First Breeding Atlas 1968 - 1972
Full details can be found in the introduction of The Atlas of Breeding Birds in Britain and Ireland (Sharrock 1976) and brief extracts are given below.
The aim in covering a square was to record every species breeding in it during 1968-72 and, if possible, to prove breeding for each one. To this end, the main guidance given to observers was that they should visit samples of every different habitat in the square during the main breeding season of April to July, to make visits outside this period for certain species (e.g. in February or March for owls) and to make dusk and night visits for crepuscular and nocturnal species.
We were fortunate that two other national breeding bird surveys coincided or overlapped with the BTO/IWC Atlas project. First, the Wildfowl Trust's survey of breeding and summering wildfowl in Britain, covering 1965-70, was also based on 10-km squares. Secondly, the Seabirds Group's 'Operation Seafarer', the aim of which was to obtain counts and estimates of all seabirds breeding in Britain and Ireland, was carried out mainly in 1969-70.
The three grades of evidence of breeding - possible, probably and confirmed - are shown on the maps in the book as three increasing sizes of dots. The maps reproduced here on the website show just two levels of breeding evidence for simplicity (shown as colours). The absence of a dot indicates that the species was not reported in suitable breeding habitat during the breeding seasons of 1968-72. Such blank areas will often accurately reflect the species' absence, but this cannot be assumed, as it is only in certain cases that truly negative records can be obtained. It is usually safe to assume, however, that the species is not common in such blank square.
Winter Atlas 1981/82 - 1983/84
A full account of the different stages of the project including figures and other details is to be found in the introduction and on page 431 of the Atlas of Wintering birds in Britain and Ireland (Lack 1986). Brief extracts are given below.
Two kinds of records were accepted: first, the results of visits made to 10-km squares specifically to do fieldwork for the Winter Atlas; and second, any casual records (termed Supplementary Records) of individual species. For a specific visit, observers were asked to spend a minimum of one hour in a 10-km square and to count the number of birds of each species seen and/or heard. Only records of birds actually seen or heard were wanted, not those which observers 'knew' to be present. Therefore provision had to be made for the recording of birds at times other than during a special visit. For such records observers filled in a Supplementary Record Sheet.
After analysis and discussion of the results of the pilot Survey, it was decided to use as the assessment of abundance 'the number of birds seen in a day'. It was further decided that if there was more than one count of a species in a square the highest figure would be used. A 'day' was defined as six hours in the field, this being the longest that most observers would be likely to be able to devote to fieldwork in a day. In fact only about 3.5% of all cards received were for periods longer than this. All timed counts of more or less than six hours were standardised to this standard 'day length'. This was to permit better comparisons of areas which might have counts of only one or two hour duration with those which had six-hour counts. The species maps produced in the book used three sizes of dot, the larger dots indicating more birds. On the website, sizes are replaced with colours. The divisions between the different dot values were chosen so that 50% of the dots were in the lowest category, 30% in the middle category and 20% in the highest category, and the divisions were made to the nearest whole number of birds.
Second Breeding Atlas 1988 - 1991
Full details can be found in the introduction of The New Atlas of Breeding Birds in Britain and Ireland: 1988-1991 (Gibbons et al. 1993). Extracts are given below.
Fieldwork for the New Atlas was conducted between April 1st and July 31st in each of the four years 1988-91. Initially, it had been hoped that coverage would be complete in three years, but this proved impossible and the survey eventually occupied four seasons. Fieldwork was coordinated by a network of volunteer regional organisers and was undertaken by members of the BTO, SOC, IWC and others (again mostly volunteers) who followed a set of instructions issued prior to the beginning of each breeding season, in brief, the instructions were as follows. Observers visited, either alone or as a coordinated team, a minimum of eight tetrads of their own choice within each 10-km square. Two hours were spent in each tetrad, and a species list was compiled for each. From these timed visits an index of abundance of each species in each square was calculated. Additional supplementary (non-timed) observations were also requested, to ensure that the species lists for each 10-km square were as complete as possible.
A total of 551,370 10-km square records was submitted to the Atlas. These were based on 320,595 records collected during time tetrad visits, and 230,775 supplementary records. The 10-km square timed visit records were themselves based on 1,262,231 tetrad records from 42,736 tetrads (34,601 in Britain, The Channel Islands and the Isle of Man, and 8,135 in Ireland) in 3,672 10-km squares (2,720 in Britain, the Channel Islands and the Isle of Man, and 952 in Ireland).
A comprehensive account of the method of map production is given in the Introduction and Methods. The present section is provided to enable readers to interpret the maps readily without recourse to the Introduction and Methods.
The Distribution map
This shows the distribution of each species recorded during 1988-91. Two different dot sizes are used on the map. A large dot refers to a 10-km square with evidence of breeding (the ‘breeding’ category; see Introduction and Methods). A small dot refers to presence during the breeding season, but with no stronger evidence of breeding (the ‘seen’ category). These data came from both timed and non-timed observations, and thus do not take into account variation in observer effort. Only 10-km squares with evidence of breeding are presented for seabirds (see Introduction and Methods). All dots are placed centrally with each 10-km square and for coastal squares may occasionally fall in the sea.
The Change map
This map documents the recorded change in distribution between 1968-72 and 1988-91. The maps produced in the book used four different types of dot, the definitions of which were as follows:
Small open dot = possible breeding in 1968-72, not recorded in 1988-91
Large open dot = probably or confirmed breeding in 1968-72, not recorded in 1988-91.
Small filled dot = not recorded in 1968-72 but present, although no evidence of breeding, in 1988-91.
Large filled dot = not recorded in 1968-72, but evidence of breeding in 1988-91.
The maps here on the website are reproduced as coloured dots (light and dark for small and large respectively) with open dots replaced with red dots and filled dots with green dots. 10-km square in which the species was recorded during both time periods are not shown on the map, even if the level of proof of breeding had changed. For seabirds, only 10-km squares with evidence of breeding (i.e. probably or confirmed for 1968-72, breeding for 1988-91) were used in the generation of Change maps. There are thus no small dots on seabird Change maps.