In 2008 I organised the Shropshire Rookeries Survey (SRS08) and I thank again all the 160 or so recorders, notably John Harding for all his hard work both in the field and especially for putting the sites online for access to us all. The survey logged 540 sites that year and since then the total has crept up to 557. I have done nothing with the data until now, unable to find a way to deal with the fact that I have always believed that the true total must be closer to 1,000 and I would like to produce a coherent account of the species in the forthcoming Avifauna. I have now started to make progress and my thanks go to John Arnfield for County atlas data for the breeding seasons 2008 to 2011; they have freed up my thinking. I have made a spreadsheet of Shropshire’s 870 tetrads and compared data from the 1975 BTO survey with SRS08 and the first four of the six years of the Shropshire Atlas.
The 1975 survey found 460 rookeries at an average of 1.58 per occupied tetrad*. SRS08 recorded 557 rookeries at an average of 1.60 per occupied tetrad – so not much difference between the two figures.
How do the atlas data help? First, to help the atlas project, SRS08 adds 91 tetrads with rookeries. More significant is the fact that the atlas has proved breeding (here presumed to mean a rookery or rookeries) in 147 tetrads from which SRS08 has no records. If I presume that these 147 harbour sites at the same density as those In SRS08 they add 232, giving me a total of 789 which is 42% up on the SRS08 figure and much nearer my postulated 1,000. The extrapolated figure will increase as I add in atlas confirmed breeding records from 2012 and 2013.
Taking the model further, and way too far, if the SRS08 rookery density of 1.60 is applied to all the tetrads so far with no SRS08 records the county total would be 1,425 sites. This is of course unreasonable; many tetrads are untenable because they are urbanised, have no nesting trees or just no suitable feeding areas. But is does suggest an absolute ceiling for a total which will continue to increase over the coming survey season. I will continue to hone up the figures ready for the Rook account in the Avifauna and my thanks again to all SRS08 recorders – your work is proving invaluable.
My calculations make some assumptions, namely:
- The numbers and locations of rookeries are, over the period being studied, relatively stable. I certainly know of some changes but their effect on the overall numbers will be marginal.
- Where a site’s grid reference falls on the northing of a tetrad boundary, the site is allocated to the tetrad to the north and, where on an easting, to the tetrad to the east.
- True tetrads are 4 sq km in extent but the County’s bordering or edge squares are considered ‘in’ Shropshire if ≥50% of the square is within this county. In this analysis all are regarded the same way, as complete.
- Noting that it was a survey of rookeries, not of tetrads, some rookeries will be missing from tetrads – there will be others so the average per tetrad will in fact be higher than the 1.60 currently used.
* The data and analyses above are provisional and should not be cited.