BTO Second Atlas of Breeding Birds in Britain and Ireland: 1988-1991.
The dataset was used to produce the second BTO/SOC/IWC Breeding Atlas 1988-91 (Gibbons, D.W., Reid, J.B. & Chapman, R.A. (1993) The new atlas of breeding birds in Britain and Ireland: 1988-1991. T. & A.D. Poyser).
Volunteer (mostly) observers compiled as complete a list as possible of breeding bird species in each 10-km square and noted whether the species was simply Seen -- in suitable potential breeding habitat; or Breeding -- a series of standard codes were used to categorise these. In addition an estimate of relative abundance in each 10-km square was compiled from standardised timed visits to at least eight tetrads (2-km squares) within the 10-km square.
The dataset comprises: a) records of species presence in 10-km squares with an attribute of Seen or Breeding; b) records of species noted in the timed (2 hour) visit to those tetrads visited as part of the relative abundance assessment. Note that only a proportion of tetrads received such visits.
All of Britain and Ireland and including, for the original survey although not on currently available through the NBN Gateway, including the Channel Islands. All 10-km squares with more than a very small amount of land were visited. In the more populated (by humans) parts a high proportion of the 2-km squares in each were visited for the standard time but in the less well-populated areas only the minimum eight were usually visited.
Note that the specified purpose was to compile as complete a list as possible at the 10-km square resolution. By definition the species list in tetrads is not complete but is only those recorded in the two-hour time slot. Also of course only a proportion of the tetrads were visited.
To compile as complete a list of breeding species as possible for each 10-km square.
To compile a species list for a tetrad in a two-hour time slot (note that this is unlikely to be a complete list of species which occur in the tetrad).
The list and number of species recorded in each 10-km square are as complete as they can be. Clearly those squares visited for less time (especially the more remote areas) are likely to have fewer species recorded and it is the rarer and more elusive species (eg nocturnal) ones which will not be recorded in these circumstances. However the BTO believes that the maps as published are a true representation of the distribution of the species at a national level, while accepting that there will be some gaps in individual squares.
The list and number of species in each tetrad, even those visited, will not be complete by definition -- see above.
Specific fieldwork was conducted by mainly volunteer observers although professional help was used in remoter areas. Two kinds of survey work were carried out:
1) Timed Tetrad Visits (TTVs): Observers visited a tetrad (2-km square) for two hours (and only two hours), with a stated preference for this period to be split into two one-hour visits one early in the season and one late. During this time a species list was compiled and the individuals of some (specified) species were counted. Tetrads were eligible for coverage if their centre was on land. Coverage of a minimum of eight such tetrads in each 10-km square was requested although the choice of which ones was left to the observer(s) with the proviso that they should aim to represent the major habitats within the 10-km square. Where there were fewer than eight eligible tetrads, all tetrads were surveyed. These lists were then used to calculate the proportion of tetrads visited in which each species was recorded in the two-hour visit, a figure which was then used as an estimate of relative abundance in the 10-km square. For the specified count species, the relative abundance index was based on the mean count across surveyed tetrads. Note that in some remote areas the TTV comprised a single visit of two hours.
2) Supplementary Records: In addition to the TTVs records of any other species in the 10-km square were requested. Observers were particularly asked to look for elusive species such as nocturnal ones which were likely to be missed during TTVs.
In all cases observers were asked to note whether the species was 'Seen' or 'Breeding' in the 10-km square. 'Seen' was defined as being seen in the breeding season in suitable habitat.'Breeding' was defined by a series of specific activities, for example apparently holding territory, nest found, recent fledglings seen. A full list of activities considered to be 'Breeding' is in the book (p.4).
In some cases records from such as Local Bird Reports, seabird colony counts and other surveys were added to the dataset to provide a more complete species list and/or more accurate abundance information for each 10-km square.
For 10-km squares the dataset records the maximum category of breeding evidence obtained in each 10-km square. For tetrads the dataset records the species recorded during the TTV.
For a more comprehensive description of the Methods see http://www.bto.org/volunteer-surveys/birdatlas/previous-atlases/previous-methods and the Introduction to the published book.
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