of Long-eared Owls in north-east Ireland By J. S. Fairley INTRODUCTION AND METHODS ALTHOUGH THE Long-eared Owl Asio otus is the most common owl in Ireland, detailed knowledge of the nature of its food is limited to a single analysis of pellets (L. E. Adams, in Standen 1897). Adams col­ lected these, so far as is known, from the Ballycastle area, Co. , in May 1897. The present study is intended to give a picture of the food of this owl in north-east Ireland as a whole, allowing for possible local differences and to indicate any seasonal variation of the prey. Pellets were collected from the 16 sites listed in table 1. Sites 1 to 6,13, 14 and 16 were in coniferous woodland; site 8 in a peripheral area of pines Pinus spp. in mixed woodland; sites 7,11 and 12 in isolated conifers in mixed woodland strips; site 10 in a series of isolated yews Taxus baccata; site 15 in laurels Laurus nobilis, and site 9 in yews and laurels, in an overgrown garden. Thus all were associated with evergreens and usually with conifers. Times of collection were irregular since some of the sites are some distance from and could be visited only occasionally. In conifer forests pellets were normally found in large numbers around a few adjacent trees and in smaller numbers under those further away. Unfortunately, Long-eared Owls seem to shift their main roosts now and again, and in a large forest or in undergrowth these are consequently difficult to locate. It is certain in some cases that owls left an area for some months. In a few instances, the material was sent to me by other people and thus the precise date of the collection from in 1963 is uncertain. Pellets were dried on a hotplate and then broken up for analysis. The numbers of mice and were estimated from the maximum number of left or right dentaries (lower jaw bones) present in a batch of pellets. Initial counts of crania showed that these never exceeded the number of right and left jaw-bones, probably because of their tendency to fragment and lose teeth. Counts of both crania and jaws were, on the other hand, made for shrews as shrew dentaries are minute and can be missed. Birds were taken as being equal to the highest of three figures which were, in fact, almost always approximately the same: (1) the total number of upper and lower halves of the bill divided by two, the answer being taken to the nearest whole number; (2) the total number of right humeri; and (3) the total number of left humeri. As the prey species are by no means all of the same weight, use was made of 'conversion factors' to create a more realistic picture of the

130 FOOD OF LONG-EARED OWLS IN IRELAND Table i. Contents of pellets of Long-eared Owls Asia otus collected from 16 sites in north-east Ireland, April 1963 to November 1964 The sites were located as follows: Co. Antrim (1) North Cam Forest, ; (2) Magherahoney, Armoy; (3) , ; (4) Moyknock, Portglenone; (5) Eden, ; (6) Shane's Casde, ; Co. Doam(j) Newtownards; (8)Killynether, Newtownards; (9) Carro wdore Castle, Carrowdore; (1 o) Ringdufferin, Toye; (11) Ballymacashen, Killinchy; (12) Saintfield; (13) Castleward, Strangford; (14) Castleward again; (15) Moume Park, Kilkeel; Co. Londonderry (16) Moyvanagher, Kilrea. The 'conversion factor' takes a 20-gram as standard and thus corrects for differences in size (see text)

Site number Wood House Brown Pygmy and date Mouse Mouse Shrew Birds

(7) summer 1963 6 1 4 4 (16) April 1963 5° 1 2 1 3 (7) December 1963 1 1 (8) December 1963 25 2 2 (16) January 1964 267 22 9 6 (1) January 1964 1 1 (9) January 1964 15 (9) February 1964 2 1 2 (8) February 1964 40 1 1 4 (2) February 1964 7 (10) February 1964 40 3 3 2 (11) March 1964 5 2 4 (12) March 1964 74 1 3 (13) March 1964 8 1 (14) March 1964 41 4 7 4 (2) March 1964 28 r 3 (2) March 1964 33 3 3 5 (15) April 1964 9 15 2 1 (13) April 1964 66 4 1 1 (8) April 1964 6 (12) April 1964 6 1 2 (8) May 1964 6 1 (15) May 1964 9 4 1 (3) June 1964 1 1 (16) June 1964 77 3 1 1 (4) JuIy 1964 6 1 (5) July i?64 16 4 1 3 (6) August 1964 33 1 1 1 (16) October 1964 7 4 (5) October 1964 19 4 5 (2) November 1964 23 2 (8) November 1964 2 1 (13) November 1964 30 4 8 4

Total prey 955 79 56 10 57 Conversion factor XI XI X5 X0.2 XI Prey units 955 79 280 2 57 Percentage prey units 7°% 6% 20% — 4%

!JI BRITISH BIRDS total prey each species represents. This method of correcting difference, taking a 20-gram rodent as a standard prey unit, was suggested by Southern (1954) for the Tawny Owl Strix aluco. As pellets were collected at irregular intervals, it was important to obtain some idea of their maximum possible age. To do this a batch was collected from a roost which was in use at the time and, immediately afterwards, placed in another well sheltered conifer plantation and left to decay. No work seems to have been done on the rate of decomposition of the pellets of any owl. It is thought that it is controlled mainly by (a) rainfall, (b) insects feeding on the pellets and (c) also, possibly an important factor, the action of bacteria; freezing may help to break up pellets, but in dry weather and in a sheltered conifer wood frosty nights alone would have little effect. The pellets were observed over a period of eight weeks, from 12th January to 8th March 1965. Over this period one would expect insect activity to be minimal; at the same time the weather was unusually mild and under 4.7 inches of rain fell. At the end, however, they were in an advanced state of decay and were judged to be at least as old as any in the collections. It seems correct to say, therefore, that none of the pellets examined in this analysis had been regurgitated more than eight weeks before. Many were probably fresher than this.

RESULTS The results of the analyses are given in table 1. From these it is clear that Wood Mice syhatuus form the staple food of the Long-eared Owl in north-east Ireland and that the remainder is mostly Brown Rats Kattus norvegkus. House Mice musculus and birds account for only 10%. Pygmy Shrews Sorex tninutus are caught occasionally. There are several accounts of bats having been eaten in small quantities both here (Standen 1897, Ussher and Warren 1900) and in England (Ticehurst 1939), but no bat remains were recovered from the present collection of pellets. In addition to vertebrate food, the remains of beetles (mainly Geotrupes) were sometimes found. The pellets usually had numerous conifer leaves adhering to them: it is possible that some of these may have been originally ingested and this was definitely the case in the pellets from one of the roosts in yews, where yew fruits had also been consumed.

DISCUSSION An attempt was made to assess the seasonal fluctuations in the abundance of the eaten by these Long-eared Owls. The results of the collec­ tions over the periods of two months were considered as follows: (1) December-January (4) June-August (2) February-March (5) October-November (3) April-May

132 FOOD OF LONG-EARED OWLS IN IRELAND The three months of June, July and August were combined because of the lack of material; the results for the last of these months differ little from those of the other two. The corresponding percentages of prey are shown in table 2. The percentages of birds and of House Mice are small and the fluctuations can therefore hardly be regarded as significant. The two major prey species— the and the Brown Rat—do, however, show marked and significant fluctuations. The percentage occurrences for the Wood Mouse, the main food, are substantially the same for periods 1, 2 and 3. The April-May figures differ slightly because of the large numbers of House Mice in the first batch of pellets from Kilkeel (locality ijin table 1). This is the only sample of pellets that contained anything like such a high proportion of House Mice and could be explained by the destruc­ tion of buildings in the vicinity leaving the mice in the open and un­ familiar with their surroundings. In summer the proportion of Brown Rats is low, while in autumn they appear to be the main prey taken. These differences are not correlated with the annual fluctuations in the numbers of Wood Mice, since the latter are most numerous in autumn and scarcest in late spring. They are probably related to the availability of rats, for even a small increase in the numbers available will be reflected in a Long-eared Owl's diet simply because an average-sized rat will pro­ vide about five times as much food for the predator as will an average- sized mouse. A tentative explanation of the seasonal variation in prey indicated here is as follows. During the summer there is heavy cover available in the fields, both from crops and from uncultivated vegetation. In autumn, when crops are harvested and other vegetation is beginning to die down, food and cover become much scarcer. Rats, temporarily unfamiliar with

Table 2. Seasonal fluctuations, in percentage prey units, of the main foods in pellets of Long-eared Owls Asio otus collected from 16 sites in north-east Ireland, April 1963 to November 1964 All the batches of pellets listed in table 1 are included here with the exception of the first. The figures in brackets represent the results if the batch from site 15m April 1964 is omitted (see text)

Total Percentage prey units prey Wood House Brown Period units Mouse Mouse Rat Birds December-January 388 79% 6% 12% 3% February-March 375 74% 4% 16% 6% April-May 230(197) 66% (71%) 9% (3%) 22% (23%) 3% (3%) June-August 158 84% 6% 6% 4% October-November 189 43% 5% 50% 2%

133 BRITISH BIRDS their surroundings, will be moving over strange terrain in search of food and towards farm buildings, where they take up residence in ricks and other places. During this time they must be particularly vulnerable to attack from owls. By the end of the year the feral and farm populations will be more or less stable with the former minimal. The rat population in the open (and therefore available to owls) will increase as the weather improves and as the rats from the farm buildings move out in the fields again. Comparison of the present results with those of Adams (in Standen 1897) is interesting. His material contained almost 508 prey units and his results, expressed in percentage prey units, were Wood Mice 70%, House Mice 1 %, Brown Rats 22%, birds 6%, and bats and shrews 1 %—• strikingly similar to those obtained in the present survey. The food of Long-eared Owls in England has been studied in some detail by Ticehurst (1939) and, more recently, by South (1966). The results of both these authors are summarized, as calculated percentage prey units, in table 3. The former author, who examined material from seven different sites, showed that the proportions of the various prey species varied and that this was, to some extent, correlated with the type of terrain in the locality of each site. Thus, knowing the prefer­ ence of the British small (Southern 1964), it is hardly surprising to find that Field Microtus agrestis predominated where there was plenty of rough , but Wood Mice in plantations and in a park. Again, Bank Voles Clethrionomysglareolus were, most commonly found in the pellets of owls inhabiting plantations. South's material, which was obtained from coastal pine forest and mossland woods, contained more Wood Mice and Bank Voles and less Field Voles than that of Ticehurst, which was largely from owls roosting in fir coverts and hunting over more open and often cultivated ground.

Table 3. Summary, in percentage prey units, of analyses by Ticehurst (1939) and South (1966) of pellets of Long-eared Owls Asia otus in England Ticehurst South

Field 28% «% Bank Vole 2% 9% Water Vole 2% — Wood Mouse 25% 53% Brown Rat 26% 3% present Bats } 1% Pygmy Shrew — present Birds 16% 13%

Approximate total prey units c. 1,307 c. 717

134 FOOD OF LONG-EARED OWLS IN IRELAND South's study was carried out in April and May but, unfortunately, Ticehurst has indicated only 'summer' or 'winter' for times of collection. So far, therefore, information about possible seasonal variations in the prey of Long-eared Owls in England is incomplete. The differences between the English and Irish results are largely explained by the complete absence of voles from north-east Ireland, although the figures for House Mice are higher and for birds lower than one would have expected. This absence of voles, too, probably accounts for the consistency which, apart from the variation in, roughly, the latter half of the year, marks the results in the present study. The chief prey in Ireland, Wood Mice and Brown Rats, are much less specialised than voles and consequently occupy a much wider spectrum of habitat. The proportion of rats will be affected by the proximity of human dwell­ ings and water. This is illustrated by the high proportion of rats in the admittedly small sample from Newtownards; the roost in this case was in a woodland strip on the edge of marshy ground adjoining a built-up area.

ACKNOWLEDGEMENTS I should like to thank all those who made this Work possible by telling me of roosts, by helping me to collect pellets or by actually collecting pellets: J. V. Bateman, Arnold Benington, R. Carlisle, C. D. Deane, M. Gilbertson, G. Greaves, A. Green, J. Mackie, W. Stewart and especially A. McLean. Finally, thanks are due to Professor R. A. R. Gresson and Professor G. Owen for facilities provided in the Depart­ ment of Zoology at the Queen's University of Belfast.

SUMMARY Collections of pellets of Long-eared Owls Asio otus were made in Cos. Antrim and Down in north-east Ireland and analysed for prey content. Wood Mice (70% by estimated weight) were the main prey and Rats Katlusnorvegicus'(20%) the second most important. House Mice Mus mmculus (6%), birds (4%) and, rarely, Pygmy Shrews Sorex minutus were also taken. The seasonal fluctuation in prey species is indicated and the results compared with those available from England.

REFERENCES SOUTH, G. R. (1966): 'Food of Long-eared Owls in south Lancashire'. Brit. Birds, 59: 493-497- SOUTHERN, H. N. (1954): 'Tawny owls and their prey'. Ibis, 96: 348-410. ——•——• (1964): The Handbook of British Mammals. Oxford. STANDEN, R. (1897): 'Some observations by English naturalists on the fauna of and Ballycastle district. 1. General observations'. Irish Nat., 6: 173-178. TICEHURST, C. B. (1939): 'On the food and feeding-habits of the Long-eared Owl (Asio otus otus).' Ibis, 1939: 512-520. USSHER, R. J., and WARREN, R. (1900): The Birds of Ireland. London.

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