We present the results of a census of the Manx Shearwaters Puffinus puffinus nesting on the three Pembrokeshire islands of Skomer, Skokholm and Midland (formerly Middleholm), Pembrokeshire, undertaken in 2018. The breeding population estimates were largely in line with those made in 2011–2013, but differed markedly from 1998; this difference seems to be due to the different methods used in 1998 rather than any marked change in population size. Despite attempts to refine the estimation of response rate to call playback, the error of the population estimates remains large, illustrating the logistical and analytical challenges of making seabird censuses using call playback. Nonetheless, the population estimates are large and the spatial distribution of occupied burrows is consistent between censuses, and thus Wales may hold more than half of the world’s breeding population of Manx Shearwaters.

Useful estimates of population size are important components of population monitoring, to confirm the stability or increase of protected populations and signal the requirement for conservation strategies to be enacted in response to population declines. To this end, frequent sub-population monitoring is conducted (Walsh et al. 1995). Estimating the total population size of a breeding seabird colony is useful to ‘ground-truth’ sub-population estimates and to account for changing distributions, but is not always easy.

One group of birds which present particular difficulties is the burrow-nesting seabirds; these include a number of species such as some auks Alcidae and petrels and shearwaters Procellariiformes. Their nest-sites may be real burrows, such as those in soil or cavities, or crevices in rocky scree. Estimating the numbers of nesting pairs presents a number of difficulties; the nesting sites may be difficult or dangerous to access, the nesting chambers may be inaccessible, and they may be empty or occupied by a species other than the target one. An added problem confronting the surveyor is that many of the petrels and shearwaters only visit the colony at night; a daytime visitor may see no birds at all, the birds are either present in the nest-chamber or at sea.

Increasingly in recent years workers studying these species have been using call playback to estimate population size. Each species has specific calls and by playing the calls at the burrow entrances some idea of the numbers may be obtained. An added advantage is that the work can be carried out in the day, making it much safer. While this method has made it possible to estimate numbers, it is not without its own difficulties; the work has to be carried out during the incubation period, not all individuals respond to the tape, in some neither parent may be present at the time of the visit, and there are differences in the calls — and the response to them — of the two sexes.

This paper describes studies on the Manx Shearwater, carried out as part of Seabirds Count 2015–2020 coordinated by the Joint Nature Conservation Committee. The aims of this study were to census the breeding Manx Shearwater population on each of the three islands of Skomer, Skokholm and Midland (51°44’N 5°5’W) (Figure 1), by use of call playback and, if possible, to improve on the estimates made on Skomer in 2011 (Perrins et al. 2012) and Skokholm in 2012–13 (Perrins et al. 2017). These most recent censuses of the two islands estimated the number of breeding pairs on Skomer as 316,065 (95% CI: 83,534) and Skokholm as 63,564 (95% CI: 15,943). A previous census in 1998 produced markedly lower results, with 101,274 breeding pairs on Skomer, 46,021 on Skokholm and 2,990 on Midland (Smith et al. 2001). Variations in response rates between different sites were examined, and as a final aim of this paper, we reanalysed the 1998 data to make them retro-comparable with the later censuses. The census results are compared with the earlier studies.

This study could only have been undertaken with the help of many people. We are particularly grateful to the following for doing most of the fieldwork: Jack Barton, Aude Boutet, Alice Cousens, Nicci Cox, Sarah du Plessis, Connor Walsh and Tom Lloyd; and on Skokholm: Kirsty Franklin, Stephen Vickers, Amy Sherwin, Alys Perry and Zoe Deakin. The census received funding contributions from the National Trust (Wales), Natural Resources Wales (NRW), and The Seabird Group. Lizzie Wilberforce (Wildlife Trust of South and West Wales) and Patrick Lindley (NRW) assisted with funding applications and logistical support.

Alexander, M. & Bellamy, D. 2018. Aerial photography Skomer 2017: High resolution low level aerial photography May and September 2017. A report to the Wildlife Trust of South and West Wales.

Arneill, G. E. 2018. Developing and assessing methods to census and monitor burrownesting seabirds in Ireland. PhD Thesis, University College Cork.

Arneill, G. E., Perrins, C. M., Wood, M. J., Murphy, D., Pisani, L., Jessopp, M. J. & Quinn, J. L. 2019. Sampling strategies for species with high breeding-site fidelity: a case study in burrow-nesting seabirds. PLoS ONE 14: e0221625. [Crossref]

Brooke, M. de L. 1990. The Manx Shearwater. Poyser, London.

Brown, J. 2006. Census of European Storm-Petrels on Skomer Island. Atlantic Seabirds 8: 12–30.

Cummins, S., Lauder, C., Lauder, A. & Tierney, T. D. 2019 The Status of Ireland’s Breeding Seabirds: Birds Directive Article 12 Reporting 2013–2018. Irish Wildlife Manuals, No. 114. National Parks and Wildlife Service, Department of Culture, Heritage and the Gaeltacht, Ireland.

Kroodsma, D. E., Byers, B. E., Goodale, E., Johnson, J. & Liu, W.-C. 2001. Pseudoreplication in playback experiments, revisited a decade later. Animal Behaviour 61: 1029–1033. [Crossref]

Murray, S., Shewry, M. C., Mudge, G. P. & Spray, S. 2003. A survey of Manx Shearwaters Puffinus puffinus on Rum, Inner Hebrides in 2001. Atlantic Seabirds 5: 89–100.

Mitchell, P. I., Newton, S. F., Ratcliffe, N. & Dunn, T. E. 2004. (eds.) Seabird Populations of Britain & Ireland: Results of the Seabird 2000 Census 1998–2000. Poyser London.

Newton, S. F., Thompson, K. & Mitchell, P. I. 2004. Manx Shearwater Puffinus puffinus. In: Mitchell, P. I., Newton, S. F., Ratcliffe, N. & Dunn, T. E. (eds.) Seabird populations of Britain and Ireland: Results of the Seabird 2000 Census 1998–2000: 63–80. Poyser, London.

Perkins, A. J., Douse, A., Morgan, G., Cooper, A. & Bolton, M. 2017. Using dual-sex calls improves the playback census method for a nocturnal burrow-nesting seabird, the Manx Shearwater Puffinus puffinus. Bird Study 64: 146–158. [Crossref]

Perrins, C. M., Wood, M. J., Garroway, C. J., Boyle, D., Oakes, N., Revera, R., Collins, P. & Taylor, C. 2012. A whole-island census of the Manx Shearwater Puffinus puffinus breeding on Skomer Island in 2011. Seabird 25: 1–13.

Perrins, C .M., Wood, M. J., Gilham, J., Brown, R., Eagle, G. & Taylor, C. 2017. Manx Shearwater census on Skokholm Island 2012 and 2013. Report to Wildlife Trust for South and West Wales.

Smith, S., Thompson, G. & Perrins, C. M. 2001. A census of the Manx Shearwater on Skomer, Skokholm and Middleholm, West Wales. Bird Study 48: 330–340. [Crossref]

Walsh, P. M., Halley, D. J., Harris, M. P., Del Nevo, A., Sim, I. M. & Tasker, M. L. 1995. Seabird monitoring handbook for Britain and Ireland: a compilation of methods for survey and monitoring of breeding seabirds. JNCC/RSPB/ITE/Seabird Group.