The status and habitat of the Nightjar Caprimulgus europaeus in coastal Suffolk

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1 Bird Study ISSN: (Print) (Online) Journal homepage: The status and habitat of the Nightjar Caprimulgus europaeus in coastal Suffolk N. O. M. Ravenscroft To cite this article: N. O. M. Ravenscroft (1989) The status and habitat of the Nightjar Caprimulgus europaeus in coastal Suffolk, Bird Study, 36:3, , DOI: / To link to this article: Published online: 24 Jun Submit your article to this journal Article views: 321 View related articles Citing articles: 8 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 05 December 2017, At: 11:21

2 Bird Study (1989) 36, The status and habitat of the Nightjar Caprimulgus europaeus in coastal Suffolk N.O.M. RAVENSCROFT* Suffolk Wildlife Trust, Park Cottage, Saxmundham, Suffolk IP17 1 DQ, LIK Numbers of male Nightjars Caprimulgus europaeus were counted annually on the heathlands of the Suffolk coast (the Sandlings), and within coniferous plantations of the same region. The population of Nightjars here has shown a dramatic increase since a national survey in 1981 owing to the colonization of recently cleared coniferous plantation. The response of the population to the availability of habitat suggests that the reason for an earlier decline in numbers is habitat loss. Nightjars now exist in higher densities within plantations than on heathland. They prefer areas replanted for between 1 and 5 years, and occupy areas freshly felled up to 8 years old. The use of herbicides to suppress the growth of invasive plants is believed to be aiding Nightjar populations by prolonging the suitability of the habitat, The smallest size of clearing occupied by birds was 10 ha, and density was greatest when the shape of a clearing approached that of a circle. It is argued that restocked plantations resemble areas of preferred habitat which is often confined to the edges of heathland. Since the beginning of this century large areas of Great Britain have been afforested with commercial species of coniferous trees. Such plantations support a number of bird species, 1 ' Z many of which breed more usually in broadleaved woodland in Britain. One species that has taken to breeding in clearings created in plantations is the Nightjar Caprimulgus europaeus. This is traditionally a bird of dry, open country with scattered trees, 3 but one which has bred in a range of habitats such as chalk downland and even shingle beaches. 4 It is also a species that has undergone a severe contraction in range in the British Isles over the past fifty years or more. s' 6 A survey by the British Trust for Ornithology (BTO) in 1981 showed that the population of Britain and Ireland was unlikely to exceed 2100 pairs, and that the contraction in range from the north and west has continued.' *Present address: Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen AB9 2TN. Today, the strongholds of the Nightjar are the heaths of Surrey, Sussex, Dorset and the New Forest and the heaths and Breckland of East Anglia,' although, even in these areas, declines have been noted. s' s In Suffolk, Ticehurst9 found the species to be widespread on heaths, commons and over the whole of the Breckland. Since 1945, however, it has become less numerous in the county and has disappeared from many of its former haunts. 1 Large areas of heathland have been converted to coniferous plantations in the Breckland and the coastal heathlands: the Sandlings. Initially, these consisted of Scots Pine Pinus sylvestris, but recently areas have been restocked with Corsican Pine P. nigra. Planting has taken place on the poorest soils least suited to agriculture (land use capability classes 5 and 6). 71 As a result of these and other developments the Sandlings heathland is now very fragmented. As part of a programme of research and management on the Sandlings, the Suffolk Wildlife Trust (SWT) has investigated the status of a number of heathland species, including the

3 162 N.O.M. Ravenscroft. c ^esr^,. _r-i riss on Irit3^ 20 km Figure 1. The changes in the extent of Sandlings heathland over the past century (left and centre) and the present area of coniferous plantations (right). I = Ipswich, W = Woodbridge, A = Aldeburgh and S = Southwold. R, T and D = Rendlesham, Tunstall and Dunwich Forests. Sources of data: Sandlings Project (SWT) and Forestry Commission. Nightjar. This paper reports on its status and ecology in coniferous plantations and heathland in coastal Suffolk. Study area The Sandlings was formerly an extensive belt of lowland heath that stretched from Ipswich in the south almost to Lowestoft in the north along the sandy soils of the eastern coast of Suffolk (Fig. 1). At the turn of the century, there was an estimated 7000 ha of this habitat which had been reduced to about 1800 ha by Much of the original heathland has been planted with conifers (29%), but much has also been lost to agriculture (29%) and urban Pre Decade Figure 2. The area of coniferous plantation planted on the Sandlings between the late nineteenth century and the present (dark stipple) and the amount since felled and restocked (light stipple). development (14%), especially in the Ipswich region. Lack of management has also caused extensive loss, and much of the present heathland is dominated by bracken Pteridium aquilinum (33%) or covered by trees (16%) and scrub (13%). Two major tracts of heathland remain: the Sutton and Hollesley heaths east of Woodbridge (c. 450 ha) and those of Westleton, Dunwich and Minsmere south of Southwold (c. 350 ha, Fig. 1). There are 3 major coniferous plantations on the Sandlings: at Dunwich (470 ha), Tunstall (1100 ha) and Rendlesham (1400 ha), and their total extent is almost double that of remaining heathland in Suffolk (Fig. 1). Altogether, coniferous plantations form nearly 50% of all woodland` and cover ha. These have increased since 1947 by over 4500 ha. 13 Most of the planting of these forests took place in the 1920s and 1930s (Fig. 2). Much has now reached felling age and large areas have been cleared and restocked during the past decade. These represent high proportions of the original planted areas: Rendlesham 28.3%, Tunstall 18.9%, Dunwich 20.4%, combined 23.6%. METHODS In the TO census most areas of Sandlings heathland and restocked plantations were surveyed. Subsequently, annual surveys

4 were undertaken on these and additional areas from 1983 to Numbers of birds for other areas of heath regularly counted (e.g. by the Royal Society for the Protection of Birds at Minsmere) are included in this paper. In each year, the major areas of heathland and coniferous plantations were surveyed (except Rendlesham in 1983 and 1984) although it was not always possible to encompass all small pockets. Recording Observers were asked to mark registrations of 'churring' male Nightjars and sightings of birds onto a 1: scale map of the site being surveyed. Sites were divided prior to survey into small compartments by landmarks or according to felling programmes within a forest, and pairs of observers patrolled each compartment. Up to 30 ornithologists were required to survey the larger plantations. Each year a site was surveyed for one or more evenings between and h GMT in late June and early July following the methods described by Cadbury' } and Gribble. 7 Individual birds were recognized from simultaneous registrations by a single observer, registrations by 2 or more observers or by a sequence of time records for individual birds occurring in different positions. The detection rate of birds was high, as revealed by monitoring certain sites over a season. Cadbury'`' expected at least 75% of males to be recorded on a single visit to a site. Vegetation sampling The vegetation of Forestry Commission clearfell and restocked areas was surveyed during Nightjars in Suffolk 163 the summer of All compartments in two forests (Rendlesham and Tunstall) up to 10 years old (8 years after planting) were covered and the planting age of each (U = unplanted, 0 = planted since the previous summer, 1 = one year after planting, etc.) used to indicate differences in ground cover. Each compartment was sampled by random 5 m x 5 m quadrats, the number of which varied according to its size to a maximum of 5 for a 20 ha compartment. The ground cover and height of each species or category (including trees) was recorded using 100 points within each quadrat and the density of Scots Pine, Corsican Pine and Silver Birch Betula pendula was determined by counting the number per quadrat. For each planting year, an average of all quadrats within compartments of the same age is taken as representative of the vegetation present. Other data The density of birds is expressed as the number per 10 ha of habitat. Available habitat is defined as either the total area of a particular age-class of restocked plantation or the area of a clearing. Areas and circumferences of compartments were measured, using a planimeter, from Forestry Commission maps and those of heathland from past and present Ordnance Survey maps. RESULTS Population size and distribution The number of Nightjars breeding on the Sandlings has undergone a rapid increase since 1981 (Table 1). Some of this increase is Table 1. Number of male Nightjars estimated from surveys of major sites on the Sandlings, Sutton and Hollesley heaths Minsmere, Westleton and Dunwich heaths Other heaths and commons Rendlesham Forest Tunstall Forest Dunwich Forest Total not surveyed

5 164 N.O.M. Ravenscroft Table 2. Comparison of Nightjar numbers (males) recorded from the two habitat types of the Sandlings, using heathland data from sites surveyed in every year (forestry total for 1983 and 1984 is minus Rendlesham) Heathland total Forestry total 4 (5) (8) %U coniferous due to improved coverage of sites but most is due to the numbers breeding in coniferous plantations. Comparison of the population recorded from heathland sites studied in every year with the total recorded in forestry areas indicates the dramatic nature of this increase (Table 2). One heathland population also increased over the same period (Table 1) although the extent of interchange between this and small surrounding areas sporadically surveyed is unknown. Other populations show slight fluctuations but little indication of any decrease. Habitat preferences of Nightjars in restocked areas During a combined total of 105 churring males was recorded in plantations and these form the basis for further analyses. Vegetation of woodland age-classes The vegetation of the clear-felled and restocked areas occupied by Nightjars showed large variation according to age-class (Fig. 3). Immediately after felling, Bramble Rubus fruticosus agg. and Bracken became dominant. Their cover was greatly reduced in age-classes 1-5 where grasses (Festuca ovina, Agrostis stolonifera, Holcus lanatus and H. mollis), Silver Birch, heathers (Calluna vulgans and Erica cinerea) and gorse Ulex europaeus all became more established. Beyond Year 5 the trend was reversed and Bramble and Bracken once again became dominant. The amount of bare ground decreased to a minimum by Year 5. At that stage debris produced by felling and management practises was also small. Willowherb Chamaenerion angustifolium increased steadily and became well established in the later stages. Most vegetation remained short until age-classes 3-4 when Silver Birch and pines (both Scots and Corsican) started rapid growth. The density of trees increased greatly from the planting density of 2600 ha -1 to over ha -1 2 years after planting (Fig. 4), reflecting the immense amount of natural regeneration of Scots Pine that occurs through seeding, even within Corsican Pine plantations. Eight years after planting, this almost returned to initial planting density as a consequence of management. Age class of plantations Nightjars exhibited a preference for certain age-classes of restocked plantations. The most favoured age-classes lie within the range of 1 to 5 years after replanting and the highest densities occurred in age-classes 3 and 4 (Fig. 5). The preferences for these age-classes appeared to become less marked by 1987 when there was an increased use of the youngest and oldest clearings. It seems that clearings become unsuitable at about 10 years after planting although few older restocked areas were available for survey. Effect of size and shape of clearing Further analyses of the use of clearings by Nightjars were made for those clearings in the preferred age-classes of 1-5 years. The peak numbers that occurred in each of these clearings in the years of study were used as the data is successional rather than stable. The size of a clearing was defined as that enclosed within a continuous boundary of mature plantation and thus may encompass more than one age-class. As might be expected, the number of birds and the size of an area occupied showed a

6 Nightjars in Suffolk 165 (a) Pines as Silver Birch Grasses 60 Heather & Gorse Bracken Bramble 40 -El Willowherb 20 3 U 0 i ^ "' (b ) > ^ Age-class of clearing (years) -^_.^ ^'/, ^;i - _ La Debris L Moss Silver Birch Pines Bracken ^ Willowherb /^ Bramble / - ^ ^<--^ -^ \ \ ^ Grasses 0 ' C.\,%-'1^ Gorse U 0 I Age-class (years) Bare ground Figure 3. The pattern of vegetation change on the ten age-classes of restocked plantation studied: (a) average cover afforded by vegetation categories and (b) their average heights. strong relationship (correlation coefficient r 12 = 0.86 P < 0.001, Fig. 6). Density of birds (two or more), however, did not bear any relationship to the area of a clearing (r7 = 0.42). There appears to be a minimum size that birds will occupy, about 10 ha, below which no birds were found. Similarly, there was a clear dividing line between the areas occupied by 1 and 2 birds, at about 23 ha. At higher population levels it becomes increasingly difficult to predict the numbers that will occupy a given area (Fig. 6). The maximum density recorded was 1.26 males 10 ha -1 (7.9 ha bird- ') and the minimum ha- i (16.8 ha bird -1 ). The circumference of clearings that contained 2 or more birds was not correlated with the number of birds within them (r7 = 0.57). However, there is a significant negative relationship (r7 = 0.74, P < 0.025) between

7 166 N.O.M. Ravenscroft i O 20,000 - s ^ ^ = 10,000 ó ^ 0 0 I Age- class (years) Figure 4. The densities of Pinus sp. (dark stipple) and Silver Birch (light stipple) recorded from different age-classes of restocked plantation (no data for ageclass 7) i 0 o 0 á o 0-4 o , ^ ^ ^ U Age-c l ass (years) Figure 5. The densities of birds occupying plantation age-classes varying from unplanted (U) to 8 years old in Rendlesham, Tunstall and Dunwich Forests combined. = 1985, 0 = 1986, = Nightjar density and the circumference/area ratio of clearings. For a given area, as the shape becomes more irregular, the circumference/ area ratio increases. As a clearing's shape departs from a circle or square, so its Nightjar density decreases (Fig. 7). DISCUSSION Contrary to the reported pattern of a national decline in numbers and distribution,'''' $ the population of Nightjars on the Sandlings region of Suffolk has undergone a large increase in recent years. It is probable that some of this can be accounted for by enhanced survey efficiency, but largely represents the colonization by Nightjars of new habitat in the form of clearings in coniferous plantations. Moreover, there appears to be no evidence of a continued decline in populations inhabiting heathland as noted by previous authors for Suffolk. 910 Accordingly, areas of coniferous plantations that have begun their second rotation are becoming, numerically, the most important habitat for Nightjars in the county. This is entirely due o ^ 09 T 07 ^ I I Area of clearing (ha) Figure 6. The areas occupied by differing numbers of Nightjars within all three forests (correlation coefficient r 12 = 0.86, P < 0.001). Dashed lines (a) and (b) indicate 95% confidence limits of regression line (eqn: y = 0.07x+0.13) and lines (c) and (d) show 95% confidence limits for predicting numbers of Nightjars from the area of habitat. s. 'I I I i Shape o` cleanng (circum'erence/area, b) Figure 7. The densities of Nightjars in different shaped clearings showing a decrease with increased eccentricity (correlation coefficient r 7 = 0.74, P < 0.025). Dashed lines indicate 95% confidence limits of regression line (equation: y = x). to the increased availability of such habitat as immense stretches of mature plantations created on heathland in the 1920s have been felled and restocked in the past decade. Within plantations, Nightjars exhibit a preference for areas that have been felled, restocked and growing for between 1 and 5

8 Nightjars in Suffolk 167 years. Although earlier stages may support birds, they do so at a low density similar to that of the latest successional stages occupied (6-8 years after planting). The use of these extremes appears to be increasing, perhaps reflecting breeding success in more preferred clearings which are at, or approaching, capacity. Few areas of older age classes existed during the study, but it seems that clearings become unsuitable about 10 years after planting. At this age, pines and Silver Birch are above 3 m in height, which Berry' also regards as unsuitable for Nightjars on heathland. Hughes' s found that Nightjars breeding in plantations in Sussex most often frequented areas with trees less than 2 m in height, and that records of birds in areas above 3 m were scarce. In the present study, the height of trees in the preferred zones was between 0.5 m and 1.75 m and the density of trees, especially of Silver Birch, was at its greatest. It is difficult to analyse the preference of the Nightjar for a particular vegetation type, but it appears that the overall mosaic and the presence of bare ground and heathers are important for nesting. 3 ' 16 Plantations 1-5 years old are the most diverse in terms of vegetation structure, species composition and topography: they are notable for the presence of grasses, heathers, Silver Birch and the low height of vegetation apart from trees. These characteristics may play important roles in the provision of food (e.g. Silver Birch is a food plant of several abundant moths such as the Common White Wave Cabera pusaria) and other features attractive to the Nightjar, such as song posts on surrounding mature pines or standards retained within clearings. The development of the vegetation in restocked areas is largely a consequence of management. In the early stages of growth and, indeed, immediately prior to planting, regenerating and invading ground flora is a major problem for the Forestry Commission. In coastal Suffolk, several applications of herbicide (usually glyphosate (Round-up) or occasionally Asulam for bracken) are applied to control this. Bramble and bracken are reduced and the amount of open ground and other species increases. Later, mechanical swiping is used to control the large amounts of regenerating Scots Pine and Silver Birch. Some areas, however, are left to regenerate naturally and receive little management. These are characterized by rapid and dense growth of vegetation, especially Scots Pine, the height, density and cover of which greatly exceed those of similar aged managed plantations of Corsican Pine (Table 3). None of the regeneration areas of any age supported Nightjars in the study. It seems, therefore, that the management procedures adopted by the Forestry Commission are benefiting Nightjar populations by prolonging the suitability of the available habitat. Table 3. Vegetation characteristics of managed Corsican Pine plantations and little managed, regenerating Scots Pine areas of the same age Managed plantations Natural regeneration Corsican Pine Corsican Pine Scots Pine 1983 Pines: density cover 12.2± ± ±4.9 height 0.9± ± ±0.1 Silver Birch: cover 0.3± ± ±1.5 height 1. 5± ± ±0.2 Bracken: cover 2.0± ± ±4.2 Bramble: cover 13. 2± ± ±3.3 Other herbs: cover 60.7± ± ±3.7 Open ground: cover I 1.6± ± ±8.3 Data expressed as mean + se, covers in %, height in m and density numbers ha-'.

9 168 N.O.M. Ravenscroft That there is a minimum area of clearing that will support Nightjars (c. 10 ha) is obviously important when considering management for the species. Furthermore, there is no indication that larger areas support greater densities of birds. Rather, the shape of a clearing seems to be instrumental in determining density. The minimum area required was 8 ha per churring male in plantations, the maximum was 17 ha; birds usually occupied between 11 and 14 ha each. In a Bavarian forest Bruenner" found birds occupied 11 ha each. Compared with heathland studies, a preference, in terms of bird density, for plantations is evident. On heathland, Lack 18 thought about 12 ha per bird to be normal while Cadbury 14 calculated 21.3 ha per bird. Berry 3 found values between 6 and 25 ha. On Sandlings heathland, an average of 21 ha per bird was recorded over 5 years at one site. 19 In the year of least numbers this became 26 ha and at maximum 17 ha. Nightjar numbers also appeared to be determined by the amount of tree edge present on the site: the more present, the greater the number of birds. This correlates well with the known association of the bird with such habitat. 3' 14 The edge of heathland in Suffolk is typically an invasive zone of pine and Silver Birch. The vegetation of restocked plantations may, therefore, resemble large areas of habitat often confined in extent on heathland, an effect heightened by the occasional retention by the Forestry Commission of broadleaved and coniferous standards in clearings. This may explain the preference of birds for certain shapes of clearings as the more irregular the shape then the closer birds are to a mature tree edge (in plantations the reverse is the case; the less tree edge the better). In such clearings, dense vegetation is characteristic of edges as invasion by the ground flora of surrounding mature plantations takes place, in particular by bramble and bracken. 20 The apparent preference of the Nightjar for zones of regenerating woodland presents problems for conservationists who are striving to maintain existing areas of heathland and restore others. It may also partially explain the past decline of the species, as heathland eventually reverts to woodland through lack of management, a factor responsible for much loss of this habitat. Despite this, the decline of the Nightjar has previously been thought to be climatic in origin' 16 and this has also been used to explain the decline of other bird species. 21,22 However, the response of Nightjars to large areas of suitable habitat in the form of plantation clearings suggests that the major cause of the original decline, at least in Suffolk, is habitat loss. The Forestry Commission had projected a felling programme for the Forests of Dunwich, Tunstall and Rendlesham of 80 ha per year until 1991, 70 per year until 2001 and thereafter about 50 per year. This would have ensured a succession of habitat continuing to support a large population of the Nightjar in Suffolk. However, the destruction of large areas of these forests by the storm of October 1987 will result in premature clearance of much timber. If these areas are restocked simultaneously they will temporarily create an abundance of suitable habitat, but eventually this will greatly decrease; an effect similar to that of the original planting in the 1920s. It seems likely, however, perhaps owing to the scale of the damage, that areas will remain for some time within the plantations for breeding Nightjars. ACKNOWLEDGMENTS Many people, too numerous to mention individually, helped with the surveys, in particular employees of the SWT, and members of the Suffolk Ornithologists Group. Thanks are extended to all. The survey owes much to the enthusiasm of the late M. Cavanagh and was organized as part of the Sandlings Project of the SWT of which C. Fitzgerald, E. Fish, M. Rose and Dr M. Auld are to be thanked for their involvement and for the provision of data on heathland loss. J. Sorensen (RSPB) and C. Waller (NCC) supplied additional survey data, and Drs D.E. Glue and R.J. Fuller useful discussion. Drs R.J. Fuller and C.F. Mason and two referees kindly commented on the manuscript. Thanks are due to the Forestry Commission for maps of restocked plantations and additional information. REFERENCES 1. Moss, D. (1978) Songbird populations in forestry plantations. Quarterly J. For. 72, Bibby, C.J., Phillips, B.N. and Seddon, A.J.E. (1985) Birds of restocked conifer plantations in Wales. J. Appl. Eiol. 22,

10 Nightjars in Suffolk Berry, R. (1979) Nightjar habitats and breeding in East Anglia. Brit. Birds, 72, Bannerman, D.A. (1955). The Birds of the British Isles, Vol. IV. Oliver & Boyd, Edinburgh and London. 5. Stafford, J. (1962) Nightjar enquiry Bird Study, 9, Parslow, J.L.F. (1973) Breeding Birds of Britain and Ireland. Poyser, Berkhamsted. 7. Gribble, F.C. (1983) Nightjars in Britain and Ireland in Bird Study, 30, Sharrock, J.T.R. (1976) The Atlas of Breeding Birds in Britain and Ireland. Poyser, Berkhamsted. 9. Ticehurst, C.B. (1932) A History of the Birds of Suffolk. Gurney & Jackson, London. 10. Payn, W.H. (1962) The Birds of Suffolk. Ancient House, Ipswich. 11. Bibby, J.S. & Mackney, D. (1969) Land Use capability classification. The Soil Survey, Technical Monograph No Ravenscroft, N.O.M., Ireland, E.M. & Pankhurst, T.J. (1987) Suffolk Habitat Survey-results of a survey of the botanical resources of Suffolk. Report to Nature Conservancy Council. Suffolk Wildlife Trust, Saxmundham. 13. Forestry Commission (1982) Census of Woodland and Trees, County of Suffolk, Cadbury, C.J. (1981) Nightjar census methods. Bird Study, 28, Hughes, S.W.M. (1980) Nightjar habitats and status in Sussex. Brit, Birds, 73, Berry, R. & Bibby, C.J. (1981) A breeding study of Nightjars. Brit. Birds, 74, Bruenner, K. (1978) Two year study of a population of the European Nightjar Caprimulgus europaeus in Northern Bavaria. Anz. Ornithol. Ges. Bayern, 17, Lack, D.L. (1932) Some breeding habits of the European Nightjar. Ibis, 74, Ravenscroft, N.O.M. (1987) Numbers and habitat of the Nightjar population of the Sandlings, Suffolk Suffolk Wildlife Trust, Saxmundham. 20. Hill, M.O. (1979) The development of a flora in even aged plantations. In The Ecology of Even- Aged Forest Plantations, (Eds E.D. Ford, D.C. Malcolm & J. Atterson), pp Institute of Terrestrial Ecology, Cambridge. 21. Monk, J.F. (1963) The past and present status of the wryneck in the British Isles. Bird Study, 10, Ash, J.S. (1970) Observations on a decreasing population of Red-backed Shrikes. Brit. Birds, 63, , ,(MS received 23 May 1988; revised MS received 7 November 1988)