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Volume 72, 1942-43
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A Study of the Little Owl, Athene noctua, in New Zealand.

Department of Zoology, University of Otago.

[Read before the Otago Branch on July 14, 1942, received by the Editor, July 20, 1942; issued separately, December, 1942.]

Introduction.

The Little Owl, Athene noctua, or, as it is often called in this country, the German Owl, was introduced by the Otago Acclimatisation Society some 36 years ago, in an endeavour to control the numbers of the small introduced birds. According to Thompson, 28 owls were imported from Germany in 1906. Further liberations followed—39 in 1907, 80 in 1908 and 72 in 1910. Owls are also reported to have been liberated in North Canterbury, and in 1911, 66 were obtained by the Waimate County Council, at a cost of £50. A pair was also liberated in the North Island at Rotorua, about the same time, but does not appear to have become established. The bird soon began to increase and spread in the South Island, and before long was accused of being a serious menace to the native birds. At the present time, these accusations are widespread, and rewards are offered for the destruction of the owls.

This history is closely parallel to that in England. The owl, though widespread in the Palearctic region, had only been recorded as a rare straggler in the British Isles. It was introduced into England towards the end of last century, and is at present widespread in England and Wales, except in the north. It is now, if not actually decreasing, probably increasing much less rapidly than formerly. Soon after its introduction, it was accused of killing song-birds and the chicks of game and poultry. It was consequently selected by the British Trust for Ornithology for special study, and a long and detailed report on the investigation by Miss Hibbert Ware appeared in 1938. The result of this investigation was to show that only negligible destruction of game, poultry and wild birds took place, and that the owl fed almost wholly on such insects, other invertebrates and small mammals as can readily be picked up on the ground. The sweeping accusations were shown to be without foundation, though it is pointed out that the feeding habits may have been somewhat different when the numbers were increasing rapidly than at the time of the investigation, when the rate of increase was less. However, a previous investigation of the food, carried out by Collinge in 1918, gave the same results. The bird is regarded as beneficial on the continent of Europe, and is protected in some countries.

When the owl was brought to New Zealand, it was placed in a different environment, and changes, especially in its feeding habits,

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might be expected to have taken place. It was therefore decided to study the bird and its feeding habits, taking advantage of the fact that as it was being killed a large number might be obtained, and also that the report of the English investigation was available for comparison. It is not intended to enter into the controversy as to the habits and value of the owl in this country, which has been carried on without any adequate basis of fact. It has been suggested that the present investigation is an attempt to prove that the owl does not eat native birds. No such object is intended, the whole investigation is concerned solely with the accumulation of as many facts as possible concerning the owl and its food, from an examination of the actual birds and of their nesting places and castings. These facts may prove the basis for future discussion. On the other hand, many of the statements which are frequently made about the degree to which the owl destroys other birds, show a remarkable lack of reasoning power or appreciation of the value of evidence. For instance, when it is stated that, as the owl increased, the native birds decreased, it is apparently not realised that these changes, which would need actual numerical data to substantiate them, may be wholly unrelated. Similarly, when it is stated that the screams of birds being murdered by the owls are frequently heard, there is no evidence that the birds really are being murdered, and if so, by whom. A great deal of the so-called “evidence” is of this type. Statements of this kind have been excluded from the present paper, only those giving definite statements of fact being included.

Many of the statements bear so close a relation to ones which remain wholly unsubstantiated in England, that they appear to originate in the English press. It is perhaps worth mentioning some points which were studied in England. The owl is chiefly a twilight and night feeder, and does not habitually feed by day as is often supposed, though it does so occasionally. It is a ground feeder, as is shown by its food. The so-called “larders” are not used for storing excess food, but are more in the nature of “carving holes.” Very little evidence of nest raiding has been recorded, and no eggs of other birds have been found in the owl's nests or food. There is no evidence that the owl kills prey and leaves the body to attract beetles which the owl will return and eat later on. These conclusions were reached in England and, though it is of course possible that the owl changed its habits as it became adapted to life in New Zealand, this has not yet been demonstrated. Any reliable information about its habits here would be valuable and should be collected.

Finally, it should be pointed out that arguments as to the economic value or otherwise of the owl are worthless, as none of the requisite data is available. To determine whether the owl is beneficial or not, it would be necessary to know, not only the number of owls present and their food, but also the numbers, food and interrelations of almost all the other animals in the district, as well as the effect of fluctuating conditions of season, climate, etc.

I wish to record my indebtedness to the Royal Society of New Zealand for a grant from the Hutton Fund to defray the cost of

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the investigation; to Dr Miller, of the Cawthron Institute, for identifying insects; to Mr Ferguson, of the University of Otago, for assistance with statistical matters, and to all those who assisted by sending in material or information during the course of the investigation.

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.

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All the material collected during this investigation is preserved in the Department of Zoology of the University of Otago, so as to be available, if necessary, for more detailed study and comparison in future.

Stomach Contents.

The ideal in investigating the diet of any animal is to be able to determine the relative amounts of the basic food materials, carbohydrates, fats and proteins eaten, and the source from which they are derived. In most cases this is impossible, as the foods have not been analysed. Another method is to express the different items as percentages of the total by weight or volume. This method clearly has its disadvantages owing to the large proportion of certain foods which are indigestible—for instance, the soil in the intestine of an earthworm, or the cellulose in that of a caterpillar, but it is more satisfactory than the mere listing of individuals. It is, however, only possible when the food can be recovered unchanged from the gullet or crop of the bird. The great majority of food items recovered from the stomachs of the owls have only been recognised from microscopic fragments, and any attempt to estimate the weight or volume of the very diverse animals represented would be quite valueless. The only possibility, therefore, is to deal with the numbers of individual animals, bearing in mind, in considering their relative importance, both difference in size and frequency of occurrence.

Of the 242 owls whose stomachs were examined, only one contained no food remains of any sort, though several had only one or two minute fragments of beetle skeleton or a few earthworm chaetae. Very few solid particles, even small ones, pass into the intestine, but they are retained in the stomach and periodically ejected as the so-called castings or pellets. Counting each occurrence of chaetae as one worm, which will presumably be an underestimate, 5,443 individual items of food were found, an average of 22.6 per owl. Though insects naturally predominate, the owl exhibits an extreme catholicity of taste, and representatives of all the groups of terrestrial animals are present. Also noteworthy is the small size of many of the animals eaten. The smallest, an oribatid mite smaller than an ordinary pin's head, and a pseudoscorpion, may have been eaten accidentally together with other food, but small spiders, whose length cannot have been much more than a quarter of an inch, were commonly eaten.

It is usual to identify all the species found in such an investigation, but in the present case this was felt to be impossible, and moreover would not be of any great value when only occasionally occurring species are concerned. Only one or two very common species have therefore been identified, and in Table I a number of species will be found included under such headings as “araneomorph spiders,” “orthoptera” or “caterpillars.” As all the material is preserved, complete identification can be attempted in future should it become necessary.

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Table I.
The Numbers of Animals Found in the Stomachs of 241 Owls.
Type of Food. No. of Individuals. No. of Owls Containing them. Average No. per Owl. Average No. per Total Owls. % of Owls In Which They Occur. Greatest No. In Any One Owl.
Caterpillars 1936 126 15.3 8.0 52.3% 186
Earwigs 821 26 31.5 3.4 10.8% 267
Lamellicorn Beetles 796 90 8.8 3.3 37.3% 73
Araneomorph Spiders 271 94 2.9 1.1 39.0% 18
Carabid Beetles 179 60 3.0 0.7 24.9% 27
Mygalomorph Spiders 131 18 7.3 0.5 7.4% 57
Lepidoptera 95 41 2.3 0.4 17.0% 10
Weevils 79 23 3.4 0.3 9.5% 21
Cicadas (Adult) 46 11 4.2 0.2 4.5% 14
Opiliones 21 16 1.3 0.08 6.6% 4
Birds 19 22 0.8 0.08 9.1% 1
Total Beetles 1639 211 7.7 6.8 87.5% 74
Total Spiders 404 98 4.1 1.6 40.6% 57
Total Orthoptera 116 16 7.2 0.5 6.6% 53
Total Cicadas 49 14 3.5 0.2 5.8% 14
Total Individuals 5443 22.5 296
Invertebrates 5403 22.4 296
Vertebrates 40 0.16 7
Insects 4871 20.2 296
Arachnids 427 1.7 57
Earthworms (93) (0.4)
Myriapods 7 0.03 1
Crustacea 5 0.02 1
Molluscs 3 0.01 3

Invertebrate Food: Caterpillars.—The commonest insects eaten are caterpillars, which occurred in about half of the owls. Several species are represented but two are commonest and sometimes of large size, up to an inch and a-half or so in length. One owl contained 186 caterpillars in addition to 10 other animals. Most of the bulk of a caterpillar consists of chewed vegetable matter in its capacious intestine, and this material being indigestible by the owl, forms a very conspicuous part of the stomach contents of birds which have been eating caterpillars. Of the caterpillar itself usually only the jaws remain. Most of the caterpillars appear to belong to species which live on the ground, the largest and commonest being the larva of the moth Persectania ewingi.

Caterpillars not unnaturally show a marked seasonal periodicity in their occurrence in the stomachs. During the winter, between about March and September, they form an important part of the diet, reaching their peak in June or July, while during the other five months few are to be found.

Beetles.—Nearly as many individual beetles as caterpillars were found, and they occurred at all times of the year, appearing in 87.5 per cent. of the owls. Many different kinds occurred, the most noteworthy being the Lamellicorns, the Carabids and the Weevils,

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of which 796, 179 and 79 respectively were found. The Lamellicorns were mostly Odontria striata the adult of one of the well known “grass grubs,” though another similar but smaller species also occurred. The weevils were Platyomida sp. Amongst the others may be mentioned five Elaterid beetles and 12 Longicorns, the large Huhu.

Earwigs.—Twenty-six owls contained earwigs, 14 of them coming from southern bush areas in the Catlins and near Tuatapere. These contained 64 earwigs. Four owls were received from Pembroke and they contained 6, 178, 245 and 267 earwigs respectively, the last two being by far the greatest number of individuals of any Animal found in a single stomach. These large numbers are of interest in connection with the opinions expressed by certain fruit-growers in Central Otago that the owl is beneficial to them because of the numbers of earwigs which it consumes. It is perhaps worth recording that the sex ratio of the 690 earwigs in three Pembroke owls was 60.7 males to 100 females.

Spiders.—Spiders formed a common, though not as a rule conspicuous part of the diet, 404 being recorded from 40% of the owls. Most of them were represented only by isolated chelicerae, but had evidently been quite small. The most interesting feature was the occurrence of the remains of 131 male trapdoor spiders. Those from north of Dunedin were mostly Arbanitis gillesii, while those from the south were A. huttoni. The males of these are difficult to obtain, but evidently they emerge from their burrows in autumn and are captured by the owls. One stomach contained 57 and another 37 of them. The females apparently do not emerge and none were recorded.

Of the animals only found occasionally, a few deserve special mention. Only five millipedes were found, which, in view of their abundance, is peculiar and suggests that they may be distasteful to the birds or do not emerge while they are feeding. The same applies to woodlice, of which only five were found. One owl contained three slugs, the only molluscs which were encountered, but as it is doubtful if any recognisable remains would be left after they had been in the stomach for any length of time, it is possible that they may be eaten more often than this number suggests. Earthworm chaetae occurred in 93 of the owls, 38% of the total. Anything from one or two to very large numbers were found, but never any trace of the soft parts of the worm.

Comparing the invertebrate food of the owl in New Zealand with that in England as found by Miss Hibbert Ware (1938), a number of differences are apparent. Five insects were found in England to stand out on account of their great abundance. They were a crane-fly (Tipula sp.), and earwig (Forficula auricularia), a carabid beetle (Pterostichus madidus), a dung beetle (Geotrupes stercorarius) and a cockchafer (Melolontha vulgaris). In New Zealand only one tipulid was found, and if some unidentified black eggs were assumed to belong to these flies, the number would only be raised to five. The earwigs, carabid beetles, and lamellicorns, the last being similar to, though smaller than, the European cockchafer, have already been discussed, while a common dung beetle, corre-

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sponding to the English Geotrupes, does not occur in this country. Caterpillars occur in the English lists but are not specially mentioned and evidently do not form such an important part of the food as they do here. Other differences are that woodlice, millipedes and small snails are recorded as being common, while in this country they are noticeably infrequent. The work in England was done for the most part on pellet material, but the examination of stomach contents is stated to have given essentially the same results, and the general conclusion was that “the Little Owl feeds to a great extent on what is common on the ground at dusk and by night.” This statement apparently holds good for New Zealand also.

It has been suggested that the owl kills birds and leaves the bodies to attract necrophorous insects, visiting the bait from time to time in order to eat the insects. This fantastic story is discussed by Miss Hibbert Ware (1938), who points out that apart from the impossibility of crediting the owl with a degree of intelligence entirely unknown amongst birds, she found only 75 burying beetles in 2,460 pellets and 76 nest-holes. Necrophorous insects are far less conspicuous in New Zealand than in England, so that this habit is. if possible, even less likely to be found here. One stomach contained 28 maggots, but an insectivorous bird such as this is likely to eat carrion insects as well as others should it happen to find them.

Vertebrate Food.—Representatives of all classes of terrestrial vertebrates were found in the stomachs. Two owls contained small frogs, seven in one and one in the other. One contained four small lizards. Twenty-two owls contained remains of birds (three young owls from the same nest containing remains of one bird), nine remains of mice and one unidentifiable pieces of meat with no hairs, feathers or bones accompanying them. Fragmentary rabbit remains were found in three young birds, all sent in together, so probably from the same nest.

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Table II.
Species of Birds Whose Remains Were Found in the Stomachs of the Owls.
Thrush 6 and 1 doubtful
Blackbird 1
Starling 1
Sparrow 1
Greenfinch 1 doubtful
Redpoll 1
Hedgesparrow 1
Lark 1
Fantail 1 doubtful
Claw 1
Gizzard 1
Unidentified feathers were found in three owls.

Table II gives the bird remains, arranged according to species. The individuals of thrush and greenfinch listed as doubtful are represented only by fragments which are too small to make identification certain. The lining of the gizzard is unidentifiable, but probably belonged to a finch, and three stomachs contained feathers which

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did not belong to the owl itself but which could not be identified. The doubtful fantail is represented only by what may be the minute horny sheath of the upper mandible, while the isolated claw belonged to a bird as small or smaller than a white-eye. It will be seen that almost all the birds whose remains were found belong to ground-feeding species.

Vertebrate food occurred most commonly in spring and summer, possibly not only because young birds which are easy to catch are found then, but also because large objects may readily be carried to the nest to feed the young. Fourteen birds occurred between October and January, three between February and May, and two between June and September. Six mice occurred between September and December, three between January and April, and none between May and August. The lizards and frogs occurred in November and December.

Miscellaneous Objects Found in the Stomachs.—A good deal of debris and vegetable matter was found in the stomachs, most of it presumably eaten accidentally with the food. Small stones up to about a quarter of an inch in diameter occurred in 13 owls, and sand or mud in 19. Blades or small tufts of grass were found in 16 birds, and curiously enough 20 contained one or two clover leaves and six contained thistle spines. Another curious feature was the presence of 124 small seeds of various kinds in 35 stomachs. They seem too numerous to have been there accidentally, and yet they were only about 2 mm. long as a rule. Five owls contained seeds with hooked spines belonging to the bidibidi, Acaena sp.

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Table III.
Numbers of Individual Vertebrates Found in the Various Collections of Nest Material and Pellets.
Sparrow. Thrush. Starling. Pipit or Lark. Blackbird Hedgesparrow. Goldfinch White-eye. ? Species. Mouse. Rabbit. Frog.
Wallacetown 16 1
Otatara 1 1 1
Orepuki 1 1 1
Otekura 3 1 1 1 1
Otekura 1 1 1
Pounawea 1 1
Kaka Point 1
Mosgiel 2 1
Patearoa 4 3 6 2 5 1
Parnassus 1 3 1 1 1
Kimbell 1
Kimbell 2 1 2
Prebbleton 6 2 2 5
Kaiapoi 1 1
Kaiapoi 7 1 1
Kahuika 2 1 2
? 1 1
Totals 36 14 12 7 6 2 3 1 1 16 6 2
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Castings and Nest Contents.

A number of castings and the contents of nests were received, and as explained above, only the bones in these were studied. It is possible that one or two of the castings may belong to the Morepork, not the Little Owl, but the numbers are not large enough to have much effect on the result. As will be seen from Table III, the remains of 82 birds, 16 mice, 6 rabbits and 2 frogs were obtained. Much the commonest bird-remains were those of House Sparrows, of which 36 were recorded, though it is possible, owing to the great similarity between them, that bones of some of the finches have been included among them. The largest collection of nest material, containing 16 sparrows, was collected by Mr Sorensen at Wallacetown, near Inver-cargill. The nest was in the wall of a shed in the middle of stock-yards, and was occupied for several years in succession. Most of the lark or pipit bones come from Patearoa, a tussock district.

Experiments With a Captive Little Owl.

Mr Sorensen, the Director of the Southland Museum, kindly provided some detailed notes on the feeding habits of a captive Little Owl, together with a collection of all the castings produced by it during about six weeks. The owl was fed on earthworms, dead birds, mice and rabbits, and no difficulty was experienced in getting it to eat dead animals. In eating birds it usually removed and ate the head first, but finished the remainder later, leaving only in some cases the primaries. The feathers are more or less completely disintegrated in the stomach of the owl and surround the bones in the castings as a uniform greyish mass. The castings were 1.5 cm. in diameter, and 3 or 4 cm. long, as a rule, though the length was rather variable. A live young rabbit was not touched by the owl, but it ate dead ones, leaving part of the skull, vertebral column, hind leg bones, and stomach.

Similar results were obtained at the London Zoo in experiments carried out by Miss Hibbert Ware (1936). In order to find out-what would be done with food too large to swallow she presented her owls with a dead pigeon, but they would not touch it. Mr Sorenson's owl had no hesitation in eating dead young rabbits.

Parasites.

No special efforts were made to collect ectoparasites from the owls, but a few specimens of fleas and lice were collected. I am indebted to Miss Clay, of the Department of Entomology of the British Museum of Natural History for identifying the louse, which was Philopterus cursitans (Nitzsch 1861), a species found on A. noctua in Europe also. Dr Karl Jordan, of the Zoological Museum, Tring, kindly identified the flea, Ceratophyllus gallinae (Schrank 1803).

The intestine and caeca of 24 birds were opened and their contents examined for endoparasites. The only one found was an extremely slender nematode worm, kindly identified for me by Dr Bayliss, of the British Museum, as Capillaria tenuissima (Rudolphi, 1803). Of these 24 birds, 9 had no parasites, 9 had them in the caeca only, 5 had them in the intestine only, and 1 had them in both, a solitary one in the intestine and many in the caeca. The caeca of owls are large and are peculiar in having the closed end expanded into a pear-shaped sac. They usually have dark coloured glutinous contents,

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unlike that of the intestine. The caeca only were examined in 20 owls, of which 6 contained nematodes. Infections were not very heavy. The 6 infected intestines contained only 15 worms, while 15 pairs of infected caeca contained 135 worms. This nematode is normally found in A. noctua in Europe. Two specimens of the Morepork, Ninox novaeseelandiae were also examined.*

It is interesting to find that while the introduced bird has only its introduced parasites, the native owl contained a cestode and two species of nematodes, one a Capillaria sp.

Though the numbers are not large enough to be conclusive there is a suggestion that the nematode is more common in the bush districts of South Otago and Southland than in the more open and drier Central Otago and Canterbury. Five birds were received from the Cromwell district and seven from localities in Canterbury. The intestine and caeca of two and the caeca only of the rest were examined and none contained nematodes. As will be seen from the figures given above, among the 32 southern birds examined, one individual out of every three had the parasites in its caeca.

In Europe the owl is known to be the host of certain Haemosporidia. No special attempt was made to look for the presence of these, but the blood of one or two specimens was examined. No blood parasites were noticed.

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Table IV.
Mean Monthly Weights of the Owls.
Male. Female.
No. of Birds. Mean Weight. Lightest. Heaviest. No. of Birds. Mean Weight. Lightest. Heaviest.
June 1 219 1 200
July 3 230 203 253 2 233 227 238
August 3 200 198 201 2 201 175 227
September 2 176 166 186 6 174 141 189
October 6 165 143 187 4 192 174 231
November 4 156 142 180 2 209 189 229
December 9 157 148 175 6 161 139 177
January 2 166 142 191
February 6 175 139 203 13 170 163 181
March 7 185 141 216 2 147 143 151
April 7 173 142 192 8 180 148 210
May 6 172 146 191 9 201 173 220
June 4 213 212 215 9 230 142 298
July 2 215 206 225 3 193 167 210
August 2 217 208 227 1 176
September 1 170 4 178 164 211
October 1 203
November 1 146
December 2 148 145 152 2 155 149 161
January 1 146 1 173
February 1 148 1 167
March 6 159 148 178 5 157 147 169
April 17 169 148 193 17 175 154 206
May 1 158 1 184
June 1 273 2 228 223 234

[Footnote] * It should be noted that neither of these Moreporks met its death because of the investigation. One was found dead and the other was killed in mistake for a Little Owl on the ground that it destroyed native birds.

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Graph showing the monthly changes in mean body weight of the two sexes during the period June, 1938, to June, 1940.

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Weight.

The weights of the owls varied considerably during the year, being highest in winter, when in some cases an extraordinary amount of fat was to be found not only around the viscera but also forming a thick layer below the skin. The two sexes are similar in weight, as they are in size. The weights of 93 males ranged from 139 to 273 grms., while the limits in the case of 104 females were 139 and 298 grms.

The birds were weighed just as they were received, so that some variation in weight is due to the amount of food in the stomach. In order to find out how far this affected the result, the stomach contents of two birds with unusually full stomachs were weighed. The total weights of the birds were 174 and 170 grms, and the weights of their stomach contents were 4.3 and 4.5 grms. respectively. Clearly the weight of the food in the stomach will not interfere with the use of gross body weights in the study of the seasonal variations, which are of the order of 100%.

The changes in weight from month to month are shown in Table IV and the graph. The figures plotted are the mean weights, and it will be seen from the table that the monthly number of birds varies from 1 to 17, the mean being about 4. In spite of the smallness of the samples the shapes of the graphs for the two years are similar, and so it seems probable that the true changes are shown.

In the male the weight falls steadily from a maximum about July until November, when it remains low and more or less the same for two or three months. Then it rises again towards the winter maximum. There is, however, a minor loss in weight about April or May, interrupting the upward trend. The curve for the female is of similar shape except for a sudden and very marked increase in the spring, about October or November. The minor loss in weight in the autumn occurs one or two months earlier in the female than in the male. As the moult takes place in autumn it is reasonable to assume that this loss in weight is associated with it. This is confirmed by the fact that of the birds recorded as being in moult before March 1, eight were females and four were males; of those after March 1, seven were females and 15 were males. The high average weights of the females in October and November are partly due to the presence of a few very heavy birds, but even if these are excluded the average weight of the remainder is above that of the males. This increase in weight of the females is clearly connected with the changes in metabolism during the development of the ovary, and perhaps also to the period of enforced inactivity during incubation.

The same general form of curve showing the seasonal changes in weight has been found in a number of different birds. Nice, working with the Song Sparrow, Melospitza melodia, found a winter maximum for both sexes in January. The weight decreased during the spring in both sexes, but the females showed an increase during laying and incubation, as in the owl. Similar seasonal changes are recorded in other passerine species.

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Table V.
Mean Monthly Weights of Ovaries and Testes.
Ovary. Testis.
No. of Birds. Mean Weight. Lightest Heaviest. No. of Birds. Mean Weight. Lightest Heaviest.
June 1 0.050 1 0.030
July 2 0.075 0.07 0.08 3 0.026 0.02 0.03
August 2 0.070 0.02 0.12 3 0.036 0.03 0.05
September 6 0.175 0.06 0.29
October 4 0.565 0.23 1.50 5 0.242 0.10 0.34
November 3 0.576 0.14 1.21 3 0.113 0.04 0.21
December 4 0.077 0.04 0.12 5 0.024 0.01 0.04
January 1 0.040
February 13 0.027 0.02 0.07 6 0.013 0.01 0.02
March 1 0.020 7 0.022 0.01 0.05
April 10 0.031 0.02 0.05 9 0.016 0.01 0.02
May 9 0.041 0.02 0.08 6 0.024 0.01 0.06
June 10 0.054 0.04 0.08 3 0.026 0.02 0.04
July 3 0.066 0.03 0.09 2 0.020 0.02 0.02
August 3 0.053 0.04 0.06
September 3 0.150 0.10 0.21 1 0.180
October 1 3.520
November 1 0.050
December 2 0.045 0.04 0.05 1 0.050
January 1 0.050 1 0.020
February
March 3 0.023 0.02 0.03 1 0.010

Seasonal Changes in the Gonads.

In Table V are shown the mean weights of the gonads of the owls for each month. It will be seen that the usual avian changes are shown, the ovaries and testes about February or March being completely regressed and about equal in weight. Up to August the testes remain about the same, while the ovaries increase slightly. Then comes a tremendous increase in size and weight to the maximum in October or November, followed by an equally sudden decrease. The limits of weight are 0.02 and 1.50 grms. in the case of the ovary, and 0.01 and 0.34 grms. in the case of the testes. By comparison with the graph of body weight it is seen that the spring increase in weight of the female accompanies the increase in the ovary. It is not caused directly by it since the ovary only weighs some three grammes, and the mean body weight of the female at this period is at least 30 grammes more than that of the male. The increase in size of the testes is not accompanied by increased body weight in the male, nor are the changes in body weight accompanying the moult reflected in changes in the gonads.

At its smallest, the ovary measures roughly 8.0 × 4.0 mm. The eggs in it vary in circumference in different birds, those in which they are less conspicuous being possibly young birds which have not yet bred. Nine ovaries were recorded with dark spots on them, which

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might have been due to the degeneration of eggs or egg-follicles after breeding. Four individuals were noticed in which the right ovary was present, but it was always small, measuring about 3.0 × 1.5 mm. In two of them, three or four small eggs were visible.

The size of the testes varied between 2.0 × 1.5 mm. and 3.5 × 2.0 mm. for the right and left organs respectively, to 9.0 × 7.0 mm. and 11.0 × 8.5 mm., the left always being larger than the right. Normally the testes are pale coloured, even when in the regressed condition, but four exceptions were noted. In two, one testis was black, in one the testes were dark coloured, and in the other they had black spots on them.

Miscellaneous.

The numbers of the two sexes obtained, including both immature and adult birds, were 114 males and 125 females. This is a sex ratio of 91.5 males to 100 females.

The breeding females had a large bare brood-patch, about 120 × 65 mm. Laterally the knees were bare and the apterion extended up to the axilla. Seven females were received having the brood-patch, one apparently in the middle of laying. One male (15/1/40) had what appeared to be the remains of a brood-patch, and was assumed to be a female until it was dissected.

Six eggs were received, their measurements being as follows:— 33.6 × 28.8, 33.9 × 29.6, 34.7 × 29.0, 35.6 × 28.7, 35.8 × 29.0, 37.6 × 28.7 mm. The first two came from one nest, the rest from another. These measurements are within the limits given by Witherby for 67 British eggs.

Some of the birds were measured. The wing was measured by laying it along a ruler and straightening and flattening it as much as possible, the length being taken from the carpal bend to the tip of the longest primary. The tarsus was measured from the upper side of the base of the middle toe to the back of the heel, the measurement being therefore somewhat greater than the length of the tarsal bone itself. The middle toe was measured from the upper side of the base to the tip of the claw. The beak was measured from the tip to the cere, a region referred to below as the culmen, and from the tip to the feathers, a distance referred to as the beak.

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Table VI.
External Measurements of Owls.
Number Measured. Mean. Limits. Standard Deviation.
mm. mm.
Wing 45 166.7 157 -179 55.90
45 169.1 158 -180 17.97
“Tarsus” 44 39.9 38.0- 42.0 1.050
43 39.8 38.0- 42.0 1.145
Toe 47 30.6 27.0- 33.0 1.220
45 30.6 29.0- 32.0 0.283
Beak 45 19.3 17.0- 21.0 0.238
41 19.2 18.0- 20.0 0.175
Culmen 39 14.8 13.5- 16.0 0.197
36 14.6 13.0- 16.0 0.189
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The details of all these series of measurements are shown in Table VI. The male is in each case more variable than the female. The variation is especially marked in the case of the wing, where the distribution of lengths shows little or no resemblance to the normal distribution curve. It is possible that immature birds may have been included though they were omitted whenever recognisable. There appears to be no difference in size between the two sexes as shown by the mean lengths of these various parts, the greater length of the female wing being obviously not significant. (Actually the probable error of the difference is 5.8, while the actual difference is only 2.4). Witherby gives the following measurements from British specimens: Wing—male 153–165 mm. and female 156–165 mm., there being 20 and 18 specimens respectively. Beak—male, 18–21 mm. The upper limit of the New Zealand wing measurements is considerably higher and the lower limit slightly so, but the numbers are small, and the difference may be accounted for by a slight difference in the method of measuring.

For comparison it may be stated that Hicks, studying the variations of a large number of Starlings, S. vulgaris vulgaris, in Ohio found that the males had a somewhat greater range of size variation than the females for all measurements. Out of 10,000 birds, he found that 5.35% had physical deformities. Only one abnormal owl was found in the present investigation. In it the hind claw, instead of being curved and about 10 mm. long, was quite straight and 7.5 mm. long.

Only one diseased bird was found. It appeared to be in good condition, though perhaps moulting late, and was heavy for the time of year, but the liver and spleen were pale and much enlarged and thickly covered with fatty-looking nodules. There was a good deal of soft yellow fat in the region of the post-hepatic septum, and a fatty adhesion on the right side, apparently at least partially obliterating the thoracic air sac. The testes were rather larger than usual for the time of year but looked normal, as also did the intestines and kidneys. Dr Sutherland, of the Department of Pathology, kindly examined this specimen and reports that it was suffering from tuberculosis, large numbers of tubercle bacilli being visible in the lesions.

Information Received from Correspondents.

In response to appeals in the press for information about owls, some 60 letters were received, and of the considerable number of questionnaires sent out, 26 completed forms were returned. The information from these sources has been analysed below, but it must be remembered that it is evidence of a different nature from that provided by the actual stomach and nest contents. It is accordingly kept in a separate section.

The information is not sufficiently extensive to give a very detailed knowledge of the distribution of the Little Owl, but it is predominently in the south-eastern part of the South Island. It is reported to be present at Puysegur Point, in the south-west, and at Parnassus, in North Canterbury. All the other records except two lie to the south of a line joining the two places, the exceptions being at Pembroke, just west of the line, and at Inchbonnie, in Westland. The more mountainous regions lie to the north of this line, but

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whether the absence of records is due to the absence of owls or of observers is not at present certain. It is stated to have been absent at Lake Kanieri, in Westland, in 1929. It is reported to be common both in bush districts, such as the Catlins, and to the west of Inver-cargill and in the treeless Central Otago. It is also frequently found in the willow trees along the banks of rivers.

In the North Island a pair of owls was liberated at Rotorua about 1910, but soon vanished. There seem to be no specimens from the North Island, and the owl is usually stated to be absent. Four letters have, however, been received stating that it is present in the southern part of the North Island, and the matter requires further investigation. One observer states that it had been present at Te Marua, 23 miles north of Wellington, for four years in 1938, and had been seen eating snails. The Morepork was stated to be also present. Another observer, who had known the bird in England, states that he saw it in 1934 at the mouth of the Otaki River, and in 1935 near the Ohau River. Another records it as not plentiful, but present for 15 years, in the Waitohu Valley, three miles from Otaki, and another possible record is from Palmerston North.

General impressions as to the changes in numbers of birds from year to year are of little or no value, only the results of actual counts are to be relied upon, and none of these have been made. Nine out of 23 informants state that the owl is increasing in their district, but four reports are contradictory. These districts are towards the edge of the inhabited area, while there are three reports of decrease towards its centre. Though little reliance should be placed upon such reports, it may perhaps be suggested that the owl increased and spread rapidly soon after its introduction, and that at present both its numbers and its range are only increasingly slowly. From the majority of places the bird is reported to have been present 15, 20 or even 30 years, and it is only about 35 years since it was introduced.

Collaborators were asked to state whether they had actually seen the owl catch birds. The numbers of reports of different birds seen to be taken were as follows:—Sparrow, six; Blackbirds, four; White Eye, two; Thrush, Goldfinch, Chaffinch, Starling and “Tit,” one each. One observer had seen an owl chasing a sparrow, and another chasing “birds.” One recorded an owl taking the bodies of blackbirds which had been shot, and another an owl feeding on a dead hare.

The nesting sites recorded are as follows:—Hollow trees, 12 hollow logs, rabbit holes and stacks, 4 each; clay banks, 3; crevices in rocks and the walls of old sheds, 2 each; caves, 1. The different numbers of eggs or young in a nest were found the following number of times:—Two eggs or young, 7 times; three eggs or young, 9 times; 4 eggs or young, 10 times; 5 eggs or young, twice. Miss Hibbert Ware states that of 17 records in England, two eggs or young were found 6 times; three eggs or young, 6 times; and 4 eggs or young, 5 times. She also states that there is reason to believe that more eggs were layed some years ago, and that this diminution is connected with the slower rate of increase of the owl compared with that which it had during the first few years after its introduction into England.

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Table VII. List of Vertebrates Mentioned in the Text, with Their Scientific Names.
Frog, Hyla aurea.
Lizard, Hoplodactylus sp.
Morepork, Ninox novaeseelandiae.
Little Owl, Athene nootua.
Pipit, Anthus novaeseelandiae.
“Tit,”? Petroica macrocephala.
Fantail, Rhipidura fuliginosa.
White Eye, Zosterops halmaturina.
Greenfinch, Chloris chloris.
Chaffinch, Fringilla coelebs.
Goldfinch, Carduelis carduelis.
Sparrow, Passer domesticus.
Thrush, Turdus ericetorum.
Blackbird, Turdus merula.
Hedge Sparrow, Prunella modularis.
Lark, Alauda arvensis.
Starling, Sturnus vulgaris.
Mouse, Mus musculus.
Rabbit, Oryctolagus cuniculus.

Summary.

  • 1. The stomach contents of 242 Little Owls, A. noctua, were examined. Remains of 5443 animals, belonging to practically all terrestrial groups, were obtained. Caterpillars and beetles formed the most numerous constituents. Remains of 20 birds, 9 mice, 4 lizards and 8 frogs were found. With two doubtful exceptions, none of the birds were native species.

  • 2. Vertebrate remains were collected from 17 samples of castings and the contents of nests. They contained 82 birds, 16 mice, 6 rabbits and 2 frogs. All the birds were introduced species, with the exception of 1 white-eye and perhaps 7 individuals which were either larks or pipits.

  • 3. Some owls were examined for endoparasites. Only the Nematode C. tenuissima was found.

  • 4. The seasonal variation in weight was studied, the maximum being in June or July.

  • 5. The seasonal variation in gonad weight was studied.

  • 6. The sex ratio was 91.5 males to 100 females.

  • 7. Only one abnormality, a straight claw, and one diseased bird were noted. The diseased individual was suffering from tuberculosis.

  • 8. Measurements of about 45 birds of each sex are given. There is no sexual difference in size.

  • 9. Replies from correspondents with reference to distribution, nesting and feeding are analysed. Out of 17 birds seen to be taken, 3 only were native species, 2 white-eyes and 1 “tit.”

References.

Hibbert, Ware A., 1936. Report on an investigation of the food of captive Little Owls. British Birds, XXIX, pp. 302–305.

—– 1938. Report on the Little Owl Food Inquiry. British Birds, XXXI, pp. 162–264.

Hicks, L. E., 1934. Individual and Sexual Variations in the European Starling. Bird Banding, V, pp. 103–118.

Nice, M. M., 1937. Studies in the Life History of the Song Sparrow. Trans. Linn. Soc. of New York, IV.

Thomson, G. M., 1921. The Naturalisation of Animals and Plants in New Zealand. Cambridge Univ. Press.

Witherby, H. F., etc., 1920. A Practical Handbook of British Birds. London. Witherby and Co.