Exmoor Moorland Breeding Bird Survey 2008
Andy Stanbury
During the spring and summer of 2008, a breeding bird survey, funded by the Exmoor National Park, Natural England and South West Water, was carried out on moorlands within Exmoor National Park. The main objective was to provide current data on the breeding populations of moorland birds, and put these into both a national and regional context. The survey also aimed to highlight trends in species abundance and distribution since previous surveys in 1978, 1992/93 and 2002, and investigate habitat associations relating to these trends.
Four RSPB field surveyors worked between April and July covering 171km2 of Exmoor moorland to within 100 metres. The methodology broadly followed that used during the 1992/93 and 2002 projects.
Nationally important populations in the survey area included nightjar, whinchat, stonechat, grasshopper warbler and Dartford warbler, with merlin and possibly tree pipit being regionally significant.
The results showed that avian diversity tended to be higher around the edge of the moor, within the valleys, and on the coastal heaths. Some areas of grass moor held very few species. Species showing significant increases since previous surveys included stonechat, grasshopper warbler, Dartford warbler, linnet and reed bunting.
The 2008 survey showed particularly encouraging signs for grasshopper warblers with a 25-fold increase in the species since 1992/93. Although this may be partly due to more early morning visits, they appear to be expanding across many parts of the moor, particularly within squares at higher altitudes and those with taller swards and a greater frequency of large grass tussocks.
Both Dartford warbler and stonechat have also seen overall increases in numbers since the previous surveys. Having only colonised the area in the mid-1990s (Ballance & Gibbs 2003), Dartford warblers have continued to increase in population. Stonechats have similarly shown significant increases since previous surveys. Both species have also moved altitudinally over time. The median territory height of Dartford warbler has increased by nearly 50 metres in just six years, while stonechat have moved up over 60 metres since 1992/93. Both species appear to be declining on the coastal heaths and moving to colonise higher areas within the main moorland areas.
Snipe numbers had also increased but this was quite possibly due to improved survey times rather than to an actual increase in numbers. Their population would therefore appear to be relatively stable.
A number of species had undergone significant declines since previous surveys. Kestrel, curlew and wheatear have all suffered severe declines, and red grouse (introduced to Exmoor in 1916), lapwing and ring ouzel have totally disappeared from the area. Many of those to suffer declines could be described as classic upland species. These are being replaced by species more associated with lowland habitats in the past.
Wheatears appear to have declined by over 80% since 1992/93. The evidence suggests that this may be more due to habitat change rather than climate change. Redstarts have declined by around 50% since 1992/93. Over this time, their median territory altitude has declined significantly, suggesting that they are contracting back off the moor to their core habitat zones..
The survey highlighted declines in both meadow pipit (- 7%) and skylark (-19%) since previous surveys. The data analysis suggests that these declines may indeed be higher than suggested. Skylarks seem to have declined more within squares with a higher proportion of rank Molinia and scrub, while meadow pipit appear to be faring worst within squares that are in a favourable SSSI condition assessment.
During the course of the fieldwork, data were collected on a number of habitat variables in order to give a general impression of the habitats present.
Significantly, skylark counts tend to increase with the cover of heather, Molinia, other grasses, higher grazing pressure, altitude and the presence of cattle. They also appear to show a preference for flat ground, but have a significant association with west and south-facing aspects. Increased sward height, bracken, European gorse, scrub, woodland, wetness and the presence of fences all tend to have a detrimental affect on the number recorded.
Whinchat counts within areas with taller swards, increased heather, bracken, Juncus, grazing pressure, wetness, slope and the presence of sheep and ponies tend to be higher than elsewhere. It is also evident that whinchats are closely associated with the valleys. Areas with a favourable SSSI condition assessment also tend to have higher counts. Gorse, hedges, woodland, south-facing aspect and the presence of cattle all appear to have a detrimental affect.
Grasshopper warblers show a preference for areas with taller swards and increased amounts of wetness, Juncus, heather, bracken cover and large grass tussocks. They also appear to prefer higher altitudes and, like whinchat, they are associated to some degree with the valleys.
The 2008 Breeding Bird Survey has shown that the moors within Exmoor National Park continue to hold important bird populations, but highlighted the mixed fortunes undergone by bird communities over the past 30 years. A changing climate is likely to have significant impacts on the area’s biodiversity. There is evidence that at least three species have moved altitudinally since the early 1990s. Both Dartford warbler and stonechat have moved to colonise the higher moor. It seems likely that these are either a direct (milder weather) or an indirect (changing vegetation) climate response. Continued management, particularly appropriate grazing, is vital if a range of bird communities is to be conserved.
Acknowledgements
The author would like to thanks all those who have contributed to the 2008 Breeding Bird Survey, particularly the three other field surveyors, Lee Dark, Mary Richardson and John Tayton. Thanks go to Exmoor National Park Authority, Nature England and South West Water for funding the project, and to RSPB and Exmoor National Park Authority Staff, in particular to Helen Booker (RSPB), Ali Hawkins (ENPA) and Matt Sully (ENPA) for their assistance. The author would also like to thank David Ballance for sharing his knowledge about the bird communities in the area.
References
Ballance D K & Gibbs B D (2003). The Birds of Exmoor and the Quantocks. Isabelline Books, Falmouth.
Davies S & Jarman R (1978). Exmoor Moorland Ornithological Survey. Unpublished RSPB and Somerset Trust for Nature Conservation report.
Chown D & Robins M (1994). Exmoor Moorland Breeding Bird Survey 1992/93. Unpublished RSPB report.
Geary S (2002). Exmoor Moorland Bird Survey 2002. Unpublished RSPB report.
Stanbury A, Dark L, Richardson M & Tayton J. Exmoor Moorland Breeding Bird Survey 2008. Unpublished RSPB report.