Dartford Warbler

Copyright

This report has been prepared for and funded by The Verderers of the New Forest Higher Level Stewardship Agreement AG00300016. The HLS partners shall have the sole right to publish the report and results of the survey, with an appropriate acknowledgement of the work or material contributed by the Contractor.

This report should be cited as: Hampshire Ornithological Society 2023, New Forest Dartford Warbler (Sylvia undata) survey. Higher Level Stewardship Agreement The Verderers of the New Forest AG00300016.

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SUMMARY

Hampshire Ornithological Society (HOS) was commissioned by Forestry England on behalf of the New Forest Higher Level Stewardship (HLS) Scheme partners to survey breeding Dartford Warblers in the New Forest. The area surveyed was primarily the New Forest Special Protection Area (SPA), including land managed by Forestry England, National Trust, Hampshire County Council and Wellow Parish Council. Some small inland heathland areas elsewhere within the New Forest National Park were also surveyed.

213 one kilometre squares were surveyed over the period April-June 2023 using the national methodology. These included all squares known to have suitable breeding habitat and/or where Dartford Warblers had been recorded in the preceding six years.

Analysis of data indicated a total of 544 Dartford Warbler breeding territories within the New Forest SPA and a further 27 territories outside of the SPA. This is the highest ever recorded in the New Forest SPA; twice as many as in 2014 when 268 territories were found and four times as many as in the last survey in 2018, when only 135 were found following unusually cold weather during February and March 2018.

Analysis of breeding density in different locations and years suggests that The New Forest could hold even more Dartford Warblers in the future if additional suitable habitat develops and if winters continue to be relatively mild.

1 INTRODUCTION

1.1 The New Forest and its designations

The New Forest is one of the largest tracts of semi-natural vegetation in the country and holds three international wildlife site designations.

It is classified as a Special Protection Area under Article 4.1 of the European Birds Directive, recognising that it is an internationally important site for breeding and over-wintering bird species. This includes an internationally important population of Dartford Warbler (Sylvia undata).

The New Forest is also designated as Special Area of Conservation (SAC) for its habitats and non-avian species of European importance, in accordance with the European Habitats Directive.

The New Forest is also listed as a Ramsar site, for its importance as a wetland, with flora and fauna of international importance.

The New Forest Site of Special Scientific Interest (SSSI) is the national wildlife designation recognising the national scientific and biodiversity value of the site.

1.2 The Higher Level Stewardship Scheme

The Higher Level Stewardship (HLS) Scheme is awarded to The Verderers of the New Forest by the Department for Environment, Food and Rural Affairs (Defra) through Natural England.

The HLS Scheme is delivered in partnership with Forestry England and the New Forest National Park Authority. This includes commissioning surveys of bird species for which the New Forest SPA is designated. The Dartford Warbler is surveyed at about five year intervals and the last survey was done in 2018.

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On behalf of the HLS partners, Forestry England commissioned HOS to survey breeding Dartford Warblers on land covered by the HLS Scheme, New Forest Crown Lands outside the scheme managed by the Forestry Commission and New Forest heathland areas managed by National Trust, Hampshire County Council and Wellow Parish Council. The primary survey area was the New Forest SPA, so that comparisons with previous years could be made.

1.3 Dartford Warblers in the UK and New Forest

In Britain, Dartford Warblers prefer to breed in lowland heathland with thick heather (Calluna vulgaris and Erica spp.) and scattered gorse (Ulex spp.). They occur widely in France and Spain where temperatures are usually higher. Although some disperse locally in late summer and autumn, Dartford Warblers are unusual among British warblers, in that they don’t migrate south to spend the winter in warmer countries with longer winter days.

Situated in the south of England with a relatively mild climate and extensive heathland, The New Forest is recognised as a stronghold for the Dartford Warbler. In recent years the species has extended its range northwards from its former southern coastal distribution and is now present as far north as North Wales, Staffordshire and Norfolk.

Dartford Warblers eat insects and spiders and these are hardest to find in cold winters when day length is short and nights are long. When winters are mild, survival is usually good and the population grows; when winters are cold and especially if there is prolonged snow cover, mortality is high and their density and range contract (Bibby, 1978; Bradbury et al., 2011). Habitat quality is also thought to be important: dense gorse and heather provide better feeding and roosting conditions than more exposed areas (Murison et al., 2007).

National surveys of the UK breeding population of Dartford Warbler were undertaken in 1974, 1984, 1994 and 2006, with some variation in sampling methods and data recording over that period.

Nationally, numbers fell to an historic low after the severe winters of the early 1960s, but recovered over subsequent years, aided by milder winters, with a total population estimated at 3,142 territories during the 2006 survey.

The New Forest SPA was surveyed as part of the 2006 national survey, recording 453 territories (420 when corrected for accuracy and within the SPA) (Fearnley et al., 2012). In 2014, a reduction to 268¹ territories was found (Gates, 2014), but evidence from limited survey work in 2017 suggested that numbers were back at around the 2006 level. Cold weather with lying snow for two periods in February/March 2018 are thought to have severely affected the population, both within the New Forest SPA where only 135² territories were found (HOS, 2019) and elsewhere in the species range.

Supplementary advice on conserving and restoring site features for the New Forest SPA (Natural England, 2019) includes the following target: ‘Restore the size of the breeding Dartford Warbler population to a level which is above 538 pairs whilst avoiding deterioration from its current level as indicated by the latest mean peak count or equivalent’. The supporting and explanatory notes explain that this objective is ‘subject to natural population variations in response to climatic factors’.

1. The total population within the New Forest SPA in 2014 will have been greater than 268 pairs because the survey did not include West Wellow Common, nor National Trust land around Hale, Plaitford Common and Bramshaw.
2. The 2018 survey also found three territories in National Trust land at Foxbury and five on Burton Common, which are outside the New Forest SPA.

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2 METHODS

2.1 The area surveyed

The 2023 survey was organised for HOS by Rob Clements and Nigel Matthews.

The boundary of the area within the New Forest to be surveyed for breeding Dartford Warblers was primarily the New Forest SPA, which is covered by the New Forest HLS Scheme. Most of the SPA is New Forest Crown Lands managed by Forestry England, with smaller areas managed by Hampshire County Council (HCC), National Trust and Wellow Parish Council. As was done in 2018, small inland heathland areas outside of the New Forest SPA were also surveyed: around Badminston Common near Fawley, Burton and Poors Commons near Bransgore, Newlands Plantation near Rockford Common, Foxbury managed by National Trust near West Wellow and Gorley and Hyde Commons managed by HCC³.

Heathland areas within the boundary were identified using the 2014 and 2018 Dartford Warbler surveys, the HOS species database, a study of relevant OS maps and through fieldwork. All 1km squares surveyed in 2014 and 2018 and all with April-June records in 2017-22 were considered. OS maps showed additional heathland, including small areas adjacent to woodland, and clearings within plantations. Including some squares with small amounts of potentially suitable habitat that were checked by the survey organisers, a total of 213 1km squares was checked, and where appropriate surveyed during the fieldwork season.

2.2 Survey methodology

HOS members were invited to survey 1km squares through a dedicated website. The same website provided survey instructions and enabled each volunteer to download a recording form for each of their squares (each with a map of their 1km square at 1:25,000). 85 volunteers responded and subsequently surveyed one or more squares each. During the survey period, two other people reported a total of five Dartford Warblers territories (via Going Birding or BirdTrack) in locations where surveyors did not find them.

Most volunteers knew the New Forest well and were experienced in doing bird surveys. Training (on two separate mornings) was offered to all volunteers and 25 less experienced volunteers took advantage of this.

The survey methodology was based on the national survey methodology (Wotton et al 2009). Volunteers were instructed to survey areas of potentially suitable habitat at least twice; preferably three or four times if Dartford Warblers were located (with a minimum of 10 days between all visits). The first two visits were done between April 1st and May 15th, and third and fourth visits between May 16th and June 30th. Where possible, visits were carried out between dawn and mid-morning, in calm, dry weather.

Wet and windy weather during the early weeks of the survey period meant that ideal weather conditions were relatively few but the large number of surveyors meant that best use of these was possible. Additional visits were made if initial visits had to be done during less favourable conditions and also where several pairs were found to be nesting closely together.

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Routes followed within the allocated squares were chosen individually by each volunteer to ensure all suitable habitat was surveyed.

The start date of April 1st fitted well with recent experience that Dartford Warbler breeding activity has started by this time. The end date of June 30th avoided the possibility of recording dispersing Dartford Warblers in squares where they had not bred.

2.3 Determination of territories

The locations of all Dartford Warblers were recorded on the maps provided. Annotations showed singing males (especially those singing simultaneously with neighbouring birds), calling birds, birds carrying food and birds in flight.

The definition of a territory followed that used in the national survey (Wotton et al., 2009, Bibby et al., 2000) as follows:

• Singing male (simultaneously singing males were regarded as separate territories as were further singing males recorded at least 200m apart)
• Pair exhibiting breeding behaviour (nest, mating, displaying etc)
• Individuals present on more than one occasion and/or two individuals present.

In most cases, where three or four visits were made, the consolidated map clearly showed a pattern of discrete territories.

A centre point (six-figure grid reference) for each territory was chosen, based on the distribution of registrations. Since numbers were so high this year, in several squares where 10+ pairs/territories were located, there were concentrations of breeding pairs with centres of territory around 100m apart, much closer than recorded in 2018.

Territories near the edge of the squares were checked against territories near the edge of adjacent squares to ensure there was no duplication.

3 RESULTS

3.1 Breeding population of Dartford Warblers in the New Forest SPA in 2023

Within the New Forest SPA, 544 Dartford Warbler territories were found in 142 1km squares, many more than in the last survey, in 2018, when 135 territories were found in 74 squares.

Outside of the New Forest SPA, territories were found at Foxbury (7 territories), Burton Common (4), Poors Common (1), Newlands Plantation (1), Badminston Common (11) and Hyde Common (3) bringing the total inland New Forest population to 571 pairs.

Figures 1 and 2 show the distribution of individual territories located across the New Forest during the 2023 survey.

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Figure 1. Distribution of Dartford Warbler territories found in 2023 – western half of survey area. Green areas: New Forest SPA; brown areas: lowland heathland; pale blue lines: National Trust land; orange lines: Wellow Parish Council land; dark blue lines: HCC land. See footnote on page 9 for copyright information.

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Figure 2. Distribution of Dartford Warbler territories found in 2023 – eastern half of survey area⁴. Green areas: New Forest SPA; brown areas: lowland heathland; pale blue lines: National Trust land; orange lines: Wellow Parish Council land; dark blue lines: HCC land. See footnote on page 9 for copyright information.

⁴ Note that coastal parts of the New Forest are not covered in this report.

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3.2 Breeding density and distribution

The table and bar chart below⁵ show that in 2023 many squares had much higher breeding densities than in 2014 and 2018.

⁵ Data used here include territories found outside the New Forest SPA in all three years.

Figure 3 shows the distribution and density of territories across the New Forest during the 2023 survey (excluding the coast).

Figures 4 and 5 compare the distribution with in 2023 with 2014 and 2018. In comparison with both 2014 and 2018 Dartford Warblers were found in many more squares in 2023; they were ‘missing’ from just a few squares.

The number of territories found in each 1km square in 2014, 2018 and 2023 is shown in Appendix A.

Six-figure grid references for each territory found in 2023 are provided in Appendix B.

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Figure 3. Number of Dartford Warbler territories found in each 1km square surveyed in 2023, with stronger colours indicating higher densities. Copyright information for all maps⁶.

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Figure 4. Changes in the breeding distribution of Dartford Warblers between 2014 and 2023

• Green square: Present in 2023 but not in 2014
• Grey square: Present in both 2014 and 2023
• Red square: Present in 2014 but not in 2023

See footnote on page 9 for copyright information.

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Figure 5. Changes in the breeding distribution of Dartford Warblers between 2018 and 2023

• Green square: Present in 2023 but not in 2018
• Grey square: Present in both 2018 and 2023
• Red square: Present in 2018 but not in 2023

See footnote on page 9 for copyright information.

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4 EVALUATION AND DISCUSSION

4.1 Fluctuations in breeding populations

The table below gives the Dartford Warbler population estimates for the New Forest SPA in each of the surveys completed since 1994 (Gibbons and Wotton, 1996, Fearnley et al., 2012, Gates, 2014, HOS, 2019).

The marked fluctuations are consistent with variations in winter weather conditions, with low population levels after hard winters and rising populations after mild winters. Although changes in habitat condition may have helped, the data suggest that the series of relatively mild winters with minimal snow cover since 2018 has enabled the New Forest Dartford Warbler population to reach its highest ever recorded level.

The 2023 population is higher than the target of 538 pairs for the New Forest SPA (Natural England, 2019).

In north-east and east Hampshire annual surveys on the Thames Basin Heaths and Wealden Heaths SPAs have shown Dartford Warbler numbers to be strongly dependent on winter conditions. The cold winters in 2008-09 and 2009-10 caused a major collapse. Numbers recovered in subsequent years until the 'Beasts from the East' in early 2018 resulted in another fall. The following run of mild winters has allowed the populations to reach record levels in recent years, peaking in 2022 (J Eyre & J Clark, pers. comm.).

During the last national survey, in 2006, the UK breeding population of Dartford Warbler was estimated at 3,142 territories. Of this total, the New Forest population of 420 territories represented around 13% and the estimated Hampshire population of around 800 pairs was about 25%.

The six SPAs designated for Dartford Warbler (including the New Forest) held a similar population in the 1990s and 2006 (1,681 and 1,654 pairs respectively). In the 1990s, these SPAs held virtually all the UK population. However, subsequent range expansion meant that by 2006 they held only 52% of the national total, an example of the changing conservation significance of protected areas with a species that is expanding its range, perhaps as a result of climate change.

4.2 Breeding densities and habitat management

In the past, the density of breeding Dartford Warbler in the New Forest compared with other heathland SPAs in southern England has been considered to be relatively low and Gates, 2014, tabulated and compared densities in various locations, habitats and years.

In 2006, the density recorded on dry heathland (their preferred habitat) in the New Forest was 0.03 pairs per hectare. This was much lower than the Dorset Heaths and Thames Basin Heaths SPAs, which had 0.11 and 0.10 pairs/hectare respectively.

During this survey, some New Forest 1km squares held up to 19 territories (0.19 pairs/hectare), a density which is compatible with the national survey methodology that most pairs are likely to nest 200+m apart, and greater than densities found in the Dorset and Thames Heaths. These squares had large amounts of good Dartford Warbler habitat (thick heather growth and scattered clumps of gorse).

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Across several adjacent squares, the New Forest habitat is typically more varied, with areas of unsuitable (or less suitable) mires, short or non-existent heather, over-mature gorse, bracken or scrub. This ever-changing mosaic of habitats suits a wide range of plant and animal species and is achieved through grazing and occasional direct habitat management (controlled burning, heather cutting etc.). It follows that the average Dartford Warbler population density across the whole New Forest is much lower than in the small proportion of squares that temporarily and coincidentally have ideal Dartford Warbler habitat.

Volunteers were not asked to grade the habitat in their allocated survey squares, so direct comparisons with previous surveys is not possible. However, examination of OS maps and online aerial views enabled a rough estimate to be made of the proportion of each 1km square surveyed that was open heathland and mire (i.e. after wooded areas, roads, farmland and residential areas had been removed). Across all 1km squares which had at least one Dartford Warbler territory there was an average of 0.09 pairs/hectare, much more comparable with the other SPAs, especially as some of these squares had wet mires which are mostly unsuitable.

Data from the RSPB Heathland Survey in Dorset, covering 31 sites mainly in east Dorset, showed a decline from 450 territories in 2017 to 369 in 2018, followed by a rise to 563 in 2022 (Chris Dyer pers comm.). These fluctuations are much smaller than had occurred in the New Forest, probably because the Dorset heathland is often managed to maximise the quantity of gorse and heather whereas New Forest habitats are more influenced by grazing and the desire to maintain and improve a wider range of rare and wildlife rich environments.

Although the New Forest may not provide wide, contiguous areas of ideal Dartford Warbler habitat and will therefore be vulnerable to spells of cold weather, its large size and mix of habitats means it is capable of sustaining a significant Dartford Warbler population. In the event of widespread cold weather and persistent snow cover, the New Forest population is likely to remain a valuable reservoir from which the national population would recover.

Although the number of ponies grazing the New Forest is fairly stable at around 5,500, the number of cattle registered to be depastured (usually for only part of the year) has reduced in recent years, from over 8,300 in 2019 and 2020 to just over 4,000 in 2022 (The Verderers of the New Forest). Resultant reduced grazing may, over time, allow heather and gorse to grow more densely and thereby increase the amount of good Dartford Warbler habitat.

Further heathland is being created as selected plantations are felled as part of the New Forest Design Plan. Initially, these areas tend to have very little heather or gorse, but over time these species will become established and thereby extend the area which is suitable for nesting Dartford Warblers.

Even during the 2023 survey, some volunteers noted that Dartford Warblers were not found in all apparently suitable habitat so, even without additional areas of habitat, numbers could increase still further if mild winters continue into the future.

5 CONCLUSIONS

A full survey of breeding Dartford Warblers undertaken in 2023 across the New Forest SPA identified a healthy total of 544 Dartford Warbler territories (and a further 27 territories within the New Forest but outside the SPA). This was more than had been found in any previous survey.

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The total of 544 territories is twice as many as in 2014 when 268 territories were found and four times as many as in the last survey in 2018, when only 135 were found following unusually cold weather during February and March 2018.

Compared with both 2014 and 2018, Dartford Warblers were present in many more 1km squares and they were breeding at much higher densities. The major factor in this increase is very likely to have been the series of mild winters since 2018.

The survey resulted in a dataset that documents the distribution and breeding densities of Dartford Warbler in the New Forest in a ‘good year’. If mild winters continue, it is likely that their numbers will increase further.

Changes in grazing and heathland creation through clear-felling of plantations could enable habitats preferred by Dartford Warblers to expand, thereby enabling a further increase in the population.

The New Forest Dartford Warbler population is significant within Hampshire and within the UK. Even though the habitats are not managed specifically for the Dartford Warbler, The New Forest is likely to provide a reservoir from which the species would recover if a series of hard winters were to occur.

The survey fulfils the commitment of the HLS board, under the agreement for the HLS scheme, to provide accurate and current population information for Dartford Warbler, one of the species for which the New Forest SPA is designated.

7 REFERENCES

Bibby, C.J. (1978) Conservation of the Dartford Warbler on English lowland heath: A review. Biological Conservation 13, 299-307

Bibby, C.J, Burgess N.D, & Hill D.A. (2000) Bird Census Techniques. Academic Press, London

Bradbury, R.B., Pearce-Higgins, J.W., Wotton, S.R., Conway, G. and Grice, P.V. (2011) The influence of climate and topography in patterns of territory establishment in a range-expanding bird. Ibis 153, 336-344

Fearnley, Hoskin R, Liley D, Whire J, & Lake S. (2012) Urban development and the New Forest SPA. Footprint Ecology/New Forest National Park Authority

Gibbons, D. W. and Wotton, S. (1996) The Dartford Warbler in the United Kingdom in 1994. British Birds 89, 203-212

Hampshire Ornithological Society, 2019. New Forest Dartford Warbler Survey Report 2018. Higher Level Stewardship Agreement The Verderers of the New Forest AG00300016

Murison, G., Bullock, J.M., Underhill-Day, J., Langston, R., Brown, A.F. and Sutherland, W.J. (2007) Habitat type determines the effects of disturbance on the breeding productivity of the Dartford Warbler Sylvia undata. Ibis 149, 16-26

Natural England, 2019. European Site Conservation Objectives: Supplementary advice on conserving and restoring site features New Forest Special Protection Area

Wotton, S. Conway, G. Eaton M. Henderson I & Grice P. (2009) The status of the Dartford Warbler in the UK and the Channel islands in 2006. British Birds 102: 230-246

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Gates, N. (2014). New Forest Dartford warbler 2014 Survey Report. Higher Level Stewardship Agreement The Verderers of the New Forest AG00300016

6 ACKNOWLEDGEMENTS

Thanks are due to Keith Betton and Ron Taylor for assistance with the planning of the 2023 survey and writing this report. Robert Klaschka kindly produced the maps. The following 87 observers contributed records to the survey (including two who sent records via BirdTrack or Going Birding).

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Appendix A. Number of territories found in each 1km square in 2014, 2018 and 2023

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Appendix B. Locations of all Dartford Warbler territories found within the survey area in 2023. Those in red font were outside the New Forest SPA.

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