Worcestershire Record No. 16 April 2004 pp. 36-41
A Common Bird Census (CBC) was carried out in Trench Wood, Sale Green, to measure bird population in 1987 and 1990, soon after Worcestershire Wildlife Trust (WWT) had purchased their part of the wood from Harris the paint brush manufacturers (the rest is private, owned by Mr M. Parker & Mrs C.L. Parker
Habitat and Management:
The wood had been planted with broad leaved trees by Harris for the production of brush handles. The planted trees were Sycamore, Italian Alder, Beech and a few conifers in the centre. The conifer and the beech crop had pretty well failed leaving areas of natural scrub re-generation of Birch, Field Maple, Ash, Oak etc. The scrubby nature of the whole wood gave a very high density of scrub loving warblers in 1987/90.
The wood at that time also had key species of butterflies – Marsh Fritillary, Grizzled Skipper and Pearl Bordered Fritillary. This was taken into account when considering management.
The management of the wood was the subject of a great deal of discussion by the management committee, comprising of WWT and BC members. After a time a management plan was agreed.
This was basically:
|Centre section of the wood to be coppiced on a 10 year cycle to leave continuous scrub.|
|A part of this section to be flailed regularly for regeneration of Devil’s-bit Scabious and other nectaring plants for butterflies.|
|Other parts of the wood to be “thinned” on a semi-commercial silvicultural basis|
|The section in the north of the wood to be left as a non-intervention area.|
|Rides to be mowed and flailed on a yearly basis according to a detailed plan supplied by BC.|
The general principles of this plan have been followed. However far more flailing was done on the coppice plots than planned, leaving large areas which developed into coarse grassland. The coppiced areas have a large number of standard trees left uncut spaced at between 3 to 10 meter centres, changing the character of the section from scrub to an area of thinned small trees (see habitat map)
A small pond was also added to the reserve.
The Common Bird Census was the method used by British Trust for Ornithology (BTO) for measuring bird population nationally from 1967 until 1992. The method involves visiting the area eight to ten times in the breeding season and recording territorial behaviour using a series of codes on a visit map. The visit maps are transferred to species maps to enable the number of territories to be assessed. The results were then tabulated and the 2003 results compared with those for 1987 and 1990. As many bird populations have changed nationally since the original survey, it would not be expected that Trench populations would remain constant irrespective of habitat change brought about by natural development or management. The BTO population trends have also been included in the table of results as a comparison. The BTO figures shown are from 1975 to 2000 for England and give an indication of general national trends.
The results in Table 1 show a considerable decrease in bird population in all species apart from Great Tits. This increase is due to the fact that nest boxes were added to a wood with very few natural nest holes – Blue and Great Tits have taken advantage of this.
The typical Scrub nesting species i.e. Willow Warbler, Blackcap, Garden Warbler, Nightingale and Bullfinch have all reduced by far greater amounts than national trends. The 1987 and 1990 figures for these species were exceptional and were never going to be maintained as the wood naturally matured. However it was hoped that maintaining the centre section of the wood as scrub would at least hold a reasonable number.
We know that most of Worcestershire has lost all its nightingales as part of a general contraction of the bird’s range. Bullfinches have been decreasing for years for various reasons. The number of Garden Warblers is still high in Trench but nowhere near the quantity previously recorded. Blackcaps are increasing nationally and are quite tolerant of larger trees providing the under storey is dense – the decrease in this species is therefore surprising.
The real shock is the reduction of Willow Warbler – this bird was the main feature of Trench Wood. The territory map for 1987 showed solid Willow Warbler throughout the wood with 68 breeding pairs. This compares with only three in 2003. The BTO figures only show a 44% reduction in England between 1975-2000. Willow Warblers reach maximum density in scrub between six and 15 years old.
Key Species lost since 1990 – Breeding Woodcock, Turtle Dove, Nightingale, Grasshopper Warbler and Willow Tit. All these species have declined nationally.
Looking at the density of birds generally, we get a figure of 141 breeding pairs per 100 acres compared with 388 in 1990. As a typical oak wood holds about 350 pairs per 100 acres it can be seen that we have gone from a high population density to an extremely low one.
Table 2 shows the butterfly transect results from 1988 to 2002 inclusive. The graphs show how the total count and some species counts have varied over the period. Note:- these figures have not been adjusted for weather conditions or missed observations.
The general trend shows a steady increase in the total count since 1988 with peaks in 1995 and 1996. The 1995 peak is caused by an unprecedented number of Small Whites and the peak in 1996 is due to a very high count of Gate Keepers. Analysis of the results shows that the species responsible for the increase in the overall count are Meadow Brown, Ringlet and Gate Keeper.
The overall count has gone up by 1538 – of which Meadow Brown contributed 620, Ringlet 253 and Gatekeeper 160. These are common grassland species.
The key butterfly species which have been lost are Marsh Fritillary – lost and reintroduced a number of times, Pearl Bordered Fritillary and Grizzled Skipper.
Bird population in Trench Wood has decreased significantly. The diversity of species has also diminished. We were never going to maintain the 1987 and 1990 incredibly high numbers as the wood matured and the understorey thinned out. Although a tremendous amount of management work has been carried out in Trench Wood it has not maintained the scrubby-loving population of birds.
By widening rides, regular flailing of rides and scrub, large areas of coarse grassland have developed which favours three common grassland butterfly species. Efforts to maintain the populations of the key species have failed.
Many factors affect bird and butterfly population apart from the breeding habitat. Things like weather, wintering conditions, migration conditions, surrounding farmland and disturbance by visitors to the reserve will all contribute. By measuring the populations regularly we can review our management plans and make sure that the breeding habitat is the best it can be to achieving our goals.
The CBC was carried out by J K Tilt, Tom Haynes and Neil McLean. The Butterfly data was obtained from Neil Gregory and the Butterfly Transect done by Ken Thomas et al
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