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

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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. 1 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 3 Crynodeb Gweithredol 1 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. 2 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 3 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 4 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.
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