Status and habitats of the Dartford Warbler, Whitethroat and Stonechat in Dorset in 1959-60 JV. W. Moore
INTRODUCTION From 1954 to i960 I made a study of the flora and fauna of the Dor set heathlands in order to determine the conservation requirements of the area. This involved an intensive study on the distribution of certain indicator species in the summers of 1959 and i960; the species included the Dartford Warbler Sylvia undata and the Stone- chat Saxicola torquata (Moore 196a). I also studied the Whitethroat S. communis which occurred commonly on the same heaths. Con current studies on the habitats of all three species were made, but the work was not published since I hoped to add to the rather slender data at a later date; this proved impossible owing to my move from the district in i960. Since i960 the heaths have been further reduced and the status of the three species has changed. The Dartford War bler and Stonechat were nearly exterminated in Dorset by the severe winter of 1962/63, but have recovered since. In 1969 the White- throat declined throughout Britain and its population remains much reduced (Winstanley et al. 1974). Therefore it now seems worth putting on record a description of the situation which existed 15 years ago, despite the inadequacy of the data. For the Dartford Warbler, this account also provides some background information to the more extensive recent studies of Bibby and Tubbs described in the foregoing paper.
THE BREEDING BIRDS OF THE HEATHLANDS During 1959-601 visited all the main blocks of heathland from Black Down, five miles (8 km) west of Dorchester, to the heaths lying immediately west of the River Avon (then in west Hampshire). At that time they composed 27 entities which varied in size from about 1,330 acres (538 hectares) to about 40 acres (16 hectares). All con tained dry heath, of which common heather or ling Calluna vulgaris, bell heather Erica cinerea, bristle bent Agrostis setacea and one or more of the three gorse species Ulex europaeus, U. minor (dwarf gorse) and U. gatlii (western gorse) were characteristic. All the heaths supported some trees, notably Scots pine Pinus sylvestris and birch Betula. I recorded all the species of birds observed during the breeding season (May to July) on these 27 heaths. Stonechats were found on 24 heaths, Meadow Pipits Anthus pratensis on 22, Dartford Warblers on 19, Linnets Acanthis cannabina on 18, Whitethroats on 17, Black-
196 IBrit. Birds, 68: 196-802, May 1975] Dartford Warbler, Whitethroat and Stonechat, ig§g-6o 197 birds Turdus merula on 15, and 35 other species on fewer than half of the heaths. Some of the more rarely observed species were clear ly marginal, living on the edges of the heath; probably they were only partly dependent on heathland ecosystems. These results give no more than an approximate indication of the relative abundance of the species at the time, because the heaths differed greatly in size and variety of habitat, and because some were visited more often than others.
HABITATS The Dartford Warbler was virtually confined to lowland heathlands, which were also the preferred habitat of the Stonechat, whereas the Whitethroat was numerous in a wide range of other habitats in the district. Table 1 shows that the three species were closely associated and often occurred on the same heath, the association between Dartford Warbler and Stonechat being closer than that between Dartford Warbler and Whitethroat.
Table I. Association of Dartford Warbler Sylvia undata, White- throat S. communis and Stonechat Saxicola torquata, Dorset heaths, i959-6» Number of heaths with Association association indicated
Dartford Warbler + Whitethroat + Stonechat 11 Dartford Warbler + Stonechat 6 Dartford Warbler + Whitethroat I Dartford Warbler only I Whitethroat + Stonechat 5 Whitethroat only o Stonechat only 2 None of three species present 1
TOTAL HEATHS VISITED 27 Whenever I saw one of these three species, I recorded the prin cipal plants within two yards of where I first saw it perched; these records are summarised in table 2. They show that the habitats of the Dartford Warbler and Stonechat were floristically extremely similar; common gorse U. europaeus and common heather (in some cases with small amounts of bell heather) were the preferred species, 77% of the Dartford Warblers and 69% of the Stonechats being seen in a mixture of heather and gorse. The Whitethroat was found with a wider range of plants; gorse and birch were its preferred species. Heather was often present, but only 27% of the Whitethroats were seen in mixtures of heather and gorse. It should be emphasised that practically all the sightings of the three species were within a few metres of the common heathland shrubs gorse and heather. Never- 198 Dartford Warbler, Whitethroat and Stonechat, ig§g-6o
Table 2. Plant species associated with Dartford Warbler Sylvia undata, Stonechat Saxicola torquata and Whitethroat Sylvia communis in Dorset, 1959-60 In some cases common heather was accompanied by small amounts of bell heather Erica cinerea Dartford Warbler Stonechat Whitethroat (5a records) (4a records) (45 records)
Gorse Ulex europaeus 5° 36 41 Common heather Calluna vulgaris 43 30 13 Birch Betula 10 6 21 Scots pine Pinus sylvestris 3 4 3 Sallow Salix 2 1 2 Moor grass Molinia caerulea - ~1 - Bristle bent Agrostis setacea I Undetermined grasses - h }» Hawthorn Crataegus monogyna I J 1 1 Western gorse Ulex gallii I 1 - Holly Ilex aquifolium I - 1 (dead) Bramble Rubus - 2 10 Bracken Pteridium aquilinum - 1 1 Rhododendron Rhododendron ponticum - 1 - Oak Quercus robur - - 2 Blackthorn Prunus spinosa - - 1 Hazel Corylus avellana - - 1 Grey poplar Populus canescens - - 1 theless, there is a clear indication that the Whitethroat occurred in the more marginal, and hence more complex, habitats within the heathland ecosystems, whereas the Dartford Warbler and Stone chat were associated with pure, and hence simpler, heathland habitats. This is in accord with the general distribution of the species in Dorset, as mentioned above. The heights of heather, gorse and trees at each place where birds were sighted were also recorded. The results are shown in fig 1. They indicate that: (i) The Dartford Warbler was usually found in treeless situations in which gorse was about four feet (1.2 metres) high and heather about two feet (0.6 metres) high. These observations are consistent with those of Tubbs (1963) on the habitat of Dartford Warblers in the New Forest. In no instance in Dorset were Dartford Warblers found in trees greater than eight feet (2.4 metres) high. During the period of these observations many conifer plantations with taller trees were also visited, but no Dartford Warblers were ever seen in them during the breeding season (see page 201). (ii) The pattern shown by the Stonechat was very similar to that of the Dart ford Warbler, but in five instances Stonechats were observed on trees more than eight feet (2.4 metres) high. Eight birds were also seen in places where the heather was only one foot (0.3 metres) high; only one Dartford Warbler was recorded from such low heather. The observations on die height of the gorse bushes are in accord with the statement of Johnson (1971) that Stonechats require a perch up to 1.5 metres in height, from which they drop on' to insects. ISJ-H-I i'S'S'S-g r 83,3.8 .a
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W a-1 » « P -3 1 ji S t $ i S3 200 Dartford Warbler, Whitethroat and Stonechat, ig§g-6o (iii) The Whitethroat differed considerably from the other two species in that most of the records were from habitats containing trees. No fewer than 20 birds out of the 45 recorded were in or close to trees more than eight feet (3.4 metres) high. Most of the Whitethroats were seen away from heather, but where they occurred in heather and gorse this tended to be where these plants grew to the heights preferred by Dartford Warblers and Stonechats. Theoretically, the patterns of distribution of the birds in the different categories of vegetation height could reflect availability of habitat rather than habitat preference. However, this is cer tainly not the case in so far as the lower values are concerned, because most of the heather and gorse on the heaths was lower than two and four feet (0.6 and 1.2 metres) respectively. Although there was plently of gorse and heather at heights greater than the 'prefer red' levels, the scarcity of records at the extreme upper limits—in four-foot (1.2-metre) heather and seven- to eight-foot (2.1- to 2.4-metre) gorse—might have been influenced by the relative scarcity of vegetation in these categories. However, in general, it is safe to asjume that the maxima for heather and gorse in fig 1 indicate height 'preferences'.
CONCLUSIONS AND DISCUSSION Whereas the Dartford Warbler was wholly, and the Stonechat mainly, confined to true heathland consisting of gorse and heather, the Whitethroat was found principally in the small islands of more complex habitats in which other plant species, including trees, had supplanted the heathland vegetation. Therefore, it is unlikely that the Whitethroat was a significant competitor with its close relation the Dartford Warbler or with the Stonechat. The relationship between Dartford Warbler and Stonechat is an interesting one. Not only were they usually found on the same heath (17 out of 27), but also they were frequently found in the same patch of scrub. In fact, a good way to discover Dartford Warblers in new terrain is to search those places where one has seen the much more conspicuous Stonechat. The breeding season, nesting and gene ral food requirements of the two species overlap to a large extent, but I have never observed any interspecific fighting between them, such as that recorded by Phillips (1970) between Stonechats and Whinchats Saxicola rubetra. Nor were fights observed between either of the two species and the Whitethroat. The Dartford Warbler feeds largely in heather and gorse and the Stonechat on the ground; the overlap in food requirements is almost certainly more apparent than real. The severe winters of 1961/62 and 1962/63 nearly exterminated the Dartford Warbler and Stonechat in southern England (Tubbs 1963, Dobinson and Richards 1964, Bibby and Tubbs 1975). If a Dartford Warbler, Whitethroat and Stonechat, ig
ACKNOWLEDGEMENTS My thanks are due to Mrs Margaret Haas for assistance with figure I and to John Parslow for helpful criticisms of the manuscript.
SUMMARY In 1959-60 the Dartford Warbler Sylvia undata, Whitethroat S. communis and Stonechat Saxicola torquata were present on most of the heaths of Dorset. Dartford Warblers and Stonechats preferred mixtures of two-foot (o.6-metre) common headier Calluna vulgaris, in some case.- mixed with small amounts of bell headier Erica cinerea, and four-foot (i.2-metre) common gorse Ulex europaeus in treeless areas. In die breeding season no Dartford Warbler was seen in habitats containing trees more dian eight feet (a.4 metres) high. Whitethroats were found mainly in richer habitats widiin die heaths, where there was a greater variety of plant species and where trees were present. There is no evidence to show tiiat the diree species competed under the conditions of the time, nor diat recent changes in the status of odier species are attributable to changes in die status of diese three.
REFERENCES BIBBY, C. J., and TUBBS, C. R. 1975. 'Status, habitats and conservation of the Dartford Warbler in England'. Brit. Birds, 68: 177-195. DOBINSON, H. M., and RICHARDS, A. J. 1964. 'The effects of die severe winter of 1962/63 on birds in Britain'. Brit. Birds, 57: 373-434. JOHNSON, E. D. H. 1971. 'Observations on a resident population of Stonechats in Jersey'. Brit. Birds, 64: 201-213, 267-279. 202 Dartford Warbler, Whitethroat and Stdnechat, 1959-60 MOORE, N; W. 196a. "The heaths of Dorset and their conservation*. J. Ecol., 50: 369-39I- PHILLIPS, J. S. 1970. 'Inter-specific competition in Stonechat and Whinchat'. Bird Study, 17: 320-324. TUBBS, C. R. 1963. 'The significance of the New Forest to the status of the Dartford Warbler in England'. Brit. Birds, 56: 41-48. WDJSTANLEY, D., SPENCER, R., and WILLIAMSON, K. 1974. 'Where have all the Whitethroats gone?' Bird Study, 21: 1-14.
Dr N. W. Moore, Nature Conservancy Council, 19-20 Belgrave Square, London swix 8PY