British Birds
VOLUME 8 3
NUMBER 4
APRIL 1 9 9 0
Breeding biology of the
Grasshopper Warbler
in Britain
David E. Glue
ecretive, mouse-like, with a boldly striated plumage that blends
S
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]
131
132
Breeding biology ofthe 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
0- 100
101- 200
201- 300
301- 400
401- 500
501- 600
601- 700
701- 800
801- 900
901-1,000
Totals
m
No. of nests
%
(0- 31)
(32- 62)
(63- 92)
(93-123)
(124-154)
(155-185)
(186-215)
(216-246)
(247-277)
(278-308)
63
36
58
20
10
6
1
1
2
1
31.8
18.2
29.3
10.1
5.1
3.0
0.5
0.5
1.0
0.5
198
100.0
Breeding biology ofthe 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 ofthe 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
Scrub
Farmland
Heath and moor
Marsh
Coast and estuary
Garden, park, habitations
Totals
15
50
56
38
45
11
44
5.8
19.3
21.6
14.7
17.4
4.2
17.0
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 ofthe 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
9
Reed
2.7
Gorse
7
2.1
7
Willowherb
2.1
Nettles
6
1.8
Deciduous trees
5
1.5
Coniferous trees
3
0.9
3
Man-made supports
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. 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
cm
No. of nests
%
Ground level
74
33.0
1- 6
1-15
83
37.1
16-30
53
7-12
23.7
13-18
31-45
5
2.2
19-24
46-60
4
1.8
25-30
61-75
3
1.3
31-36
76-90
0.9
2
Totals
224
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 ofthe 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 Ripariaripariaand 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 nestfinding 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 ofthe 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. T h e 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).
T h e 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 attempts 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
Total
Mean
clutch
size
Standard
error
3
3
1
0
52
39
34
10
5.60
5.49
5.35
4.90
0.11
0.14
0.14
0.31
7
135
5.45
0.07
Time of
laying
2
3
4
5
6
7
24/4-15/5
16/5-31/5
1/6-30/6
1/7-1/8
0
0
0
0
1
1
1
1
3
4
3
2
15
12
14
4
30
19
15
3
Totals
0
4
12
45
67
CLUTCH SIZE
Incubation and fledging
T h e 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.
T h e 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.
T h e 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.
142
Breeding biology ofthe 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
Mean
BROOD SIZE
brood
Time of
1
3
4
5
6
7
laying
2
Total
size
S.E.
24/4-15/5
16/5-31/5
1/6-30/6
1/7-1/8
1
1
0
0
1
0
1
2
4
0
4
1
3
2
6
7
11
6
12
3
20
9
4
2
1
3
0
0
41
21
27
15
5.10
5.43
4.52
4.13
0.20
0.29
0.20
0.31
Totals
2
4
9
18
32
35
4
104
4.88
0.13
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
Breeding biology ofthe 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 ofthe 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 ofthe 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
CAMPBELL, B., & FERGUSON-LEES, I. J. 1972. A Field Guide to Birds' Nests. London.
GEROUDET, P. 1963. Us Passereaux. vol. II 2nd edn. pp. 215-223.
GINN, H. B., & MELVILLE, D. S. 1983. Moult in Birds. Tring.
GLUE, D. 1987. Seeing the season through. BTO News 149: 11.
HENRY, C. 1972. Notes sur la reproduction et la biologie de la Locustelle tachetee et de la
Locustelle luscinioide. Oiseau 42: 52-60.
HULTEN, M. 1959. Beitrag zur Kenntnis des Feldschwirls (Locustella naevia). Regulus 39: 95-117.
KLEINSCHMIDT, O. 1937. Die Singvbgel der Hernial. Leipzig.
LABITTE, A. 1949. La Locustelle tachetee en Pays Drouais. L'Oiseau 19: 31-40.
MASON, C. F. 1976. Breeding biology of the Sylvia warblers. Bird Study 23: 213-232.
MAYER-GROSS, H. 1972. Nest Record Guide. Tring.
MEAD, C. J., & HUDSON, R. 1985. Report on bird ringing for 1984. Ringing and Migration 5: 125172.
NEWTON, I. 1964. The breeding biology of the Chaffinch. Bird Study 11: 47-68.
PERRINS, C. M. 1979. British Tits. London.
RIDDIFORD, N., & FlNDLEY, P. 1981. Seasonal Movements of Summer Migrants. Tring.
SHARROCK, J. T. R. 1976. The Atlas of Breeding Birds in Britain and Ireland. Tring.
SWANBERG, P. O. 1945. Hackande gras-hoppsangare (Locustella naevia Bodd.). Vdr Fagetoarld 4:
149-174.
VAUCHER, C. 1961. Notes sur la Locustelle tachetee. Nos Oiseaux 26: 45-50.
VERHEYEN, R. 1947. Les Passereaux de Belgique. vol. 2. Brussels.
WALPOLE-BOND, J. 1934. Some habits of the Grasshopper-Warbler in Sussex. Brit. Birds 27:
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