Monitoring & Recording

DARTFORD WARBLERThe first formalised monitoring of wildlife on the Pebblebed heaths was undertaken in the 1970s, with annual monitoring beginning in the late 1980s. All records are archived at the Devon Biodiversity Records Centre. Key species for which data is recorded are the Dartford Warbler, Nightjar, Curlew, Stonechat, Southern damselfly, Silver-studded blue and reptiles.

National surveys for the Dartford Warbler have been carried out in 1974, 1984, 1994 and 2006. In addition, from 1989 staff from the Pebblebed Heaths Conservation Trust and the RSPB and volunteers have monitored this species annually using the Common Bird Census methodology. Dartford Warbler populations are adversely affected by cold winters (especially with prolonged snow cover or frosts) and persistent wet weather (especially during the breeding period when whole broods may be lost).  Conversely their numbers increase during warm, dry summers and dry, mild winters.  Populations crashed after the cold winter of 1977/78 and severe winter of 1978/79, after which it took 10 years for the species to re-appeared in any numbers.  The severe winter of 2010/11 once again reduced the population which is now in a state of recovery.  Accidental fires, especially during the breeding season can destroy territories and displace birds, with the large fire on Colaton Raleigh in 2010 reducing available habitat on this Common.

National surveys have been the primary means of monitoring the Nightjar population, with these carried out in 1974, 1981, 1992, 2004 and 2010.  RSPB staff and volunteers have monitored Aylesbeare and Harpford Commons annually since 1976, Withycombe Raleigh from 1995 and Venn Ottery East from 1996.  Surveys comprise a minimum of two visits undertaken during suitable weather conditions (ideally clear and calm) either just after dusk or one hour before dawn during mid-May to early August.  Churring males are mapped and from this probable territories calculated. Data is incomplete for the whole area of the Pebblebed heaths, but comparing the results of the National Surveys numbers of breeding Nightjars have not varied to any great extent, with 60-80 territories occupied. The amount of suitable habitat remains the same on the heaths, but that on adjacent plantations decreases as the trees grow and increases as they are felled.Screenshot_1

Southern damselfly colonies on Aylesbeare have been monitored annually since 1977,  with those on Colaton Raleigh from 1994. Following re-introduction a further colony at Venn Ottery has been monitored since 2007.

Surveys are carried out using a modified ‘Pollard’ walk whereby a standardised route is followed on each site and all individuals seen are recorded.  Ideally, counts are be started between 12 and 2pm, when the temperature exceeds 17oC and in full sun with little wind.  Counts are carried out weekly throughout the flying season from late May or early June to late July or early August.  The highest total counts (including males, females and pairs) give a comparable figure for each year.

Cattle grazing was introduced onto Colaton Raleigh Common in the summer of 1998 to improve and expand the available habitat for this species. However, the colony has moved away from being concentrated on a small stream at the base of the slope to the runnels flowing down the slope, following the repair of a leak in the pipe that supplies water to Bicton gardens.  This leak was a major source of water for the small stream. Initially the colony increased where these runnels had been brush-cut under the temporary electric fence.  This site has also been burnt during the winter to remove dead vegetation which was choking the runnels and to provide a fresh flush of Molinia for the cattle.  Southern damselflies only fly in good weather conditions and the cold, wet windy summer of 2012 resulted in a significant drop in numbers, although also, fewer counts were carried out because of these unsuitable conditions

The silver-studded blue butterfly has been monitored by various volunteers from 1988 and coordinated by the Devon branch of Butterfly Conservation. Three visits are undertaken during the flight period (early June to late July) of at least 10 days apart, during calm, hot and sunny weather, between 10.30am and 3.30pm.  A transect is walked and all butterflies seen and identified are recorded.   Areas, rather than tracks, are counted by carefully walking in parallel lines and noting where the butterflies fly, to avoid double counts.   

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Silver-studded Blue butterflies are dependent on weather conditions as well as suitable habitat.  They only readily fly on calm, sunny days.  The average lifespan of the adult is 4 to 5 days and being weak fliers, rarely move further than 20m during that time.   On the Pebblebed heaths silver-studded blues are not present in large discrete colonies but scattered in relatively low numbers. The caterpillars hatch at the end of March and feed on the young shoots of heather and gorse, before being taken or crawling underground to pupate in the brood chambers of Lasius niger, a species of black ant.  These ants also require bare soil with little vegetation, which heats up quickly.  Much work has been undertaken since 1989 to provide the bare ground with pioneer heath and areas of flowering heathers for nectaring, which the butterflies need.  Clearance work has been targeted at maintaining and linking present colonies.   Burning and mowing tends to result in increased cover of Molinia, whereas scraping the vegetation down to bare soil and pebbles results in ideal habitat for a number of years.  The dips in recorded numbers from 1997 to 2000 was mirrored in larger colonies in Dorset. 

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