binocular microscope for parasites. The length and breadth of the gonads were measured in situ, and these organs were then removed and weighed. Finally, the sternum and limbs of about 100 birds were removed, and the bones cleaned by boiling, for measurement and the study of variations. The results of this will be described in a separate paper.

For the examination of the stomach contents, the stomachs were opened and carefully washed out into a glass dish. All recognisable remains were then picked out under the binocular microscope. Naked eye examination would be useless as in most cases the food is reduced to small fragments, and all soft parts digested. Such things as the chaetae of earthworms or the jaws of small caterpillars are practically invisible to the naked eye, and many things can be recognised only with practice even when magnified. The complete examination of the contents of a full stomach frequently required several hours' work with the microscope. All the fragments, as they were picked out, were placed in another dish containing alcohol, in which they were re-examined and counted, so as to determine the number of individuals present. The material from each stomach was preserved separately in a tube of alcohol.

In addition to the birds themselves, a considerable number of castings or pellets, and debris from nesting holes were received. No attempt was made to examine the invertebrate contents of these, as it was felt that much more satisfactory material was provided by the stomachs. All bones and feathers, however, were picked out. The bones were measured and the measurements compared with a table of measurements prepared from the skeletons of native and introduced birds. The species of birds whose bones were available for comparison were as follows:—Rifleman, Pipit, Fern-bird, Grey Warbler, White-breasted Tit, North Island Robin, Fantail, Whitehead, White-eye, Bell Bird, Greenfinch, Goldfinch, Redpoll, House Sparrow, Thrush, Blackbird, Hedge Sparrow, Starling. The bones were then carefully compared with all the known bones of about the same size and so identified. It is not always possible to be certain of the species to which an isolated bone belongs, but it is possible to distinguish between the bones of different families.