Variation in the diurnal activity of breeding Short-eared Owls Asio flammeus: implications for their survey and monitoring.

Capsule The proportion of time during which breeding Short-eared Owls could be seen during daylight hours was low and assessment of seasonal variation in their detection likelihoods suggests that surveys may only be able reliably to identify pairs that successfully reach the chick-rearing stage. Aims To identify the most appropriate times of day and stages of breeding for surveying and to contribute towards the development of feasible survey protocols. Methods Fixed point watches sampled the activity of breeding Short-eared Owls in three extensive study areas in Scotland during two breeding seasons; 2006 and 2007. Results 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 stage of breeding and also of time of day within some stages of breeding on the proportion of time during which owls were visible. There was a greater than 75% likelihood of seeing owls during a combined total of four hours of watching (1) in the evenings during incubation (mid-April to mid-May), (2) mornings and evenings during chick-rearing (June), and (3) early morning and late evening during fledging (July). The likelihood of seeing key territorial behaviours was always low. The mean distance from an observer at which owls were first detected in 2006 was 495 m (95% CI: 390-601 m), and significantly greater in 2007 at 741 m (95% CI: 651-832 m). Conclusion The interpretation of field surveys of breeding Short-eared Owls should take into account the effect of season and time of day on the likelihood of detection. Surveys may only be able to reliably identify pairs that successfully reach the chick-rearing stage and, therefore, the most robust survey/monitoring unit may be pairs that successfully hatch young. 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. Evidence of a disturbance or deterrence effect of field surveyors on the owls supports the need for unobtrusive survey methods. [ABSTRACT FROM AUTHOR]