National Woodland Re-survey Project

Funded by
DEFRA, Forestry Commission, Natural England,
RSPB, Woodland Trust and the BTO

Background

Many of the specialist birds of our woodlands are in trouble. The government’s Headline Indicator of Woodland Wildbird Populations has fallen by 20% in 20 years and in a recent revision of Birds of Conservation Concern several woodland species were added to the lists due worrying declines in their populations. Marsh Tit, Willow Tit and Lesser Spotted Woodpecker are now on the Red List while Tree Pipit, Willow Warbler, Wood Warbler and Lesser Redpoll have been added to the Amber List. The situation is potentially every bit as serious as that which has occurred on farmland.

The data that these trends are derived from come primarily from the BTO’s Common Birds Census. Whilst there have undoubtedly been large declines in the species concerned within the sample plots covered by the CBC, we cannot be sure that these declines will be evident in all types of woodland and in all regions of the UK. The volunteers who have contributed to the data collection select their own plots, which means that the sites which contribute to the index are not necessarily representative of woods across Britain. For instance, woods of a certain size or type may be over-represented in the sample, whilst the distribution of plots across the country inevitably shows a strong bias to the south and east – areas where most BTO members live.

The Aims

The BTO and RSPB have teamed up to carry out a National Woodland Bird Re-survey Project which has two purposes. Firstly, it aims to improve the estimates of changes in woodland bird populations. For some species the current trends are based on observations of very few territories and by increasing the sample sizes on which the estimates are based we can be more sure about what’s happening to their populations. Also, by improving the regional balance of the data we can ensure that the overall figures are not unduly biased to well-covered regions, and see whether trends differ between parts of the country. Secondly, the project aims to make a preliminary test of some of the many hypotheses explaining the observed trends. Whereas with farmland birds there were obvious candidates to explain the declines, as agricultural practices and landscapes had undergone large changes in recent decades, the factors affecting woodland birds seem to be more obscure. The variety of species affected suggests that multiple factors may be at work. These include some intrinsic to the woods themselves (such as changes in the habitat available to birds due to changes in management, tree species composition, age-structure and deer-browsing), landscape-level factors such as woodland fragmentation and changes to surrounding farmland, and wider-scale issues such as climate change, air pollution and changing conditions on the wintering grounds of migrants. If at least some of the possible mechanisms outlined above can be ruled out at this early stage, then further research can be much more focussed.

The Project

Both the BTO and the RSPB have good quality datasets from the past which can be used to meet these objectives, comprising surveys undertaken in woods throughout the UK between the 1960s and 1980s. These sites are being revisited to estimate what changes have taken place. The BTO are returning to more than 150 old CBC plots covered during the period from the late1960s to the1980s, using a combination of trained professional fieldworkers and volunteers. These re-surveys are somewhat less time-consuming than the original CBCs since we will only be making four visits to each site in a season. This ensures that we can cover a greater number of sites - the original survey data will be re-analysed based on four visits to make sure the two surveys are comparable.

Detailed measurements of habitat characteristics will be made to look for associations between bird density and habitat and to see whether changes in habitat availability could explain changes in bird numbers. We will also assess deer impacts on vegetation (since the numbers of deer themselves are difficult to estimate) and the relative abundance of squirrels in the woods. Measures of squirrel abundance can be determined through dray counts and even counts of the animals themselves, if conducted at the right time of year. Finally, we will take into consideration a variety of other features associated with the woods we visit, such as the landscape context (e.g. proximity to urban centres, proportions of surrounding farmland), levels of recreational disturbance and factors related to woodland management.

This project will help to confirm the extent to which the population trends shown in the Headline Indicator of Woodland Wildbird Populations apply to all regions and hopefully provide some characterisation of the woods in which bird populations are most adversely affected.