Breeding Biology of the Grasshopper Warbler in Britain
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British Birds VOLUME 83 NUMBER 4 APRIL 1990 Breeding biology of the Grasshopper Warbler in Britain David E. Glue ecretive, mouse-like, with a boldly striated plumage that blends Sbeautifully with the ground herbage in which it nests, the Grasshopper Warbler Locustella naevia presents one of the hardest nest-finding chal lenges in Britain (Campbell & Ferguson-Lees 1972). Not surprisingly, therefore, the species has been little studied in Britain, though the 'Breeding Atlas' (Sharrock 1976) showed it to be widely but thinly distributed throughout most of England, Wales, southern Scotland and Ireland, except in most upland areas. [Brit.BinbS3:131-145, April 1990] 131 132 Breeding biology of the Grasshopper Warbler In Britain, the main earlier investigation was of over 200 nests found in Sussex by Walpole-Bond (1934). This paper adds further to our knowledge, by drawing on information from three major sources. Foremost, it examines 261 nest histories submitted to the British Trust for Ornithology's nest record scheme during 1941-86, then 20 nests detailed in the diaries of the late A. Whitaker during 1909-48, plus observations made by the author at six nest sites in Hampshire and Buckinghamshire. Where relevant, a comparison is made with the findings of local studies on the Continent, including those in France by Labitte (1949), Vaucher (1961) and Henry (1972), in Sweden by Swanberg (1945), and in Luxembourg by Hulten (1959). Distribution of records The 261 nest record cards cover 46 counties and most of the regions shown by the Breeding Atlas to be supporting Grasshopper Warblers (figs. 1 & 2). A few counties, notably Oxfordshire (46), but including Lancashire (33) and Somerset (19 cards), are particularly well represented, as a result of nest-finders who concentrated their efforts on this species. Wales and southern Scotland are, however, both relatively poorly covered, and Ireland not at all. Whitaker's nests were found mainly in northern England, chiefly in Yorkshire. This paper, therefore, is confined to observations made at 287 nests in mainland Britain. Altitude The majority of Grasshopper Warblers breed at low altitudes (table 1), nests from sea level up to 500 feet (154 m) accounting for 187 (94.4%) of all sites. Few breed higher, though examples were found in a range of habitats with nests up to 745 feet (228 m) in rough grass downland scrub (in Sussex in 1985), at 900 feet (277 m) in upland bog (in Radnorshire in 1972), and at 1,000 feet (308 m) on damp moorland (in Denbighshire in 1973). Breeding habitats The widespread breeding distribution of the Grasshopper Warbler in Britain (fig. 1) is in part a reflection of the bird's ability to occupy a wide Table 1. Distribution by altitude of 198 nests of Grasshopper Warbler Locustella naevia, from BTO Nest Record Cards ALTITUDE feet m No. of nests % 0- 100 (0- 31) 63 31.8 101- 200 (32- 62) 36 18.2 201- 300 (63- 92) 58 29.3 301- 400 (93-123) 20 10.1 401- 500 (124-154) 10 5.1 501- 600 (155-185) 6 3.0 601- 700 (186-215) 1 0.5 701- 800 (216-246) 1 0.5 801- 900 (247-277) 2 1.0 901-1,000 (278-308) 1 0.5 Totals 198 100.0 Breeding biology of the Grasshopper Warbler 133 Fig. 1. Breeding distribution of the Grasshopper Warbler Locustella naevia in Britain and Ireland during 1968-72 (from Sharrock 1976, by permission of the publishers, T. & A. D. Poyser). Small dots, possible breeding; medium dots, probable breeding; large dots, confirmed breeding 134 Breeding biology of the Grasshopper Warbler Fig. 2. Regional distribution of 287 nests of Grasshopper Warbler Locustella naevia covering 46 counties during 1941-86, split into the respective secondary divisions of the Euring code. Number of counties represented, in brackets spectrum of nesting habitats spanning both dry and marshy situations. On the basis of the information recorded, it was possible to allocate 259 of the nests to one or another of the broad habitat categories described by Yapp (1955). The seven fairly discrete types of terrain involved (table 2) are discussed below. The number of nests involved in each case (given in brackets) can be taken only as a very crude guide to the relative frequency with which the respective habitats are used nationwide. 1. WOODLAND (15 nests). Mature woodland is generally avoided. Blocks of deciduous woodland (10), less often conifer stands (3) or mixed forest (2), are only occasionally Breeding biohgy of the Grasshopper Warbler 135 Table 2. Broad breeding habitats occupied by 259 Grasshopper Warblers Locusteila naevia in Britain JNo. ot Habitat nests % Woodland 15 5.8 Scrub 50 19.3 Farmland 56 21.6 Heath and moor 38 14.7 Marsh 45 17.4 Coast and estuary 11 4.2 Garden, park, habitations 44 17.0 Totals 259 100.0 occupied. Most cases are in woodland which includes open glades, often where patches of bramble Rubus jrutkosus and tall grasses form a dense undergrowth. Such conditions are usually the result of uneven planting, gale damage, clearance for rides, or where extensive thinning has left scattered standards with regenerating bush scrub. 2. SCRUB (50 nests). Land covered by scrub, whether bushy shrubs or small trees 10 feet (3 m) high or less, offer prime nesting ground. Young plantations, especially at the thicket stage and where left to become part-choked with rank grasses, bramble or hawthorn Crataegus, have provided an expanding wealth of breeding sites this century. Plantations of conifers (33) are those most frequently occupied, less so mixed or deciduous ones (6), though this probably reflects only their relative abundance. Clear-felled woodland with naturally regenerating scrub (2) is a favoured nesting situation, as are tracts of bush scrub where thorn, willow Salix or birch Betula have pushed up through a grass-dominated ground flora (9). 3. FARMLAND (56 nests). Low-grade farmland, often where ill-drained or in an unkempt state, is also favoured, notably damp rough grazing, wet meadow and water-meadow (19). Grass fields left for a late cut of hay (8) are sometimes occupied, similarly uncultivated or abandoned fields (4). Witherby et al. (1943) described nests in corn, hay and clover fields. Intensively managed farms may still offer nesting possibilities; for example, where mixed arable and pastoral farmland has thick broad-based hedges and linear drainage ditches choked by herbage (10), The instances of arable farmland (15) include examples from the most intensively managed of fen and mosslands; there it is the network of drainage ditches or dykes whose sides and bottoms are clothed with common reed Phragmites australis, rushJuncus or rough grasses that offers nesting potential. 4. HEATH AND MOOR (38 nests). Pure CaUuna heathland in lowland Britain is avoided, but, where tufted grasses, bramble and gorse Ulex become established in clumps or swards, Grasshopper Warblers may breed (17). Most upland moor sites include bog with much moor-grass Molinia/Sesleria and cotton-grass Eriophorum, willow and birch scrub (10). Commons (8) and downland (3), where clothed with thick grasses, thorn and, sometimes, birch clumps, are dwindling habitats formerly much favoured. 5. MARSH (45 nests). Vegetation emerging from deep water is generally avoided. Fenland comprising very wet ground with quaking vegetation, often including tufted sedges, reeds and sweet-grass Glyceria, and the edge of extensive reed-beds (13) are prime nesting ground. So, too, are areas of marsh with dense patches of rush, often with some bulrush Typha latifotui, sedge and willow (20). The remaining fragments of mossland of northern England (11) are occupied where greater tussock-sedge Carex pamculata, meadowsweet Filipendula uhnaria and young willows intermingle. Withy or osier beds (1) are another nesting habitat formerly favoured, but now dwindling in extent (Witherby et al. 1943). 6. COAST AND ESTUARY (11 nests). Salt-laden winds help to produce swards of low, mixed grasses and stunted scrub suitable for nesting Grasshopper Warblers. Most nests on sand dunes are in the damp dune slacks (5). Coastal grassland and cliff-top sites (4) can hold high densities (e.g. Walpole-Bond 1934); especially where smothered by gorse, thorn, bramble and bracken Pteridium aquilmum. Scrub patches above the high-tide mark on saltmarsh (2) are sometimes taken. 7. GARDEN, PARK, HABITATIONS (44 nests). Industrial ground, most often where overgrown 136 Breeding biology of the Grasshopper Warbler with coarse grasses and thorn scrub (21), either awaiting development or disused, is frequently occupied. The types of management range widely from, for example, railway sidings and embankments, steel and sewage works, gravel and chalk pits to airfields. Residential ground (18) ranges from urban dereliction and domestic rubbish tips to swampy ground beside a town or city pond, lake or reservoir. Less often, the 'roughs' of golf courses (3), notably coastal links, and parkland (2) offer suitable breeding habitat. Nest sites Grasshopper Warblers require thick ground-cover in which to nest. This was most often found to be supplied by a range of coarse grasses (commonly purple moor-grass Molinia caerulea), sedges (notably greater tussock-sedge), and rushes (frequently soft rushJuncus effusus). Fewer nests were built in heather Calluna vulgaris, reed or clumps of less rigid stemmed plants such as willowherb Epilobium, nettle Urtica, meadowsweet, bracken, and thistle Carduus/Cirsium, and rarely in climbers such as honeysuckle Lonicera periclymenum, and vetches Vicia, or short annuals such as colt's-foot Tussilago farfara, sorrel Rumex and dock Rumex (table 3). Table 3. Types of vegetation wholly or partly supporting 243 nests of Grasshopper Warbler Locustella naevia in Britain Type of No. of vegetation instances % Grasses 181 53.7 Bramble 40 11.9 Sedges 28 8.3 Rushes 16 4.7 Heather 12 3.5 Reed 9 2.7 Gorse 7 2.1 Willowherb 7 2.1 Nettles 6 1.8 Deciduous trees 5 1.5 Coniferous trees 3 0.9 Man-made supports 3 0.9 Other plants 20 5.9 Totals 337 100.0 Most nests were built into the side or jammed down in the centre of a grass tuft or sedge tussock, invariably with dead foliage and where the stiffer stems and branches of bramble, or sometimes gorse or willow, were growing up and arching over the site.