The Diet of the Little Owl on Skomer Island NNR 1998 – 2003

The Diet of the Little Owl on Skomer Island NNR 1998-2003

Jane Hayden

Contract Science Report Number XXX

This is a report of research commissioned by the Countryside Council for Wales. The Council has a programme of research in scientific and other areas, which supports the development of policies and practical work and helps point the way to new countryside legislation. However, the views and recommendations presented in this report are not necessarily those of the Council and should, therefore, not be attributed to the Countryside Council for Wales. No part of this report may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the Countryside Council for Wales.

Report number: XXX Publication date: December 2004 Contract number: FC 73-01-457 Nominated officer: Catherine Gray Title: The Diet of the Little Owl on Skomer Island NNR 1998-2003 Author: Jane Hayden Restrictions: None Distribution list CCW: HQ Library, Bangor N Region Library, Bangor N Region Library, Mold SE Region Library, Cardiff W Region Library, Aberystwyth W Region Library, Pembroke Dock Catherine Gray, Bangor Charlotte Gjerlov, Aberystywth Andrea McConnell, Pembroke Dock David Worrall, Pembroke Dock Mike Alexander Roger Lovegrove, CCW Council Member

EN: EN HQ Library Peterborough

SNH: SNH Library, Edinburgh

JNCC: JNCC Library, Peterborough JNCC Library, Aberdeen

Other: National Library of Wales, Aberystwyth British Library, Document Supply Centre, Boston Spa National Assembly of Wales Library, Cardiff Madeleine Havard, Wildlife Trust for South and West Wales Juan Brown, Wildlife Trust for South and West Wales Graham Thompson, Wildlife Trust for South and West Wales Chris Perrins, EGI, Oxford Reg Thorpe, RSPB Wales, Bangor Mick Green, Ecology Matters, Aberystwyth Roy Leigh, Northwich

Recommended citation for this volume: Hayden, J.E. 2004. The Diet of the Little Owl on Skomer Island NNR 1998-2003. CCW Contract Science Report no XXX

The Diet of the Little Owl on Skomer Island NNR 1998 – 2003

Jane E Hayden, 2004

Crynodeb Gweithredol

Executive Summary

1. The diet of little owls on Skomer has been studied since 1998, to assess levels of predation on storm petrels. This study was continued in 2003, together with a census of the Skomer storm petrel population.

2. Known little owl territories and new nest sites were searched for. Pellets were collected from nest and roost sites from May to November, although none were found in the final month.

3. Three or four pairs of little owl attempted to breed in 2003, but none were seen to successfully fledge chicks. It is possible that increased human disturbance played a part in this, as other years where studies of diet have taken place have also shown lower productivity figures. This should be taken into account in further studies.

4. Seventy-two pellets were collected from four territories and were analysed to provide a sample of little owl diet in 2003. Birds and mammals contributed the vast majority of biomass to little owl diet, contributing 43.5% and 41.7% respectively. Bird prey consisted of Manx shearwater (25.1%), storm petrel (13.9%) and meadow pipit (4.6%). Two mammals species were taken in 2003, the Skomer vole (21.7%) and wood mouse (20.0%). Herptiles and invertebrate prey were found in small quantities.

5. Data on little owl diet has been collected for the last six years. Comparing the six years of data showed that the bulk of diet by biomass consists of mammals and birds in all years, though the relative importance of prey types varied from year to year.

6. Little owls predated the storm petrel in each year, and storm petrel accounted for, on average, 14.8% of little owl diet by biomass. This equates to a mean estimate of 192 storm petrels predated per year. It was shown in two years that the little owl predated birds from the breeding population of petrels on Skomer, but the status of predated birds was not known in other years. However, as storm petrels have been predated at a fairly constant rate over the last six years, and at least some of these were breeding on the island, it is suggested that discussions upon future management of the little owl on Skomer should be reopened.

Contents

Crynodeb Gweithredol 1 Executive Summary 2 Contents 3

1 Introduction 4 1.1 The Study Species 4 1.2 The little owl – storm petrel problem 5 1.3 Aims and objectives of this Report 5

2 Methods 5 2.1 Pellet Collection 5 2.2 Pellet Analysis 6 2.3 Data Analysis 6

3 Results 8 3.1 Breeding Success of the Little Owl in 2003 8 3.2 Little Owl Diet in 2003 8 3.3 Comparison of Little Owl Diet 1998 – 2003 11 3.4 Storm Petrel Predation by the Little Owl 1998 – 2003 12

4 Discussion 13 4.1 Breeding Success and Diet of the Little Owl in 2003 13 4.2 Little Owl Diet 1998 – 2003 14 4.3 Storm Petrel Predation by the Little Owl 1998 – 2003 14

5 Recommendations 15

Acknowledgements 17 References 18 Appendix 1 – Short-eared Owl Diet on Skomer Island NNR 19 Appendix 2 – Barn Owl Diet on Skomer Island 19

1. Introduction

1.1. The Study Species The little owl Athene noctua is the smallest breeding owl to be found in Britain. It has a wingspan of 54-58cm, with males weighing 140-190g and females weighing 150- 220g (Cramp, 1985). It is not native to this country. It was first introduced in 1843 to control garden pests (Hibbert-Ware, 1938), though this attempt failed. Further attempts were successful, and the owl spread widely from 1900 onwards, and is now common through England and Wales (Altringham et al, 1994). The little owl probably became established on Skomer and the neighbouring Skokholm in the mid-1920s (Smith and Sutcliffe, 1999).

In Britain the little owl prefers open country with hedges, copses and orchards, and nests most commonly in hollow deciduous trees (Glue and Scott, 1980). Due to a lack of trees on Skomer, the owls nest in holes in the walls of the old field system remaining from when the island was farmed, or in rocky areas near to the coast (pers obs).

Since 1960, estimates of numbers of breeding pairs on Skomer have been made in most years. There was one pair recorded annually from 1960-1964, but then no pairs recorded from 1965-1976. From 1977 –2002 numbers have varied from one to six pairs. Table 1 shows estimates of little owl breeding success for the years when such information was recorded. Little owls have, however, been difficult to census with any certainty on Skomer and these figures should be taken as minima. Data are summarised from Smith and Sutcliffe (1999) and from Skomer NNR annual reports.

Table 1. Minimum Estimates of Breeding Success of Little Owls on Skomer Year Minimum No. of Breeding Minimum No. Productivity Pairs Fledged (minimum) 1986 3 5+ 1.7 1988 3 2+ 0.7 1989 5 6 1.2 1995 4 4 1.0 1996 3 4+ 1.3 1997 3 7 2.3 1998 3 1 0.3 1999 3 4+ 1.3 2000 2+ 0 0.0 2001 1+ 0 0.0 2002 2+ 3 1.5

1.2. The little owl – storm petrel Problem This project was initiated to investigate the status of the little owl on Skomer Island and its impact that on the island’s population of storm petrels Hydrobates pelagicus. The storm petrel is one of the qualifying features of Skomer and Skokholm SPA (Stroud et al. 2001).

The first project of recent years to assess little owl diet was carried out in 1998 (Hayden 1999). This study showed that the storm petrel made up 15% of the owls diet by biomass (see 2.3. for methods). Sutcliffe’s earlier studies in 1988 and 1989 (Sutcliffe 1990) also showed significant levels of predation, and confirmed that, at least in two years, storm petrels from the Skomer breeding population were predated. This led to discussions of whether little owls should be removed from the island (Smith and Sutcliffe 1999). The conclusion was that there was still too little information available to be able to make well-informed decisions about removal at that time.

Studies of little owl diet continued until 2002, and showed that predation of storm petrels by little owls occurred annually since 1998 (Hayden 2003). However, it remained unclear whether this predation occurred at a level detrimental to the island’s storm petrel population, and further work was needed on both little owls and the storm petrels. This has led to a further year of little owl study, and a full and detailed estimate being made of the storm petrel population (Brown in prep).

1.3. Aims and objectives of this Report The aim of this project is to provide information on the diet of the little owl to allow informed decisions to be made on how best to manage the little owl on Skomer.

The objectives of this report are therefore: i. To provide data on the numbers, breeding success and diet of little owls in 2003. ii. To collate all current information upon little owl diet, and use this to assess the impact of little owls on the storm petrels on Skomer. iii. In light of this information, if appropriate, to provide recommendations for future research

2. Methods

2.1. Pellet Collection In early May 2003 known little owl territories were visited and possible nest or roost sites located. Potential new sites were also searched for. Pellets found on the first visit of the year were collected and discarded. This ensured that all pellets collected were definitely produced in 2003. Further visits and observations were made to confirm whether nests were active. Monthly visits were made to collect pellets from May to November. Pellets were also collected from storm petrel sub-colonies during the census.

All pellets collected were air-dried and placed individually into plastic bags. These bags were marked with the date and the site, and then kept in a safe place until leaving the island.

2.2. Pellet Analysis Each pellet was analysed individually using a hand lens. The matrix type was noted first. If it was largely fur or feather, the pellet was gently teased apart by hand and relevant material was carefully extracted. Using Yalden and Morris (1990) as a guide, small mammals were identified by their jaws and skulls. Numbers of left jaw, right jaw or skull were noted, the highest number of any one of these being taken as the minimum number of individuals (Yalden, 1985). Birds were identified and scored in this analysis from vertebrae, humeri and metatarsals. Special care was taken to ensure that each individual was only scored once, even if its remains were found in two separate pellets. Amphibians were identified by their bones, and reptiles by their jaws.

Pellets containing mostly invertebrates were crumbled between the fingers into a Petri dish. Parts used for identifying prey were separated and counted. These were the head cases of Coleoptera, Dermaptera and Hymenoptera, Woodlice ‘scales’ and Lepidoptera larvae mandibles. Invertebrate pellets were also searched for earthworm chaetae, though none were found. It is possible that some could have been missed as a hand lens was used rather than a microscope, but the sandy nature of the matrix often gives away their presence and so encourages close analysis.

2.3. Data Analysis Total numbers of each prey item found in the pellet sample are shown in the results tables, and these figures are shown as a percentage of the total sample. This gives an idea of the relative importance of each prey type, by number eaten, to overall diet.

Analysing the data using number of prey only, however, would be highly misleading. Little owl prey varies in size from ants to Manx shearwaters, and the relative biomass that these items contribute must be taken into account. Therefore, the biomass of each prey type is multiplied by the total number of that prey type found, to give an estimate of the total biomass contributed by each prey type. Converting this number into a percentage of the total sample biomass allows the relative importance of each prey type in overall diet to be assessed (Yalden & Morris 1990).

Larger food items, such as Manx shearwaters or rabbits, were treated slightly differently. This is because of the possibility that the owls eat only a proportion of the prey, due to its large size. After ensuring that all pellets from the relevant date had been analysed (as bones from large prey items are often found in several pellets), and all bones from an individual collected together, an estimate was made of the proportion of the prey eaten, depending on the number of bones present.

Table 2. shows the estimates of biomass used for each prey item, along with the source of the information. The table lists all the invertebrate prey types found, though these are grouped under larger headings in the results.

Table 2. Estimates of biomass for each prey item, with source of information

Species Estimated Biomass Source (g) C.g. skomerensis 25 Extrapolated from Fullager et al (1963) A. sylvaticus 18 Yalden and Morris (1990) P. puffinus Various Estimated using reference collection O. cuniculus Various Estimated using reference collection H. pelagicus 28 Cramp and Simmons (1997) A. pratensis 18.5 Cramp (1988) B. bufo 0.75/3.2 Wheater (1985) L. vivipara 2.5 Avery (in Altringham et al, 1994) Nicrophorus 0.29 Hayden (1999) Carabidae 0.2 Yalden and Warburton (1979) Geotrupes 1.0 Yalden and Warburton (1979) Weevils 0.1 Hayden (1999) Chrysomelidae 0.1 Hayden (1999) Elateridae 0.1 Hayden (1999) Aphodius 0.14 Hayden (1999) Staphylinidae 0.1 Yalden and Warburton (1979) Lepidoptera larvae 0.8 Yalden and Warburton (1979) Dermaptera 0.067 Hayden (1999) Formicidae 0.1 Hayden (1999) Woodlice 0.3 Hayden (1999)

3 Results

3.1. Breeding Success of the Little Owl in 2003 Figure 1 (page 9) shows the known little owl territories in 2003, and also includes known territories from previous years, which are not thought to be in use at present.

It is thought unlikely that any of the pairs in 2003 fledged chicks. The nest at Calves Park was found to have two eggs in May. Later inspection showed no eggs to be visible, but late in the season, a cold and broken little owl egg was found at the entrance to the nest. The Well Field nest site was active for much of the season, and it was thought likely that young chicks were present, but none were seen to fledge. This territory is highly visible and it is unlikely that fledging would have been missed. It is still not certain whether the other two territories marked on the map were actually separate pairs, although it is strongly suspected to be so. Both areas showed presence of little owls throughout the season, but neither showed any evidence of chicks hatching or fledging.

3.2. Little Owl Diet in 2003 A total of 72 pellets were collected and analysed in 2003. Table 3 shows the monthly breakdown of pellet collection. Table 4 shows the breakdown of pellet collection by site. Six of the pellets collected could not be assigned to a specific territory; three from storm petrel colonies, three from north of Moorey Mere hide, and so are noted here as unknown territory.

Due to the low numbers of pellets collected in each month and from each site, changes in diet through the year and differences between sites were not analysed.

The diet of the little owls in 2003 is summarised in table 5. As expected, Coleoptera were the most numerous prey, (81.0% of prey items). However, as discussed in section 2.3, biomass should be used to give a realistic idea of the importance of each prey type in owl diet.

Converting prey numbers to biomass shows that in 2003 the vast majority (85.2%) of little owl diet consisted of birds (43.5%) and mammals (41.7%). Bird prey consisted of Manx shearwater (25.1%), storm petrel (13.9%) and meadow pipit Anthus pratensis (4.6%). Only two mammals were represented in diet, Skomer vole Clethrionomys glareolus skomerensis (21.7%) and wood mouse Apodemus sylvaticus (20.0). Herptiles and invertebrate prey were present in small proportions. Figure 2 shows diagrammatically the importance of the different prey items by biomass.

Next page: Figure 1. Map of Skomer Island showing current known little owl territories (in red) and previously used little owl territories (in green). A dotted line joining two territories denotes that there is uncertainty as to whether they are separate territories.

Table 3. Number of pellets Table 4. Number of pellets collected per month in 2003 collected from each site in 2003

Month No. Collected Territory No. Collected May 6 End Wall Ridge 17 June 28 Skomer Head Cave 24 July 13 Well Field 16 August 18 Calves Park 9 October 7 Unknown Territory 6

Table 5. Summary of little owl diet on Skomer Island in 2003 Prey Categories No. Individuals % Total Number Biomass % Total Biomass A. sylvaticus 9 2.6 162 20.0 C.g. 7 2.0 175 21.7 skomerensis TOTAL 16 4.7 337 41.7 MAMMALS H. pelagicus 4 1.2 112.0 13.9 A. pratensis 2 0.6 37.0 4.6 P. puffinus 3 0.9 202.6 25.1 TOTAL BIRDS 9 2.6 351.6 43.5

Amphibia 12 3.5 16.4 2.0 Reptilia 3 0.9 7.5 0.9 TOTAL 15 4.4 23.9 2.9 HERPTILES Coleoptera 277 81.0 90.4 11.2 Other Inverts 25 7.3 5.6 0.7 TOTALS 342 100.0 808.3 100.0

Figure 2. Prey composition of little owl diet in 2003 by % of total biomass

Puffinus puffinus 25.1% Apodemus sylvaticus 20.0%

Anthus pratensis 4.6%

Hydrobates pelagicus 13.9% C.g. skomerensis 21.7%

Coleoptera 11.2% Herptiles 3.0% Other Invertebrates 0.7%

3.3. Comparison of Little Owl Diet 1998-2003 Table 6 shows a summary of little owl diet from 1998-2003. Diet is expressed as % of total biomass only, as this is the most meaningful way to assess the data. The mean values and standard deviation are also shown. Little Owl diet shows much variation from year to year, but in all years the vast majority of little owl diet is made up of mammals and birds. Other food items were found in small amounts, except in 2002 when Coleoptera made up 22.5% of the prey biomass.

Table 6. Summary of little owl diet 1998-2003 shown as % of total biomass

1998 1999 2000 2001 2002 2003 Mean Standard n=79 n=50 n=37 n=13 n=105 n=72 Dev.

Mammals 25.6 36.3 88.7 82.6 45.6 41.7 53.4 25.9

Coleoptera 5.4 7.9 3.7 5.2 22.5 11.2 9.3 7.0

Other 0.8 0.2 0.1 0.0 1.3 0.7 0.5 0.5 Invertebrates Herptiles 0.3 7.6 0.2 0.0 0.9 3.0 2.0 3.0

Storm Petrel 15.2 13.0 7.7 12.2 26.7 13.9 14.8 6.4

Other Birds 52.8 35.1 0.0 0.0 2.9 29.7 20.1 22.3

3.4. Storm Petrel Predation by the Little Owl 1998-2003 Figure 3 shows diagrammatically the importance of storm petrel in little owl diet annually from 1998 to 2003. In only one year has predation dropped below 10% of total biomass. The mean level of predation over the six-year period is 14.8% (±6.4) of total biomass taken.

Figure 3. The importance of the storm petrel in little owl diet 1998-2003, as percentage of total biomass

30 omass) i 25 l B a t o t 20 (% of 15 in diet ent 10 el cont r t

e 5 orm P t

S 0 98 99 00 01 02 03 Year

Table 7 show estimates of the total biomass of storm petrel consumed by little owl in each year 1998-2003, and an estimate of the number of storm petrels that this equates to. The mean across all years was 192 (SD±93.7) storm petrels per year. Several assumptions are made in this calculation: • That storm petrels are present on the island for 4 months (120 days) • That little owls consume 65g of food per day (Dyczkowski and Yalden, 1998, taken from data on captive and free-ranging owls) • That the storm petrels consumed had an average biomass of 28g • That the annual data used in the calculation is a fair representation of overall little owl diet in that year

The equation to calculate the amount of storm petrel biomass consumed is: (120 x (65 x a) / 100) x b Where a = the number of pairs of little owls x 2 b = the proportion of storm petrel in overall diet The number of storm petrels this equates to is calculated by dividing this figure by 28.

Table 7. Annual estimates of biomass and number of storm petrel consumed by the little owl 1998 - 2003

1998 1999 2000 2001 2002 2003 Mean St. Dev Biomass 7114 6084 2402 1903 8330 6505 5389.8 2624.0 Consumed (g) Number 254 217 86 68 298 232 192 93.7 Consumed

4. Discussion

4.1. Breeding Success and Diet of the Little Owl in 2003 The apparent failure of the little owls to fledge any chicks in 2003 is interesting. In four of the last six years breeding success was below 1.0 chick per pair, with apparently no chicks per pair in three years. Prior to 1998, productivity dropped below 1 chick per pair in only one year, and this was in 1988.

It is possible that the increased human disturbance in these years had a direct impact on little owl breeding success, and this should be taken into account in future studies. However, it should also be noted, as stated in the introduction, that little owls are difficult to monitor, and it is difficult to be certain of breeding success figures.

The number of pellets collected (72) is large enough that overall diet should be well represented in the sample. Collection was spread fairly evenly across sites, so there should be no bias towards a particular pair. Fewer pellets were collected in May and October than June to August, and none were found in November, so diet may better represent summer diet than spring or autumn.

That birds and mammals make up the bulk of diet by biomass is not a surprising result, and both groups contributed a similar amount to diet in 2003. Of the Manx shearwaters taken, one was an adult bird, but only a small part of its remains were found, and this was accounted for in the biomass estimate. Presumably the bird was taken as carrion. The other two were small chicks, and in these instances many of the bones were found, so the owls had eaten these whole. The chicks were estimated to have been 40g and 100g, and it is likely that the owls entered the burrows, since Manx shearwater chicks are not mobile at that size (C. Gray pers. comm). Storm Petrels were again predated in 2003, accounting for 13.9% of the diet by biomass.

In 2003 a note was made of pellets that contained non-prey items. Four pellets contained small stones, and four pellets contained mollusc shells, all of which were from terrestrial snails. Other papers have reported similar findings, even including the presence of marine molluscs in pellets (e.g. Hayden 1999, Heaver 1987). The stones are most likely picked up as a result of the ground feeding habit when taking invertebrate prey; indeed they were usually found in invertebrate pellets. The role of the molluscs is unclear. It could be that the shells provide a calcium supplement to the diet, but it is also possible that these items were taken as prey when the molluscs are still alive.

4.2. Little Owl Diet 1998 – 2003 The collection of data upon little owl diet over the last six years showed that mammals and birds made up the bulk of diet by biomass in all years. This is shown in Figure 4. Birds and mammals together contributed over 80% of biomass in all but one year.

Figure 4. Diagram showing proportions of birds and mammals in little owl diet

120

100

80 by biomass Mammals 60 diet Birds age of 40 rcent e P

0 98 99 00 01 02 03 Year

There is large variation from year to year in the relative proportion of birds and mammals, presumably reflecting the opportunistic nature of the little owl, but clearly these larger prey items are of greatest importance in the diet. However, in the two years where mammal prey was at its highest (2000 and 2001), the sample sizes for pellet analysis were low (see table 6, page 10), and so these samples may not have been fully representative of little owl diet. Indeed only in these two years were no other birds except the storm petrel recorded in the pellets.

Diet does not differ from year to year in presence or absence of birds, mammals or invertebrate prey, but does differ in the relative importance of these prey items (see table 6, page 10). This probably reflects the opportunistic nature of little owls, and does seem to be a common feature of little owl diets, with several authors reporting this finding (e.g. Hibbert-Ware 1938, Heaver 1987).

4.3. Storm Petrel Predation by the Little Owl 1998-2003 Storm petrels were predated in 2003, the sixth consecutive year. Over the six-year period, storm petrel has contributed a mean of 14.8% (± 6.4) diet by biomass. It is worrying that storm petrels are predated by little owls annually, and that they form a

significant part of the diet. It seems unlikely that the little owl will suddenly cease predation of storm petrels without cause, unless the owls are lost from the island altogether. Continued low breeding success could presumably lead to this, but there is no evidence as yet that this is happening.

An estimate can be made of the actual number of storm petrels the little owls have predated in each year. This is shown in table 7 (page 13). This calculation suggests that little owls could predate an average of 192 (±93.7) storm petrels each year. However, this is a somewhat crude estimate, and is based on several assumptions. Most of these are reasonable; the estimates of storm petrel weight and food intake of the little owl come from the literature, and the assumption that the storm petrel is present on the island for 120 days is, if anything, somewhat cautious. But it is also necessary to assume that the annual analysis presented here is representative of little owl diet in each year. It has already been noted (page 14) that in two years the sample size was very low, and so there can be no certainty of a representative sample in these years. Importantly, it is in these two years that storm petrel predation is recorded at its lowest, suggesting that the figure of 192 storm petrels per year may even be on the conservative side.

This average annual predation level appears to be very high, particularly when it is considered that the current breeding population on Skomer stands at just 206 pairs (Brown 2004). However, it remains unclear what proportion of storm petrels predated do in fact belong to Skomer’s breeding population, or are even prospecting, and whether this could have a long term effect on the breeding population (Hayden 2003). It could be that the vast majority of the birds taken from Skomer are merely wandering or migrating birds. However, this is unlikely, as it is the breeders and prospectors that are likely to spend the most time around the colonies and so are more likely to be predated.

In 2003 all the storm petrels predated were found in pellets in July and August, and no BTO rings were found. However in other years it has been shown that little owls did take birds from the Skomer breeding population (Hayden 2003, Sutcliffe 1990), though there is no certainty that the predated birds were actually breeding that year.

5. Recommendations

Although 3 to 4 pairs of little owl attempted to breed on the island in 2003, none apparently fledged chicks. This could be due to increased disturbance around the nest site due to the intensive study carried out in 2003, and this should be taken into account in future studies.

2003 was the sixth consecutive year in which little owls were found to predate storm petrels on Skomer. Smith and Sutcliffe’s 1999 report suggested that a lack of good scientific information at that time meant that no further decision could be made with respect to little owl management on Skomer. It would seem that a position has now been reached where good information is available, with data on little owl diet showing annual predation. This does not prove that little owls predate breeding birds annually,

but did show this in some years, and as storm petrel predation has remained at a similar level over the years it seems possible that it is the case in all years.

Although there is now a strong case for reopening discussions upon the removal of the little owl from Skomer, there is still a need for further research while these discussions take place. This research is detailed below:

Little owl • Continuation of annual estimates of numbers of breeding pairs, breeding success, and diet • Colour ringing of chicks and adults could give an idea of movements, hunting areas, and chick dispersal (although this would obviously be limited to diurnal observations). • Radio tracking could provide vital information on where the little owls go by night, and could also lead to the discovery of new storm petrel sub-colonies • Detailed information on the mainland populations should be obtained, and movements between the island and the mainland, to ensure that potential removal is realistic

Storm petrel • Continuation of the population census • Collection of information on movements of storm petrels between the two islands

Other species Both the short-eared owl Asio flammeus and herring gull Larus argentatus have been shown to predate the storm petrel in recent years, (see Appendix 1 in this report, Hayden 1999 and Rogers 2001) though apparently neither at the same level as shown in the little owl. Further research should be carried out on these species, both before and after any removal of little owls, to monitor the situation. Only small sample sizes have been used in both cases so far, therefore it would be worthwhile carrying out a more systematic study on these and other potential predators of the storm petrel, including the greater black-backed gull Larus marinus. Short-eared owls are features of the SPA, and care should be taken to ensure that research does not affect the breeding success of these or other predator species.

Acknowledgements The Countryside Council for Wales (CCW) funded this project, through sub-contract to the Wildlife Trust for South and West Wales. I am grateful to both.

The first year of study into little owl diet was carried out as part of my BSc, and a huge thank you must go to my project supervisor Dr. Yalden, who taught me how to carry out the awful task of little owl pellet analysis! Without his careful teaching and checking of my work in that first year, I am sure that much of the detail found in these pellets would have been missed.

Catherine Gray provided valuable comments on earlier drafts, and showed extreme patience while waiting for the final draft to be produced!

Juan Brown provided much advice throughout the running of this project, and collected short-eared owl pellets. Jane Matthews collected pellets from storm petrel colonies, and Mike Olczyk searched for and collected pellets in October and November when I was absent from the island.

Many thanks to Ian Porter for the free loan of a room in his house to carry out the pellet analysis, and for the loan of a laptop computer.

References Altringham, J.D., O’Brien, S. and Julian, S. (1994). Feeding Ecology of the Little Owls (Athene noctua) at an upland site in Northern England. The Naturalist, 119: 81 – 94. Brown, J.G. (in prep). Storm Petrel Census on Skomer Island in 2003. Countryside Council for Wales contract science report. Cramp, S. (1988). Handbook of the Birds of Europe, the Middle East and North Africa. The Birds of the Western Palearctic, Vol. V. Oxford University Press, Oxford. Cramp, S. & Simmons, K.E.L. (1977). Handbook of the Birds of Europe, the Middle East and North Africa. The Birds of the Western Palearctic, Vol. 1. Oxford University Press, Oxford. Dyczkowski, J. & Yalden, D.W. (1998). An estimate of the impact of predators on the British Field Vole Microtus agrestis population. Mammal Review 28: 165 – 184. Fullager, P.J., Jewell, P.A., Lockley, R.M., & Rowlands, I.W. (1963). The Skomer Vole (Clethrionomys glareolus skomerensis) and Long-tailed Field Mouse (Apodemus sylvaticus) on Skomer Island, Pembrokeshire in 1960. Proc. Zool. Soc. Lond. 140: 295 – 314. Hayden, J.E. (1999). The Diet of the Short-eared Owl (Asio flammeus) and Little Owl (Athene noctua) on Skomer Island NNR. Unpublished dissertation for Manchester University. Hayden, J.E. (2003). The Little Owl/Storm Petrel Problem on Skomer: The diet of the Little Owl on Skomer Island 1998 – 2002. Unpublished report for the Wildlife Trust of South and West Wales. Heaver, D.J. (1987). The Diet of the Little Owl on Ynys Enlli. Bardsey Island Observatory Report 1986. Hibbert-Ware, A. (1938). Report of the Little Owl enquiry. British Birds 31: 162-187, 205-229, 259-264. Rogers, D. (2001). The Diet of the Little Owl (Athene noctua) on Skomer Island NNR in 2000. Unpublished report for the Wildlife Trust, West Wales. Smith, S. & Sutcliffe, S.J. (1999). Little Owls and Storm Petrels, Skomer Island NNR. Unpublished report for the Wildlife Trust, West Wales Islands Conservation Committee. Stroud, D.A., Chambers, D., Cook, S., Buxton, N., Fraser, B., Clement, P., Lewis, P., McLean, I., Baker, H. & Whitehead, S. (2001). The UK SPA network: its scope and content. Joint Nature Conservation Committee. Sutcliffe, S. (1990). The Food of the Little Owls on Skomer Island. Pembrokeshire Bird Report 1989: 24-26. Wheater, C.P. (1985). Size Increase in the Common Toad Bufo bufo from Cheshire. Herpetological Journal 1: 20 – 22. Yalden, D.W. (1985). Dietary Separation of Owls in the Peak District. Bird Study 32: 122 – 131. Yalden, D.W. & Morris, P.A. (1990). The Analysis of Owl Pellets. Occas. Pubs. Mamm. Soc. No. 13. London. Yalden, D.W. & Warburton, A.B. (1979). The Diet of the Kestrel on the Lake District. Bird Study 26: 163 – 170.

Appendix 1 – Short-eared Owl Diet on Skomer Island NNR

The table below summarises the diet of the short-eared owl from 1998 – 2003, excluding 2000, as no data were available. Data under the annual headings is the total biomass of each prey item found in that years sample. This is totalled in the ‘All Years’ Column, and then % of total biomass is calculated in the final column. The data has been shown in this way as the yearly sample sizes are very low, but combined give a sample size of 55, which although is still low is much more respectable.

It can be seen from the table that the short-eared owl has predated the storm petrel in two years, 1998 and 2003. However, the overall importance of storm petrel in the owls diet is low, accounting for just 2.9% of diet by biomass. It would seem therefore that any role that the storm petrel plays in short-eared owl diet is minor. Continued monitoring of short-eared owl diet is, however, recommended.

The Diet of the Short-eared Owl on Skomer Island NNR 1998 – 2003 98 99 01/02 03 All Years % Biomass n=14 n=10 n=28 n=3 n=55 98 - 03 Skomer Vole 325 375 675 0 1375 47.3 Wood Mouse 18 18 36 36 108 3.7 Pigmy Shrew 8 4 0 0 12 0.4 Common Shrew 24 48 0 0 72 2.5 Rabbit 820 0 410.8 0 1230.8 42.4 Total Mammals 1195 445 1121.8 36 2797.8 96.3 Storm Petrel 28 0 0 56 84 2.9 Meadow Pipit 18.5 0 0 0 18.5 0.6 Total Birds 46.5 0 0 56 102.5 3.5 Geotrupes 0 2 0 3 5 0.2 1241.5 447 1121.8 95 2905.3

Appendix 2 – Barn Owl Diet on Skomer Island Eight pellets were collected and analysed in 2003. A summary of this analysis is shown in the table below.

Diet of the Barn Owl on Skomer Island NNR 2003 No % no Bio % bio

S. araneus 7 33.3 56.0 16.3 S. minutus 1 4.8 4.0 1.2 A. sylvaticus 6 28.6 108.0 31.5 C. g. skomerensis 7 33.3 175.0 51.0 21 100.0 343.0 100.0