Abstract from BTO Research Report No. 496:

J. Calladine, G. Garner & C. Wernham (2008)

Developing methods for the field survey and monitoring of breeding Short-eared owls (Asio flammeus) in the UK: Final report from pilot fieldwork in 2006 and 2007. ISBN 978-1-906204-32-7

INTRODUCTION

BACKGROUND
The short-eared owl is listed on Annex 1 of the EU Wild Birds Directive and is a qualifying species for six classified Special Protection Areas in the UK. There is a need to determine the appropriate methodology for national surveys of UK populations and to facilitate the most accurate and precise population estimates for the species within designated SPAs in the UK.

This report details the results of a two-year study during the 2006 and 2007 breeding seasons. The aims of the study were: (i) To identify the most appropriate times of day and stages of breeding for surveying and to translate the findings into feasible survey protocols; (ii) To carry out observations to assist in the differentiation of separate breeding territories and to translate these findings into feasible survey and analytical protocols; (iii) To evaluate the feasibility of, and encounter rates resulting from, more extensive field surveys and how these relate to numbers of breeding short-eared owls estimated from intensive observations in key study areas; and (iv) To make recommendations both for field protocols for use in producing a population estimate of short-eared owls in any given area in any given year, and for extensive methods for producing indices of abundance to monitor population changes and allow the results of a full survey in any given year to be placed in context.

MAIN FINDINGS

• The proportion of time for which breeding owls were visible during daylight hours was low (4.8% of the time for the season March – July) and there was a statistically significant effect of time of day and a marginally non-significant effect of stage of breeding on the duration of time for which owls were visible. Most reliable times were in the evenings during incubation (mid-April to mid-May), mornings and evenings during chick-rearing (June) and early morning and late evening during fledging (July). The likelihood of seeing key territorial behaviours was always low. This implies that the timing and interpretation of field surveys should take into account the variation in detection likelihoods with season and time of day and that some surveys may only be able to reliably identify pairs that successfully reach the chick rearing stage.
• Distances at which owls could be reliably detected suggest that a survey of any defined area will require vantage points separated by a minimum distance of about 1.5 km with allowance for topography and a 180° field of view.

With realistic expectations for resources, a survey that delivers a statistically robust national population estimate of breeding short-eared owls is perhaps not a practical proposition. However, robust monitoring of population trends could be achieved through surveys of key or representative areas in conjunction with more frequent but low intensity surveys that can deliver a more continuous index of abundance.

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