Processes affecting birds
in woodland and scrub

Introduction

Population densities of woodland birds and the composition of woodland bird communities are influenced by a complex mixture of factors. The processes affecting woodland birds have recently received renewed attention in Britain because several breeding species have declined substantially in recent years. Among the declining species are most of the long-distance migrants (see http://www.forestry.gov.uk/woodlandbirdsurvey for more details). These recent population changes provide an important focus for the BTO’s ongoing work on woodland birds. However, the programme is also concerned with understanding how future changes within woodland and at a wider landscape scale are likely to affect birds. Information derived from this programme is relevant to conservation management strategies in the face of climate change adaptation. Broad topics of particular interest include the following, some of which are expanded upon below:

• understanding causes of population declines, especially in migrant birds
• habitat requirements
• edge effects in woodland birds
• responses to habitat management
• effects of deer browsing on habitat quality
• implications of the creation of habitat networks
• expansion of scrub and woodland in upland landscapes
• effects of land abandonment
• the ecological value of ecotonal habitats
• interactions between non-native grey squirrels and birds

The emphasis of the programme is on Britain, but the BTO has an international remit and collaborates with partner organisations throughout Europe and beyond. The BTO led the ornithological work in the EU-funded BioAssess project which examined trends in biodiversity in different parts of Europe across gradients of land use from old forest to intensive agriculture. Other work has been carried out in old-growth forests in Poland and western Siberia.

Habitat requirements and modelling habitat associations

Both extensive and intensive studies of habitat associations are conducted; this information is vital for predicting how habitat change will affect woodland birds. A large scale project – the Scarce Woodland Bird Survey – is examining more than 1000 British woods to document habitat associations of selected woodland birds in Britain. One of these species, the nightingale, which is at the extreme edge of its European range in Britain, has been studied in particular detail, to identify the particular combinations of vegetation attributes that it selects and to assess how these are affected by woodland management and deer browsing.

Collaborative projects with the Centre for Ecology and Hydrology are examining fine scale spatial distributions within selected woods in relation to habitat structure and vegetation composition. Airborne LiDAR is being used to examine how canopy structure influences territory locations of a range of species in woods of differing habitat structure. We are also testing the capability of bird-habitat models for predicting the distribution of birds within coppiced woodland. Models based on data from the late 1980s are being compared with models for the same woodland derived from the early 2000s.

Woodland management

Most work on woodland management has been in broadleaved woods, especially on young growth habitats which tend to be especially important for breeding migrant birds. During the 1980s and 90s, we examined how birds respond to coppicing, a highly variable form of management. Recently, bird populations in young oak plantations have been studied, with the aim of assessing how weeding and thinning affects habitat quality for migrants.

Reduction of woodland management, with consequent canopy closure, loss of open space and shading of the understorey, is thought to be one of the factors affecting recent changes in woodland bird populations. A Before-After-Control-Impact (BACI) study has been undertaken in two woods over a 12 year period to assess how birds respond to the re-introduction of management. In both woods, rides have been widened and some compartments have been felled while other areas have been left unmanaged.

Impacts of deer on habitat quality

Numbers of deer have increased hugely in many parts of lowland Britain resulting in widespread changes to woodland structure. The potential consequences for birds and other wildlife were examined at a conference organised in 2000 and have been reviewed in papers published in the proceedings (Forestry 2001, Vol. 74 (3)). There is, however, a dearth of data. Potential impacts are being examined in two experimental studies. First, a long-term split-plot exclosure experiment is being conducted in coppiced woodland. Both vegetation and birds are being examined across the coppice cycle. Bird abundance is being measured by point counts and territory mapping, while standardised-effort mist-netting is used to assess habitat use in the post-breeding period. Radio-tracking of nightingales is also being carried out to assess their use of areas with and without deer. Second, in collaboration with the University of East Anglia, an experiment is being conducted to assess effects of reduction in deer density on vegetation and birds in lowland pine plantations.

Scrub and woodland in the uplands

Scrub and woodland were once far more extensive in the British uplands; much of what is now sub-montane moorland would have been covered by trees. High grazing pressure from deer and sheep, coupled with burning, now widely inhibits tree regeneration. Reduced grazing pressure could lead to more natural mosaics of vegetation incorporating large areas of scrub and woodland. In parts of the eastern and central Highlands of Scotland, especially on certain large nature reserves, regeneration of birch and pine scrub is now occurring, mainly as a consequence of reduction of deer densities. Successional trends in bird communities have been monitored at several locations since the 1990s. The data will be used to predict the effects on birds of large-scale development of scrub and woodland.

Vegetation changes have also been evident in the marginal uplands of Wales (the ‘ffridd’) in recent decades, but here the trend has been towards intensified grazing by sheep. However, changes in agricultural policy may result in considerably fewer livestock in future. These areas, which occur on hillside slopes below the open moorland, are typically mixtures of bracken, grass, scrub and scattered trees. Effects of land-use on this vegetation complex and associated birds form an ongoing project.

Recent publications

• FULLER, R.J., ATKINSON, P.W., GARNETT, M.C., CONWAY, G.J., BIBBY, C.J. & JOHNSTONE, I.G. (2006). Breeding bird communities in the upland margins (ffridd) of Wales in the mid-1980s. Bird Study, 53:177-186.
• SIRIWARDENA, G.M. (2006) Avian nest predation, competition and the decline of British Marsh Tits Parus palustris. Ibis, 148:255-265.
• FULLER, R.J., NOBLE, D.G., SMITH, K.W. & VANHINSBERGH, D. (2005) Recent declines in populations of woodland birds in Britain: a review of possible causes. British Birds 98:116-143.
• HEWSON, C.M., FULLER, R. J. & DAY, C. (2005) An investigation of habitat occupancy by the nightingale Luscinia megarhynchos with respect to population change at the edge of its range in England. Journal of Ornithology, 146, 244-248.
• WILSON, A.M., FULLER, R.J., DAY, C. & SMITH, G. (2005) Nightingales Luscinia megarhynchos in scrub habitats in the southern fens of East Anglia, England: associations with soil type and vegetation structure. Ibis, 147, 498-511.
• HEWSON, C., FULLER, R., MAYLE, B. & SMITH, K. (2004) Possible impacts of Grey Squirrels on birds and other wildlife. British Wildlife, 15, 183-191.
• JOYS, A.C., FULLER, R.J. & DOLMAN, P.M. (2004) Influences of deer browsing, coppice history, and standard trees on the growth and development of vegetation structure in coppiced woods in lowland England. Forest Ecology and Management, 202, 23-37.
• SIRIWARDENA, G.M. (2004) Possible roles of habitat, competition and avian nest predation in the decline of the Willow Tit Parus montanus in Britain. Bird Study, 51:193-202.
• DOLMAN, P. M. & FULLER, R.J. (2003) The processes of species colonisation in wooded landscapes: a review of principles. In: The Restoration of Wooded Landscapes. Pp. 25-36. Eds. Humphrey, J. et al. Forestry Commission, Edinburgh.
• FULLER, R.J. (2002) Spatial differences in habitat selection and occupancy by woodland bird species in Europe: a neglected aspect of bird-habitat relationships. Pp. 101-111. In: Avian Landscape Ecology – pure and applied issues in the large-scale ecology of birds. Proceedings 11th IALE (UK) Conference. International Association for Landscape Ecology (UK).
• VANHINSBERGH, D., GOUGH, S., FULLER, R.J. & BRIERLEY, E.D.R. (2002) Summer and winter bird communities in recently established farm woodlands in lowland England. Agriculture, Ecosystems and Environment, 92, 123-136.
• WILSON, A.M., HENDERSON, A.C.B. & FULLER, R.J. (2002) Status of the Nightingale Luscinia megarhynchos in Britain at the end of the 20th Century with particular reference to climate change. Bird Study, 49,193-204.
• FULLER, R.J. (2001) Responses of woodland birds to increasing numbers of deer: a review of evidence and mechanisms. Forestry, 74, 290-298.
• FULLER, R.J. & GILL, R.M.A. (2001) Ecological impacts of increasing numbers of deer in British woodland. Forestry, 74, 193-298.
• FULLER, R.J., CHAMBERLAIN, D.E. BURTON, N.H.K. & GOUGH, S.J. (2001). Distributions of birds in lowland agricultural landscapes of England and Wales: How distinctive are bird communities of hedgerows and woodland? Agriculture, Ecosystems and Environment, 84,79-92.
• FULLER, R.J. (2000). Influence of treefall gaps on distributions of breeding birds within interior old-growth stands in Bialowieza Forest, Poland. Condor, 102, 267-274.
• GILLINGS, S., FULLER, R.J. & BALMER, D.E. (2000). Breeding birds in scrub in the Scottish Highlands: variation in community composition between scrub type and successional stage. Scottish Forestry, 54, 73-85.
• DONALD, P.F., FULLER, R.J., EVANS, A.D. & GOUGH, S.J. (1998). Effects of forest management and grazing on breeding bird communities in plantations of broadleaved and coniferous trees in western England. Biological Conservation, 85, 183-197.
• FULLER, R.J. & GREEN, G.H. (1998). Effects of woodland structure on breeding bird populations in stands of coppiced lime (Tilia cordata) in western England over a 10-year period. Forestry, 71, 199-218.
• GILLINGS, S., FULLER, R.J. & HENDERSON, A.C.B. (1998). Avian community composition and patterns of bird distribution within birch-heath mosaics in north-east Scotland. Ornis Fennica, 75, 27-37.

Contact: R.J.Fuller
rob.fuller@bto.org

February 2007