Monitoring woodcock with acoustic recorders

Woodcock surveys are undertaken when the male woodcock are roding. © Laurie Campbell

BACKGROUND As a British breeding species, the woodcock has experienced a 56% decline in site occupancy, at the 10x10-km-square scale, between 1970 and 2010 and a 29% decline in abundance between 2003 and 2013. Automated acoustic recorders offer a cost-effective way of increasing the reliability of current survey techniques by increasing survey coverage, especially in remote areas, and by providing data that can inform the interpretation of volunteerled surveys like the GWCT/BTO Breeding Woodcock Survey. The woodcock is ‘red-listed’ in the UK, owing to severe declines in its breeding range. The drivers of the decline are not fully understood, but GWCT research has shown that larger woods with a greater diversity of stands are more likely to support breeding woodcock. The male woodcock’s conspicuous breeding display, known as ‘roding’, provides a practical means of surveying a species that is otherwise rarely seen. Roding surveys have formed the basis of national woodcock surveys in Britain since 2003, providing the most accurate indication of population size and trend to date.

Remote sound-recorders provide a new way to census a wide range of species, and a 2018 trial demonstrated the potential value of this technique for the study of woodcock. The recorders are stationed at fixed locations and record digital sound files according to a pre-programmed schedule. They are an efficient alternative to existing manual sampling as they can be used where regular access is difficult, reduce disturbance caused by human presence and maximise coverage over time through a long-term sampling regime. In this study we aim to examine the value of automated acoustic recorders and associated classification software as a way of surveying woodcock.

Surveys were conducted on fixed count points positioned along woodland rides or clearings within 16 individual woodland blocks (>10 hectares (ha) in size) in north Hampshire and took place between April and June 2018, using five SM3 acoustic detectors. At each wood, detectors were set to record over a seven-day period, twice a day: 1) 15 mins before, to 1.5 hours after sunset and 2) 1.5 hours before, to 15 minutes after sunrise. This system of monitoring has produced 784 hours of recordings representing 448 105-minute-long sessions. We have so far processed 12.5% of the total dataset, and already identified 500 woodcock recordings. The roding calls were displayed as spectrograms using Kelidoscope and SongScope, they consist of three to five low-frequency grunts followed by a high-pitched squeak, which is repeated at intervals of about three seconds (see Figure 1).

At two sites, manual roding surveys were conducted to assess the comparability of a human surveyor and an automated recorder. Initial results indicate high similarity between the two methods and in 2019 we plan to expand this comparison to encompass additional sites.

Figure 1 Woodcock spectrogram. The high ‘squeak’ and low ‘grunt’ phases are visible. The position of grunts have been marked with white arrows and squeaks are outlined using pink boxes

The data we have collected to date should provide us with comprehensive information regarding the variability in roding behaviour across different sites and survey periods. Existing interpretation of roding survey data relies upon a calibration equation that is used to convert woodcock ‘registrations’ (birds seen or heard displaying) into an estimate of the number of males present. This equation was devised using sonogram analysis that allows different individuals to be distinguished. In the future, remote sound-recorders could play a role in the refinement of these calibration equations by providing data from a wider range of woodland sites, and by monitoring change in the number of individuals detected at each site over time.

The national GWCT/BTO woodcock surveys, which have been used to estimate British breeding population size, rely on voluntary surveyors to conduct counts of roding woodcock. Generally, we recommend three visits to each survey site per year, but often this is difficult to achieve for remote sites. Demonstrating that automated sound recorders are able to detect woodcock as effectively as a human observer may mean that this technique can be used to increase survey coverage and improve its representativity.

KEY FINDINGS A 2018 trial demonstrated that acoustic recorders could be used to monitor woodcock remotely. Five hundred woodcock registrations have been identified from the 12.5% of recordings that have been examined so far. A total of 784 hours’ of data has been collected.

Niamh McHugh Chris Heward Andrew Hoodless

ACKNOWLEDGEMENTS We would like to thank the landowners who allowed us access to their land to conduct acoustic surveys. We would also like to thank Nick Bailey who assisted in the field.