Territory Mapping and the BTO Common Birds Census
In the spring of 1973 a team of volunteers, led by the late Margaret Price, began surveying the breeding birds in the wood in order to contribute to the BTO Common Bird Census. At that time, the CBC was the BTO's long-term national population monitoring operation. In the first years of ownership of the wood by the Notts. Trust, just the northern part of the wood was surveyed, but from 1976 onwards complete coverage has always been achieved.
The CBC as a national monitoring scheme was superseded by the Breeding Bird Survey. The CBC, although invaluable for recording breeding territories on a single site, has problems when used nationally – BTO staff time to produce maps, non-random, biased coverage of the country to name but two. At a single site level, however, the CBC methodology provides much more than a statement of the number of breeding territories. It provides information about density and size of territories, and location of territories within the site.
Above : Territory maps of Song Thrush (Turdus philomelos) showing fewer breeding territories present in 2015 than there were in 1976 .
It might seem like overkill to continue with CBC in the wood because the ringing programme does provide some measure of populations. However, it was decided to continue the CBC in Treswell Wood because of the long-term data set already accumulated (and computerised), because it can relate territories to coppice management and because it provides an estimate of populations quite independent of any estimates derived from ringing. In some analyses, this independent estimate of population abundance has proved vital. It also has the benefit of including all breeding birds present in the wood as there are some, such as Buzzard and Jackdaw, that we do not catch.
The TWIG data set holds a table of territory numbers for all species from 1973 onwards and the maps themselves have been committed to a GIS. From this, sets of maps can be generated and overlaid on various backgrounds including coppice age – some are illustrated here. Although these maps are interesting to browse through, the fact that they are held in GIS makes automated analyses possible and, because the grid system is the same as that used for ringing, analyses can link coppice age, territories and individuals birds.