British Birds VOLUME 83 NUMBER 4 APRIL 1 9 9 0

Breeding biology of the Grasshopper Warbler in Britain

David E. Glue

Secretive, mouse-like, with a boldly striated plumage that blends beautifully with the ground herbage in which it nests, the Grasshopper

Warbler Locustella naevia presents one of the hardest nest-finding chal­lenges 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] 1 3 1

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

Habitat

Woodland Scrub Farmland Heath and moor Marsh Coast and estuary Garden, park, habitations

Totals

JNo. ot nests

15 50 56 38 45 11 44

259

% 5.8

19.3 21.6 14.7 17.4 4.2

17.0

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 vegetation

Grasses Bramble Sedges Rushes Heather Reed Gorse Willowherb Nettles Deciduous trees Coniferous trees Man-made supports Other plants

Totals

No. of instances

181 40 28 16 12 9 7 7 6 5 3 3

20

337

% 53.7 11.9 8.3 4.7 3.5 2.7 2.1 2.1 1.8 1.5 0.9 0.9 5.9

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. Similar instances of hawthorn, blackthorn Primus spinosa, buckthorn Rhamnus catharticus, birch, broom Sarothamnus scoparius, dog rose Rosa canina and young conifers being used as supplementary nest supports and cover were few, as were man-made items such as fruit cane, fence post and wire netting (table 3).

The majority of nests (93.8%) were built low down, in closely knit vegetation situated from ground level up to 12 inches (0-30 cm) (table 4). Fewer nests were built higher, these most often being woven into the tops of tall rank grasses or sedges, frequently intertwined with bramble and rarely reaching as high as 36 inches (90 cm). Nests are uncommon above 25 inches (61 cm) without, for example, the support of grasses, reed or gorse. One-third of all nests are built at ground level or actually sunk into the top soil (table 4). Hedge-side ditches clothed with tangled weeds, and

Breeding biology of the Grasshopper Warbler 137

Table 4. Height above ground of nests of Grasshopper Warbler Locustella naevia in Britain Nest height

inches

1- 6 7-12

13-18 19-24 25-30 31-36

Totals

Ground level

cm

1-15 16-30 31-45 46-60 61-75 76-90

No. of nests

74 83 53

5 4 3 2

224

% 33.0 37.1 23.7

2.2 1.8 1.3 0.9

100.0

drainage dykes choked with marsh vegetation, are favoured sites, with nests being recorded as much as 8 feet (2 m) below the level of the field system.

The great majority of Grasshopper Warbler nests are very cleverly hidden. Just the occasional nest is situated in a relatively exposed position in the centre or side of a tuft of vegetation not unlike that typically used by a Reed Bunting Emberiza schoeniclus. 'Open' sites, with limited surrounding vegetation, are rare, usually in heather or rushes. Walpole-Bond (1934) found nests in withy beds in freely foliaged stubs. Nests are not infrequently built in the same segment of hedge, marsh or downland in successive seasons, even in the same tussock, as little as 2 inches (5 cm) from that of the previous year. Repeat nests have been measured at 8 m, 30 m and 60 m from the first attempt of the year.

78. Grasshopper Warbler Locustella naevia at nest, Norfolk, June 1942 (Eric Hosking)

138 Breeding biology of the Grasshopper Warbler

Breeding season The Grasshopper Warbler is a summer visitor to Britain. Ringing recoveries have yet to pinpoint the bird's precise wintering quarters. Numbers handled by ringers over recent years have fluctuated in tandem with species such as Sand Martin Riparia riparia and Whitethroat Sylvia communis, suggesting that the Grasshopper Warbler may also winter south of the Sahara in the Sahel region of the north tropical zone of West Africa (Mead & Hudson 1985).

In spring, migrant Grasshopper Warblers rarely reach our shores before the last week of March, but most of the British observatories record the species in early April. The major influx occurs at southern observatories, such as Sandwich Bay in Kent, Portland in Dorset and Skokholm in Dyfed in late April, preceding a peak passage through the northern observatories in early May (Riddiford & Findley 1981).

Such an arrival pattern compared with the Grasshopper Warbler's breeding season as shown by nest records (fig. 3) indicates an early start to egg-laying, soon after they reach Britain. The breeding season shown is calculated in two ways to achieve first-egg laying dates: first, where the time of laying was known, or where the observer had been able to age the young accurately; secondly, from cards lacking such detail but with eggs or young which were assumed to be half way through the incubation or fledging periods. In order to work back to the first-egg date, an average incubation period of 13 days and fledging period of 12 days were assumed (see below).

The laying season is a strikingly protracted one. First-egg dates spanned 99 days, from 24th April (in Somerset in 1948) to 1st August (in Montgomeryshire in 1954 and in Norfolk in 1983). Small broods of two young fledged successfully on 24th August (in Oxfordshire in 1985), while another with young aged 10-11 days was ready to leave the nest on 27th August (in Norfolk in 1983).

Most authorities suggest that the Grasshopper Warbler is usually double-brooded in the south and generally rears just one brood in the north (e.g. Witherby et al. 1943; Campbell and Ferguson-Lees 1972), but precise information is scarce. The spread of laying dates shown by nest record cards is a useful guide, but should be interpreted with caution.

Few clutches were started as early as April (six: 2.3%) or as late as August (two: 0.8%). The bulk were laid in May (173: 66.3%), and these included many known repeat layings from earlier losses. Fewer clutches were started thereafter, injune (57: 21.8%) or July (23: 8.8%). At this time, vegetation thickens, nests become more difficult to find, and the nest-finding effort may wane (Mayer-Gross 1972; Glue 1987). Later nests may go undetected, therefore, especially as Grasshopper Warblers become less vociferous in defence of territories when attempting later broods, injune and July, a feature also noted by Hulten (1959) in Luxembourg.

R. J. Louch and I. D. Tompson systematically searched sections of Otmoor, Oxfordshire, over several seasons during the 1980s (fig. 3) and found second broods the norm; other recorders, at sites in both southern and northern Britain, found the same. On the Continent, observers

Breeding biology of the Grasshopper Warbler 139

Fig. 3. Distribution of 261 first-egg laying dates for nests of Grasshopper Warbler Locustella mem found during 19iJ-86, from BTO Nest Record Cards. The sub-set (shaded) is fsf 46

nests found as part of an intensive local study on Otmoor, Oxfordshire

describe two broods as general, from France (Labitte 1949) and Luxembourg (Hulten 1959) to Sweden (Swanberg 1945). More recently, Henry (1972) found that three to five pairs breeding on marshes beside the Loire reared a minimum of two broods, and perhaps three. Taking known intervals of 35 and 37 days between successful broods, the best records (fig. 3) indicate that it is theoretically possible for three broods to be reared under favourable conditions. Five of the six latest clutches were started during warm or hot summers, with above average temperatures, when insect food should have been more plentiful.

Roaming families of Grasshopper Warblers become very difficult to detect in thick vegetation and in the absence of song. Departure dates are hard to confirm, therefore, though observations suggest that the great majority vacate breeding sites between mid August and mid September. The departure pattern at British observatories varies by station, but broadly involves three peaks in passage numbers: early August, late

140 Breeding biology of the Grasshopper Warbler

79. Grasshopper Warbler Locustella naevia singing, Netherlands, April 1973 (P. Munsterman)

August and mid September. These may correspond primarily with early dispersing juveniles, the subsequent movement of late broods or birds from northerly British populations. The great majority have left our shores by mid October (Riddiford & Findley 1981).

Clutch size

Clutch size was taken from cards showing no increase in the number of eggs on visits to active nests made more than 24 hours apart, or when one visit with eggs was followed by a visit with young and where the time interval between the two visits was less than the known incubation period.

Breeding biology of the Grasshopper Warbler 141

These criteria allowed 135 cards to be used and gave a mean clutch size of 5.45 eggs (table 5).

The most frequent clutch sizes were five and six eggs, occasionally four and seven, infrequently just three eggs. This is in agreement with most Continental studies. For example, in Germany, Kleinschmidt (1937) usually found five or six eggs, with seven the extreme; in Belgium, Verheyen (1947) generally found six eggs, two and seven rarely; and in France, Geroudet (1963) normally found six eggs, sometimes five, seldom four (second broods), and just occasionally three or seven.

Clutch size was examined in relation to date of laying (table 5). Clutches laid in the early part of the season (by the end of May), namely first at tempts and repeats, tend to be larger than those laid later. This progressive reduction in mean clutch size has been found for many passerines, and is probably related to reduced food availability later in the season (Perrins 1979).

Table 5. Clutch size of 135 Grasshopper Warbler Locuslella naevia nests in Britain

Incubation and fledging The first egg may be laid the first day after nest-building is completed. Eggs are usually laid on consecutive days, there being just five instances of a delay of 48 hours or more between egg-laying. One of the parents is either on or near the nest after two or three eggs, but incubation proper seldom starts before the clutch is complete.

The incubation period was known accurately in 11 cases: 12 days (4 instances), 13 days (4 instances), 14 days (2 instances) and 15 days (1 instance), giving an average of 13.0 days. Both sexes share in incubation, as also noted by F. R. C. Jourdain (in Witherby et al. 1943), who described the incubation period as usually 14 days, but with 13 and 15 also recorded.

The fledging period was known precisely at 15 nests: 11 days (4 instances), 12 (5), 13 (4), 14 (1) and 15 days (1 instance), giving an average of 12.3 days. Jourdain (in Witherby et al. 1943) noted 10-12 days in the nest as the general rule. Both parents feed the young. At some nests of first broods, though, when the young are seven to nine days old, a single parent, probably the male, may bring all the food. Parents brood the young at first constantly by day, then decreasingly after the fifth day.

Dead young are removed quickly from the nest. Addled and infertile eggs, however, are usually left, and not infrequently become enveloped in fine grasses at the nest base. Their invisibility presents a possible source of bias when calculating nest success after the brood has flown.

Time of laying

24/4-15/5 16/5-31/5 1/6-30/6 1/7-1/8

Totals

2

0 0 0 0

0

3

1 1 1 1

4

CLUTCH SIZE

4

3 4 3 2

12

5

15 12 14 4

45

6

30 19 15 3

67

7

3 3 1 0

7

Total

52 39 34 10

135

Mean clutch

size

5.60 5.49 5.35 4.90

5.45

Standard error

0.11 0.14 0.14 0.31

0.07

142 Breeding biology of the Grasshopper Warbler

One or more young may leave the nest cup during the day prior to the departure of the remainder. Young usually leave in the morning, but stay within a short distance of the nest before moving away. The interval between young leaving the first nest and the laying of the first egg in the second varies from four to 23 days, though Swanberg (1945) found one case of just two or three days.

Brood size Brood size was taken where the young had reached eight days or more of age or were known to have fledged successfully. This allowed 104 cards to be used, showing that the Grasshopper Warbler may rear from just one to the largest set of seven young, with an average brood size of 4.88 (table 6). Larger broods, often of five and six young, result from early nesting attempts in May. Those from repeat layings and second broods during June and July are significantly smaller.

Table 6. Sizes of 104 Grasshopper Warbler Locustella ncmia broods in Britain

Nesting success Newton (1964) has assessed the sources of biases affecting the analysis of nest record cards. Nests which survive for a long time are more likely to be found than those which fail quickly, thus overestimating success. Nests found during building, egg-laying or incubation have been used to determine nesting success of the Grasshopper Warbler.

Of 92 nests, 32 (34.8%) failed completely, and 60 (65.2%) were successful, with 26 (28.3%) rearing part of the clutch and 34 (36.9%) the full clutch. This success rate agrees closely with those of other scrub warblers which nest in low vegetation, such as Dartford Warbler Sylvia undata (68.4%) and Whitethroat (63.8%), exceeding that by those which tend to nest higher, including Blackcap S. atricapilla (62.0%), Lesser Whitethroat S. curruca (60.4%) and Garden Warbler S. borin (54.5%)(Mason 1976).

The Grasshopper Warbler is a particularly tenacious sitter and will tolerate considerable interference before desertion. The loss of just three clutches was attributed to wind damage or flooding, but weather is thought to be an under-recorded factor behind some of the 19 nests found 'empty' or 'deserted'. Three clutches were accidentally destroyed by man, two of these during forestry weeding operations. Of 14 nests robbed by predators, at least two were robbed by man, and a further five losses were attributed to the attention of fox Vulpes vulpes, common rat Rattus norvegkus and wood mouse Apodemus sylvaticus. A further 14 nests were lost when they

Time of laying

24/4-15/5 16/5-31/5 1/6-30/6 1/7-1/8

Totals

1

1 1 0 0

2

2

1 0 1 2

4

BROOD SIZE

3

4 0 4 1

9

4

3 2 6 7

18

5

11 6

12 3

32

6

20 9 4 2

35

7

1 3 0 0

4

Total

41 21 27 15

104

Mean brood size

5.10 5.43 4.52 4.13

4.88

S.E.

0.20 0.29 0.20 0.31

0.13

Breeding biology of the Grasshopper Warbler 143

contained young. At eight, the cause was unknown; one nest was accidentally destroyed by human beings while, of five nests robbed by predators, Jay Garrulns glandarius and Magpie Pica pica were implicated.

Discussion The Grasshopper Warbler occupies a wide spectrum of habitats in Britain. Like the Reed Bunting, and to a lesser extent the Sedge Warbler Acrocephalus schoenobaenus, it has shown an ability to occupy drier habitats this century. The surge in the conifer afforestation programme over the last 70 years has reputedly more than compensated for the loss of wetlands through drainage (Williamson 1974; Sharrock 1976), but, over the last 20 years, the number of BTO Common Birds Census plots containing Grasshopper Warblers has fallen substantially (fig. 4). This decline could reflect the continuing drainage of damp sedgy fields, carr and fenland, the loss of suitably thick hedges, maturation of many forestry plantations beyond the thicket stage, deterioration of the presumed tropical wintering habitat, or a combination of factors.

The Grasshopper Warbler requires three basic components in order to breed. The first is thick ground cover in which to nest. The second is several suitable song posts. These need not necessarily be tall: stooks of dead herbage, bramble leaders, short bushes, sapling trees, a wire fence, or even an overhead wire suffice. The third is a source of invertebrate food, most of which is collected within 50 m of the nest, though adults may forage 220 m distant. Observations and photography at three nests indicate that the young are fed largely on small caterpillars, fewer imagos (chiefly moths), and a wide range of other invertebrates including bugs (Hemiptera), aphids, spiders, beetle larvae, and flies (Diptera), with

80. Grasshopper Warbler Locustella naevia singing, Norfolk, May 1979 (Kevin Carlson)

144 Breeding biology of the Grasshopper Warbler

Fig. 4. Percentage of BTO Common Birds Census plots (farmland, woodland and special, combined) holding Grasshopper Warblers Locustella naevia during 1967-87. Includes (a) plots

with any record during the year, (b) plots with a confirmed territory

fewer mayflies (Ephemeroptera), dragonflies (Odonata) and crickets and grasshoppers (Orthoptera).

Suitable nesting conditions are found in 'dry' habitats such as young plantations, where cleared ground, sometimes enriched by artificial fertilisers, promotes a vigorous growth of fresh grasses. In 'wet' situations, there is most often a rapid growth of emergent vegetation, frequently where water stands in places throughout the summer. When conifers, thorn, gorse or bramble close to form an impenetrable block of scrub higher than a full metre, Grasshopper Warblers generally leave the site. They may be retained by initiating a cutting or burning programme, where sections are periodically cleared to create a mosaic of scrub islands enveloped by coarse grasses.

The Grasshopper Warbler has both a strikingly long nesting season and high reproductive potential. An average brood size of 4.87, fledging success rate of 65.2%, and the ability to rear two, or potentially three, broods, gives an expected output of from 6.35 to 9.46 young per pair. Such a performance may be helped by the species undertaking only a partial moult in Western Europe (by both juveniles and adults) before a complete moult later in the winter quarters (Ginn & Melville 1983). Such a strategy, and high productivity, may help this short-winged long-distance migrant to compensate for losses outside the breeding season.

Acknowledgments This paper was made possible by the dedicated contributors to the BTO's Nest Record Scheme, which receives financial support from the Nature Conservancy Council. Thanks go

Breeding biology of the Grasshopper Warbler 145

to Dr S. Baillie, R. J. Louch and I. D. Tompson who kindly commented on the first draft, to J. H. Marchant for extracting CBC data, and to Mrs E. Murray, who drew the figures.

Summary The nesting habits of the Grasshopper Warbler Locustelta naevia are examined, based upon 287 nests found in 46 counties spread throughout mainland Britain during 1909-86. The species breeds in a wide spectrum of both wet and dry habitats from sea level up to 1,000 feet (308 m), though chiefly in forms of scrub, farmland with thick hedges, the margins of wetlands, and young forests up to the thicket stage. The nest is most often built in coarse grasses and sedges, at ground level or as high as 12 inches (30 cm). The laying season is a protracted one, from late April to the start of August, two broods regularly being reared, with the potential for a third. Clutches of five and six eggs are usual, the mean being 5.45 eggs. There is a progressive reduction in clutch size over the laying period. The average length of incubation is 13.0 days (the extremes being 12 to 15 days), and the average fledging period 12.3 days (the extremes being 11 to 15 days). The size of brood ranges from one to seven young, with a mean of 4.88 young, larger families of five and six most often resulting from early nesting attempts in May. Of 92 nests found in the incubation stage, 60 (65.2%) resulted in the fledging of at least one young. The loss of the nest's contents was most frequently attributed to wind damage, flooding, and robbery by mammalian predators, chiefly fox Vulpes vulpes and common rat Rattus norvegicus, and avian predators, especially Magpie Pica pica.

References

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342-351. WILLIAMSON, K. 1974. Habitat changes in a young Forestry Commission plantation. Bird

Study 21: 215-217. WITHERBY. H. F., JOURDAIN. F. R. C , TICEHURST, N. F., & TUCKER, B. W. 1943. The Handbook

of British Birds, vol. 2. London. YAPP, W. B. 1955. A classification of the habitats of British birds. Bird Study 2: 111-121.

David E. Glue, BTO, Beech Grove, Tring, Hertfordshire HP23 5NR