Recording unit and area

It was clear that the basis for survey had to be the 2x2 km square of the Ordnance Survey grid, a standard recording unit for many local biological atlases with an area of 4km2, called a tetrad. The tetrads forming the recording area for the breeding bird Atlas are defined in three categories as follows:
A – those of which the entire land area of 4 km2 is within Cheshire and Wirral (528 tetrads)
B – those of which some area is land, bounded by the sea or the Dee or Mersey estuaries (42 tetrads)
C – those of which the land area is shared between Cheshire and another county (101 tetrads)

Categories B and C require some further explanation. In category B, some tetrads have minimal land area, or the land is subject to tidal inundation, offering few possibilities for breeding birds. In category C, there are actually 154 tetrads shared with another county, but the land area in Cheshire varies from a hectare or so up to almost all of the tetrad. We have followed the same definition as for our First Atlas and included all those tetrads of which at least 1 km2 (25% of a tetrad) is in Cheshire, but recorded the birds found in all of the tetrad. One tetrad is common to categories B and C, giving a total of 670 tetrads for the breeding season.

At the planning stage of this project, the convention for choice of category C aroused some discussion, because for some tetrads, up to three-quarters of their area was not in Cheshire and some birds recorded would not be in the county. This system was followed because of several advantages: (i) it was the same as our First Atlas; (ii) it was the same as used by other counties; (iii) all Cheshire tetrads have the same land area, allowing comparative analysis; otherwise, all edge tetrads would have had varying expectations of species totals; and (iv) practically, it avoided observers having to decide which side of mid-river a Dipper was on!

For winter recording, there is an additional complication because of birds in tetrads offshore, which do not have any permanent land area, thus being unsuitable for breeding birds. There seems to be no standard convention amongst bird atlases for how to define which offshore tetrads to include. I took advice from Mike Hodgson about the procedure followed in Northumbria, the county with the longest coastline in England (Day & Hodgson (2003) and discussed the subject with a number of local recorders especially in the north Wirral. In principle, birds up to 1 km or more from shore, perhaps even 2 km, could be recorded, and the intention was to add one tetrad going offshore from all the coasts. In practice, it turned out often to be difficult to judge if a bird was in an inshore tetrad or one farther offshore. This is not felt to be a significant drawback, however. Most of the birds were seabirds, often in flight and moving from one tetrad to another; observers wherever possible ensured that individual birds were not double-counted. The number of tetrads for winter recording was 684.

‘Cheshire and Wirral’ for ornithological purposes is the same as during our First Atlas – despite further changes in administrative local authority arrangements –  comprising the county of Cheshire and the unitary authorities of Wirral, Halton and Warrington. Although standards have thankfully advanced since the 1970s, when the county boundaries were stretched when a recorder wanted to include a desirable observation at the south end of the Dee or near Manchester, one area of uncertainty still persists, and has to be mentioned because it confuses discussion of the county status of many waterbirds. Even today, Cheshire and Wirral Bird Reports include WeBS counts from the whole of the Dee estuary, bringing in records from some sites which are 15 km or more outside Cheshire and Wirral. Brown & Grice (2005) in their Birds in England,gave two sets of counts from the border estuaries (Severn, Dee and Solway), those birds counted in England and the count for the whole of each site but, as noted with regret in several of the descriptions of the status of wildfowl and waders, CAWOS has declined to separate records in the county from those in Wales.

An important feature of this Atlas is the ability to make comparisons of bird distribution, for breeding birds between this period and that of our First Atlas, and for many species to contrast the breeding and wintering distributions. For comparisons with our First Atlas, two tetrads have been omitted, SJ27W and SJ29X because it was felt likely that records had been mis-attributed to these squares in the First Atlas. SJ27W is mainly saltmarsh but had an above-average total of 57 species listed in 1978-84, and SJ29X contains only a tiny area of land, all mown grass and roads, and cannot have held the 19 breeding species included in the First Atlas. Comparison maps, especially for the almost ubiquitous species, were distorted by inclusion of these tetrads and it was decided just to exclude them from such maps. The records from SJ27W are on the breeding and winter maps for this Atlas. If they are included on the 'change' maps they give a misleading impression for the almost-ubiquitous species that supposedly have been 'lost' from these two tetrads.