Abstract from BTO Research Report No. 502:

M. Everard & D. Noble (2008))

Association of British Breeding Birds with freshwater wetland habitats ISBN 978-1-906204-34-1

INTRODUCTION

Wild bird indicators composed of the trends of species associated with key British landscapes (such as farmland and woodland) are reported annually by the UK government and have raised public awareness about the conservation problems which many of these species face. A major objective of this project - the Wetland Bird Indicators Project - is to develop parallel indicators for breeding birds associated with different types of freshwater wetlands and waterways.

The association of species of British birds with certain habitat types at defined periods of the year is central to various routine censuses. For example, the Waterways Bird Survey (WBS) assesses populations of a defined subset of wetland-associated species found by linear waterways during the spring breeding period from late March through to July (Marchant et al., 2006). By contrast, the Wetland Birds Survey (WeBS) scheme produces indices for a number of waterfowl and waders at selected sites during the winter months (Banks et al., 2006). A number of other bird surveys are founded on habitat associations including, for example, the Garden Bird Feeding Survey, the Winter Farmland Bird Survey, the Scarce Woodland Bird Survey 2005-2006 and the Upland Breeding Bird Survey. Conversely, the Breeding Bird Survey (BBS) records numbers of a wide range of bird species in randomly-stratified quadrats across the UK (Raven and Noble, 2006) and surveys such as the Nest Record Scheme cover all habitats. The Common Birds Census (CBC) survey (Marchant et al., 1990), the predecessor of BBS, also recorded a wide range of bird species at survey sites, though these were not selected on a random-stratified basis.

The association of British bird species with habitat types is also central to various applications of these data. Habitat preferences defined in the Breeding Bird Atlas of Britain and Ireland (Gibbons et al., 1993) were used to classify species to habitat in the development of landscape-specific indicators, such as assessment of farmland bird populations (Gregory et al., 2004) and indicators for wetland, farmland and woodland birds within the UK government’s suite of Sustainable Development Strategy indicators (DETR, 1999; Defra, 2007) as well as the England Biodiversity Group’s indicators of water and wetland birds, farmland birds, coastal and marine birds, urban birds and woodland birds (England Biodiversity Group, 2006). In recent work on developing bird indicators for Scotland (Noble et al., 2006), habitat information recorded on BBS sites was used to firstly classify species as habitat generalists or specialists and, secondly, to derive habitat-specific trends for species found in more than one major habitat (e.g. farmland and woodland).

Since 2002, an indicator for Water and Wetland birds has been produced to represent one of the five landscape themes of the England Biodiversity Strategy. Composed of the population trends of a suite of wetland birds, composite trends are derived mainly from the BTO’s Waterways Bird Survey (WBS) with 1975 serving as the base year of the indicator. Since its inception, this indicator has been disaggregated into three component lines; one for birds of fast-moving waters (3 species), one for birds of standing and slow-moving waters (14 species) and one for birds of wet grasslands and marshes (4 species). As for other bird indicators, most species were classified according to their habitat categorisations in the Atlas of Breeding Birds in Britain and Ireland (Gibbons et al., 1993). These were birds of ‘lowland wetlands’ and some species classified as ‘upland’ but known to have strong associations with waterways (e.g. dipper). For three species (lapwing, pied wagtail and reed bunting) also monitored in other habitats (e.g. farmland), we used the WBS trend to represent trends in the waterways component of these populations. The latest Water and Wetland bird indicator provided to Defra in November 2007 was assembled using these categorisations, with a note to the effect that ongoing developments in waterbird indicators funded by the Environment Agency are likely to result in recommendations for a modified indicator, and a suite of sub-indicators representing a set of key wetland habitats. In the meantime, it has also been decided by UK Treasury that a new Public Service Agreement (PSA) for bird populations will be produced by aggregating the three current separate indicators for Water and Wetland birds, Woodland birds and Farmland birds (the latter already a PSA). The trend line for Water and Wetland birds will be shown alongside those for Farmland and Woodland birds to communicate information on the status of biodiversity in each sector.

The main aim of this report is to provide the background and present evidence for the categorisation of species in a revised indicator of Water and Wetland birds, including their categorisations into key wetland habitats such as fast-flowing waters and reedbeds. We also consider the representativeness and availability of key datasets for generating population trends for each of the proposed species.

This paper considers four strands of evidence leading to an overall recommendation for inclusion of bird species into a suite of indicators of five freshwater wetland types of primary interest and two of secondary interest, based upon the freshwater habitat over which environmental regulators of the UK have some degree of control. These freshwater wetland habitats include five primary types: (1) fast-flowing rivers; (2) slow-flowing rivers; (3) standing fresh waters; (4) reedbed habitat; and (5) riparian wet grassland and wetland. The distinction between ‘fast-flowing’ and ‘slow-flowing’ rivers is essentially that used in the Waterways Bird Survey (WBS), though with some subjective judgement based largely on altitude differences at either end of the survey reach of linear waterway (Marchant and Hyde, 1979 and 1980). Owing to the unique relationship in the British Isles between altitude and river slope, these ‘fast-flowing river’ birds more or less coincide with water-associated birds categorised as ‘upland’ by Gibbons; et al. (1993). Secondary habitat types of potential interest include: (6) wet woodland and (7) wet moorland. Saltmarsh and brackish habitats are excluded from this analysis which focuses primarily upon the freshwater environment.

The ‘long list’ of birds subjected to this analysis started with all 24 within the restricted list of bird species routinely indexed by WBS (Baillie et al., 2007). Other bird species were included where routine survey data (WBBS, BBS) were available and where the species were highlighted for attention by the combination of expert input from a Project Board or the literature review.

The first of the four analyses informing the choice of birds to support indicators for different wetland types in the British Isles is a ‘review of reviews’ of breeding bird associations with selected British wetland habitats. As habitat use by various bird species is known to vary across their European and wider global ranges, confounding consensus on habitat use and consistent international indicators (Gregory et al., 2005), this review is based predominantly upon British sources except in certain clearly-described instances where the literature addresses wider habitat use. For this reason, reviews addressing birds in habitats across Europe (for example Tucker and Evans, 1997) are expressly omitted from this literature search. Since there is also no consistent method of habitat classification throughout the literature, there is a degree of subjectivity and lack of transparency in determining habitat use by birds during the breeding season. Therefore, the second supporting strand of work entails analyses comparing mean bird counts in recorded habitat types in the BBS dataset as a basis for justifying the association of bird species according to their relative abundance in certain wetland types. The third evidence base is taken from an analysis of associations of bird species from the WBBS dataset with habitat features from associated stretches of river using the River Habitat Survey (RHS) methodology (Vaughan et al., 2007). Lastly, we consider the quantity or coverage of data available in order to make a confident determination of bird population trends. Habitat associations are deduced from these four evidence bases for each of a series of wetland birds of interest for indicator development. An allocation is suggested of bird species into groups potentially indicative of different wetland types, justified on the basis of their ecology and statistical association and compared to the allocation of birds to habitat classes within the England Biodiversity Group’s (EBG’s) W1: Populations of Water and Wetland Birds in England indicator (England Biodiversity Group, 2006). The process by which observed trends in population of these birds are combined into a habitat indicator, together with the statistical handling including weighting of bird species between habitat indicators and their alliance to either one or multiple habitats, will be the subject of further publications.

It should be noted that this assessment excludes analyses of species unlikely to be included in the indicator because of their rarity (e.g. black-throated diver) or the lack of reliable monitoring data (e.g. water Rail) even though these species may show very strong associations with particular habitats. The best example is bittern, a species found only in reedbeds of particular size and quality, but occurring in such low numbers and on such a restricted range of sites that it would be difficult to use it for assessing changes in biodiversity more broadly.

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