The Winter Random Sqaure Survery (WRSS) as established in 1975 as a means to record the status of common species which are otherwise overlooked by our monthly recording system.
The WRSS is conducted in the last weekend of November and February, with results combined to produce the survey counts for that winter. Observers are allocated a 1km square at random from each of the 12 10km squares of the BOS survey area so that all parts of the survey area are covered. Each observer is asked to record each species seen and estimate the total number of each species over a period of 2-3 hours. In most years we are able to field multiple observers per 10km square, thus increasing coverage.
Squares are not re-surveyed until all surveyable squares have been surveyed. Squares are omitted where there is limited or restricted access. To date we have conducted 2 complete surveys, from 1975-2000 (1120 squares, 93% of the BOS) and 2000-2016 (658 squares, 55%). A third cycle of surveys started in November 2016. Interestingly, we have found that the average number of species recorded per survey square has not changed dramatically in the 3 cycles, indicating that the species richness of the area is relatively stable. However, it appears that the abundance has now started to decline, i.e. the total number of birds seen in the surveys is now lower than it used to be.
The results from the first 3 years of surveys (1975-1977) are averaged to produce an index for each species, one for abundance and one for distribution. From that time onwards, a 3-year rolling index is calculated, which shows the relative abundance and distribution change for each species.
This enables the BOS to analyse species trends since 1975. In general the trends match national findings, but there are significant differences which allows the BOS to better understand species which are fairing better or worse within our survey area. We use this information to adjust the conservation targets on our reserves and when providing guidance to local planning.
The results of the survey can be read in our book Birds of the Heart of England and the report Bird Trends in the Heart of England. An analysis of the results of the first 20 years of surveys was published in Bird Study: Population trends of wintering birds around Banbury, Oxfordshire, 1975–96.
The charts below provide a summary of the surveys performed to date and the number of species monitored. The dashed lines show the linear trend. The charts show that the monitoring has been very consistent, which is very good considering the small membership size of our society and the amount of territory to be covered. Overall, the total number of species recorded per year has increased, which in part reflects the spread of species such as Buzzard, Red Kite and Raven which were once very rare in the area. Note that the dips in 2000 were due to the restrictions to countryside access caused by the foot & mouth outbreak.
The Average Species Count chart shows the estimated number of species in each 1km survey square - the band at the top shows the lower and upper range of the estimate. This number of species has remained relatively constant at around 25, with a recent decline. The Abundance chart shows the average number of birds seen in each survey square per year. This is showing a steady decline since the start of the survey, indicating that the total wintering population of birds in the area is declining.
The simple total counts do not really show what is happening to the birds of our region. For that we need to look at the abundance and distribution indices for different species groups. The indices start at 100 for 1977, so subsequent values since that time show if the abundance or distribution has increased or decreased. A declining distribution means that the species is occuring in fewer locations. The charts below categorise the surveyed species into 4 groups: Farmland, Woodland, Water and Birds of Prey, based on each species primary habitat. Click the "<" & ">" buttons to page through the charts. The index for each species is combined using the Geometric Mean statistical technique to produce a group index value for each 3-year period. This ensures that a large change in one species does not unduly affect the group value.
There are 17 farmland species: Carrion, Crow, Corn, Bunting, Goldfinch, Greenfinch, Grey, Partridge, Jackdaw, Lapwing, Linnet, Meadow, Pipit, Reed, Bunting, Rook, Skylark, Starling, Stock, Dove, Tree, Sparrow, Woodpigeon, Yellowhammer. The chart shows there has been a significant decline in abundance and distribution of these species, which is a major cause of concern. Around 85%of the BOS area is farmland, so these species are the core birds of the area. In contrast, Woodland species appear to be maintaining their populations. This is slightly deceptive though, since many of the 23 species in this category are generalists that have adapted to farmland and suburban habitats, such as Blackbird, Tits, Robin, etc.
The Birds of Prey trend is very encouraging, charting the rise of Buzzard, Red Kite and Sparrowhawks, Kestrel being the 4th species in this category. The 8 species in the Water category are also showing a slightly positive trend: Canada Goose, Coot, Cormorant, Grey Heron, Mallard, Moorhen, Mute Swan, Teal.
The following two maps summarise the winter survey results in the current survey cycle which started in November 2016. The map on the left shows the number species observed in each WRSS survey square - the larger the square, the greater the number of species recorded. Hover the mouse over a square to see the details of each survey. The second map shows the number of squares surveyed per 10km square. We try and ensure an even coverage across the 10km squares, but this is subject to observer availability so some parts of our area have not been surveyed to the same extent.