Thursday, 24 January 2013

Manx Shearwater Whole Island Census results 2012 - >300,000 pairs

Extracts from: A whole-island census of the Manx Shearwaters Puffinus puffinus breeding on Skomer Island in 2011 SEABIRD 25 (2012): 1–13 

Authors: Chris M. Perrins, Matt J. Wood, Colin J. Garroway, Dave Boyle, Nick Oakes,
Rose Revera, Phil Collins and Chris Taylor

Skomer Island, Pembrokeshire, Wales is believed to have one of the largest colonies of Manx Shearwaters Puffinus puffinus in the World. In 1998 a census was made of the whole island, and the adjacent islands of Skokholm and Middleholm, in order to try to establish the size of the breeding population; the Skomer population was estimated to be just over 101,000 breeding pairs. A second census was carried out in
2011. First, a set of study burrows was opened and a tape of the male call (normally only males respond to these) was played down each burrow several times during the course of incubation in order to establish the male response rate. Then the same tape was played down all the burrows in each of 288 randomly selected plots across the island and the number of responses recorded. Extrapolating responses from census plots to the whole island yielded an estimate of 125,112 (CI ± 16,445) responses. Adjusting this figure to take account of the response rate yielded an estimate of 316,070 (SE ± 41,767) breeding pairs. This figure is greatly in excess of the estimate made just 13 years earlier. Possible reasons for this are discussed.

"Comparison with the 1998 census: The results presented here differ strikingly from the results recorded in 1998 (Smith et al. 2001). A change from 101,000 to 316,000 pairs in the estimated population size requires an increase of approximately 9% p.a. For a bird with a low reproductive rate and a long period of deferred
maturity, this is a very high rate. The UK and Irish population of the Northern Gannet Morus bassana has been increasing steadily for many years, but the rates have largely been of the order of 2% p.a., though the population at the Bass Rock has increased by 4.8% p.a. for 35 years (Wanless et al. 2005). During the period when it was spreading rapidly around the UK the Northern Fulmar Fulmarus glacialis achieved rates of increase of some 8% p.a. (Tasker 2004), but this may also have included some immigration.

The only previous attempt to estimate the population size of Manx Shearwaters on Skomer using playback was made in 1995 by Gibbons & Vaughan (1998). They restricted their study to the Neck, a 23 ha area on the east of the island (Figure 1), and obtained a figure of 26,500 pairs (95% CI 21,000–32,000). In comparison, the number of responses on the Neck in 2011 was 22,891 and the estimated population size 56,801 pairs, again very much larger than the earlier estimate.

Possible reasons for this difference include:
i That the population has increased during the 13-year interval. There is some evidence to suggest that there has been an increase. Surveys of 18 0.1 ha plots have been made annually by MSc students from Oxford (Taylor et al. 2011). From 767–1,113 males have responded in the different years, with the numbers showing a significant increase with time. However, at the most, this increase has been only about 2.5% per year, not nearly enough to explain the difference recorded between 1998 and 2011.

ii That the local population has been augmented by large numbers of immigrants. This seems unlikely since, even before the 2011 survey, the Pembrokeshire islands were thought to house 40 % of the UK and Irish
population; an increase through immigration in a smaller population might well occur, but for Skomer this seems unlikely.

iii That there has been a sharp decrease in the age of first breeding. There is no evidence one way or the other, but it seems an unlikely explanation.

iv That the methods used in one (or both) of the surveys were sufficiently flawed to account for the difference, or part of it.

One difficulty with comparing the 1998 estimate with the current one is that the methods used were so different. One possible explanation for the disparity is that the technique used in 1998, of counting all the burrows in mid-winter and visiting a known percentage of these in summer, is faulty. It assumes that the ‘population’ of burrows remains the same throughout the year, whereas there is very heavy digging in spring by all three species - shearwaters, puffins and rabbits. In 2011, the number of burrows counted in the 288 plots was 13,863 and this, correcting for area (as done for responses in Table 2), yields a burrow number of 417,368, almost three times the number counted in winter 1997/98. [Burrows dug in spring are not
normally used for breeding that same year - M. Brooke pers. comm.]

Differences in tape quality or observer behaviour are other possible sources of discrepancy. However, it seems unlikely that these can explain the differences between the two studies since the response rates were similar: 1998 3,218 responses in 11,320 burrows tested (28.4%), 2011 4,475 in 13,863 (32.3%).
While the reasons for the discrepancy between the two censuses needs to be better understood, we suggest that the method used here provides a fast, relatively low-labour way of making estimates of shearwater populations. We believe that the labour involved in establishing the response rate could be greatly reduced by searching for burrows with eggs at the end of May and simply marking and playing the tapes down these without trying to establish who the occupants were. This assumes no significant egg loss prior to the end of May. All that is needed is a response rate from a known number of burrows with eggs. This would potentially enable larger samples to be made and would not only increase the accuracy of the
response rate but also reduce the Confidence Intervals."

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