Breeding of Stone-curlews at Weeting Heath, Norfolk
Typically, the Stone-curlew Burhinus oedicnemus breeds in dry sandy or stony areas and chalky downlands with sparse vegetation. Its
traditional habitats in Britain are the extensive brecklands and heaths of
[Brit. Birds 76: 291-304, July 1983]
continued...
291
292 Breeding oj Stone-curlews
129. Stone-curlew Burhinus oedicnemus, Portugal, May 1972 (Kevin Carlson)
East Anglia and the southern downlands from Kent to Dorset. Like some other southern species (e.g. Red-backed Shrike Lanius collurio, Wryneck Jynx torquilla and Nightjar Caprimulgus europaeus), it has declined markedly in range and numbers in Britain over the last 50 years, associated with loss of habitat (Parslow 1967, Sharrock 1976). Most of its old breeding haunts have fallen to cultivation, while much of the East Anglian Brecks are now afforested. Nevertheless, it shows great site tenacity and, when not disturbed, continues to breed in the same areas, even forest rides. Stone-curlews returning to sites now cultivated lose eggs and young during farming operations: an analysis of BTO nest record cards showed that 33% of eggs lost were destroyed by farm machinery (Glue & Morgan 1974). Otherwise, the breeding of the Stone-curlew has not been well studied, and this paper presents some observations made during 1971-78 on five pairs nesting on Weeting Heath National Nature Reserve, Norfolk.
Study area and methods Weeting Heath covers approximately 1.8km2, has an open calcareous sandy soil with abundant flints and chalk fragments, and is covered with short grass, mainly red fescue Festuta rubra, hair-grass Koeleria gracilis and meadow oat-grass Helictotrichonpratense (Duffy & Morris 1966). Lichens and moss are also abundant , especially on the areas heavily grazed by rabbits Oryctolagus cuniculus. The heath is enclosed by a wire-netting fence to maintain a high rabbit population and a heavily grazed turf typical of the old Brecks; the effects are particularly marked in the southern section,
Breeding of Stone-curlews 293
Fig. 1. Weeting Heath National Nature Reserve, Norfolk, showing study areas, and sites A, B, C, D where pitfall samples were collected. known as the Enclosure (fig. 1), which has always had a well-maintained fence. This area held four of the five Stone-curlew pairs.
Observations were made as often as possible, with at least one visit a week during most breeding seasons, and one a month during each winter. Each year the number of pairs present was recorded and an attempt was made to follow their breeding success, keeping disturbance to a minimum. All nests located were checked at frequent intervals. As many young as possible were caught, weighed, and individually marked with numbered BTO rings.
To obtain information on invertebrate numbers, a series of pitfall traps was operated at sites A (eight traps) and B (four traps) (fig. 1), where adult and young Stone-curlews were seen feeding in summer 1971. Additionally, four traps were placed at C in 1973 and four at D during 1975-77. Each trap was a 1-lb (454-g) jam jar with a 5-cm diameter opening, containing a
25-mm mixture of one part ethelene glycol to four parts of water. The jars were set in the ground, in rows about 4V2 m apart, checked and emptied at each visit. The captures are summarised in fig. 2.
Also, during 1973, four random samples, each one yard square (0.84m2), in sections 1 and 3 were treated fortnightly from March until July with dilute formalin (see Raw 1959) to obtain an estimate of the number of earthworms (Oligochaeta) present. Each square was watched for 15 minutes, then checked frequently for a further 10-30 minutes. As no earthworms were collected from any of these squares, this sampling was discontinued in July, though observations showed that earthworms were present on the heath, at least in some years. Presumably they were too deep to be affected by the formalin (also, samples were taken in mid morning, whereas earthworms are more active during the night and early morning).
Food and food availability Witherby et al. (1943) listed a variety of food items, ranging from various
294 Breeding of Stone-curlews
Table 1. Food items from analysis of pellets, faeces and stomach F = faeces; P = pellets; S = stomach
1 p
Nest
15 P+F
Nest
15 P
Nest
MAY
24 P
Nest
30 F'
Nest
Earthworms (OHgochaeta) Earwigs Forficula Beetle (Coleoptera) heads
elytra and other remains Beetle larvae heads Spiders (Araneae) Woodlice (Isopoda) Caterpillar skins Grasshoppers (Orthoptera) Snail shells Diptera Diptera larvae Seed: sorrel/dock Rumex
mouse-ear Cerastium Unidentified
Vegetation Chickweed Stellaria leaves
+ + 3 2 + 2 +
+ + +
1 +
+ +
9 +
+ + +
1
+ +
7 +
+
1
3 +
+ +
+
5 +
+
+
+
+
Year
1973 1974 1975 1976 1977 1978
1
Date first bird seen
April 15 April 5 April 11 April 1
March 16 NR
Interval between land 2 (days)
11 16 11 29 48
2
Date of 1st egg
April 26 April 21 April 22 April 30 May 3 May 1
3
Date of 1st egg
2nd pair
April 30 April 29 April 23 May 12 May 6 May 7
Interval between 2 and 3 (days)
4 8 1
12 3 6
4
Date of 1st egg
3rd pair
May 6/7 May 2/3 May 1
— May 16 May 11
invertebrates to voles (Microtinae) and gamebird chicks, but mainly land molluscs, earthworms and insects. In Israel, Mienis (1978) found from droppings that Stone-curlews were feeding almost entirely on the land snail Theba pisana. At Weeting, the Stone-curlews were mainly nocturnal and fed infrequently during the day. Even when they were seen feeding, it was usually impossible to tell what was taken, although earthworms were occasionally identified: in 1975, R. Pimm watched an adult carrying earthworms back to its chicks. I watched an adult feeding itself and a half-grown chick on the intestines of a rabbit which had died from myxomatosis; R. Southwood saw a Stone-curlew feeding on rabbit guts, and also saw one killing a five-day-old Lapwing Vanellus vanellus chick, and another eating a clutch of four Skylark Alauda arvensis eggs.
Further information was obtained from two young chicks (one killed by a
Table 2. Laying date of first egg for each pair of Stone-curlews Burhinus between first and
NR = not
Breeding oj Stone-curlews 295
contents of breckland, Norfolk, Stone-curlews Burhinus oedicnemus contents; + = present; + + = abundant
tractor on the Elveden Estate, a similar type of breckland habitat, the second found dead in section 1) and an adult, also from the Brecks, sent to Monks Wood Experimental Station during 1975. Their stomach contents, presumably biased towards harder-bodied invertebrates, were in line with the findings given in Witherby et al. (1943). A few pellets and faeces found around one or two nests, usually towards the end of incubation, also contained similar remains (table 1).
Most of the invertebrate groups concerned were also found in the traps, particularly woodlice (Isopoda), beetles (Coleoptera) and spiders (Araneae) (fig. 2); in most years, the number of animals increased from April or early May, to a peak during June or July. The higher totals recorded at site B were usually due to larger numbers of woodlice.
There was no obvious relationship between food supply (as measured in
oedicnemus at Weeting Heath, Norfolk, 1973-78, and laying interval subsequent pairs recorded
31 S
Pullus
+
JUNE
22 S
Pullus
1 1
+
+ 2 7
4 1
9 S
Adult
8 17
+
2
9
27 P+F
Nest
+ +
+ +
JULY
27 P+F
Nest
+ +
+
+
+
1 1
10
27 F
Standing adult
+ +
+
+
+
+
AUGUST
15 F
Pullus
++ 2
+
Interval between 2 and 4 (days)
10 11 9
13 10
5
Date of 1st egg
4th pair
Late May May 18
May 11-14 — — NR
Interval between 2 and 5 (days)
20+ 27
19-22
6
Date 1st egg pair on
Section 3
May 25/27 May 9
April 24/25 May 11 June 1 May 9
Interval between 2 and 6 (days)
29/31 18
2/3 11 29 8
Spread of
laying (days)
29/31 27
19/22 12 29 10
296 Breeding of Stone-curlews
Fig. 2. Pattern of invertebrate availability at Weeting Heath, Norfolk, 1973-77, based on pitfall samples in section 1 (Enclosure) and section 3, with egg-laying schedules (solid bars) and repeat clutches (hatched bars) for pairs of Stone-curlews Burhinus oedicnemus: dotted line indicates periods between hatching and fledging of young (also clutches which produced young). Open circle indicates date of visit prior to sighting first Stone-curlew; arrow indicates
date first individual seen
the traps) and breeding success. This would not necessarily be expected, however, if breeding failures were caused by factors unrelated to food supply, and if the birds obtained part of their food off the heath, as seemed to be the case (see Discussion).
Breeding season Five pairs were recorded during each of the first six years, and four during the next two. One of these nested at the northern end of the heath, on either section 3 or section 4, and all the other pairs in the Enclosure (fig. 1).
The first Stone-curlews were seen during the first week of April in most years, but once on 16th March. The interval between first sighting and the first egg varied from 11 days to 48 days. It was not possible to obtain precise laying dates for all clutches, but, where the hatching date was known, laying date was calculated assuming an incubation period of 25-27 days (Witherby et al. 1943). Three clutches for which both laying and hatching dates were known had incubation periods of 26, 26 and 25 days, respectively.
In most years, the first eggs were laid during the last week of April, but
Breeding of Stone-curlews 297
Fig. 3. Diagrammatic plot of annual nesting sites of Stone-curlews Burkinus oedicnemus, to show nest positions of each pair. Numbers refer to clutches listed in table 4. Solid circle: position of first pair to lay each year; triangle: second pair to lay; square: third pair to lay; open circle:
fourth pair to lay
different pairs showed little synchrony (table 2 and fig. 2), and the spread of
laying dates varied from ten to 31 days. Except in 1977, the first pair to lay
each year nested in the same place (fig. 3), and usually produced larger
eggs.
130. Stone-curlews Burhinus oedicnemus, Suffolk, June 1940 (Eric Hosking)
298 Breeding oj Stone-curlews
Table 3. Hatching and fledging success of
Year
1971
1972 1973
1974 1975
1976 1977 1978
TOTALS
No. of pairs
5 repeat
3 5
repeat 5 5
repeat 4 (+ l ) i
4 4
repeat
No. of eggs laid
10 2 6
10 2 9 9 2 6 8 8 2
74
No. of eggs hatched (%)
0 ) 4
5) 8
I) 5 6
1 ) 57
(66.7)
(66.7)
(83.3)
(88.9)
(54.5)
(83) (75)
(100)
(77)
1 One pair nested in a field just off the area and brought one hall grown young onto the heath 2 O! -\ young considered to have fledged, having survived 20 days, one was found dead on the road 3 Omitting 1972, 39%
Clutch size and weight
Stone-curlews usually lay two eggs, sometimes only one and occasionally three (Witherby et al. 1943). From 74 nest record cards, Glue & Morgan (1974) calculated a mean clutch size of 1.9. Of 38 clutches examined in the present study only two were of one egg, and the mean clutch size was 1.95; the two one-egg clutches were laid in 1974 and 1975 (table 3), perhaps by the same female. During 1976, one pair failed to lay.
All eggs were weighed when found and whenever possible on subsequent
Table 4. Weight of eggs of Stone-curlews Burhinus Numbers in oarentheses
Year
1973
1974
1975
1976
1977
1978
Mean clutch weight
Mean egg weight
(1)
(5)
(9)
(13)
(15)
(18)
1 FIRST PAIR TO LAY
Egg Clutch wt(g) wt(g)
£2) <>" S:S) «* 5£L5 | 9 9 0
48.5 1 38.5 | y 0
35.5 J
« * ) « *
S i ) 89-5
87.9±8.87
43.96±4.33
2 SECOND PAIR TO LAY
Egg Clutch wt(g) wt(g)
(2) 39.0 \ 35.0 ) 7 4-°
(6) 36.0 36.0
(10) £ S ) *» 04) S ) -.5 (16) 48.5 J 9 6 5
48.0 ) (19) 42.0 1
38.0 I 8°-°
74.6±20.34
40.7±4.34
No. of young at 31+ davs old
(fledged)
2 0
Age in days
34
Chicks fledged No. of
Not known, observation not continued 3
6 3
3 (+ l )* 4 4 2
27
31
40 31
33 202
34
as % of young fledged eggs laid per pair
16 0.4
25 0.6
67 1.2 27 0.6
50 0.8 50 1.0 60 1.5
363 0.8
(3)
(7)
(11)
(17)
3 THIRD PAIR TO LAY
Egg Clutch wt(g) wt(g)
tl) »'•" 2 5 ) •" 42.0 | 41.0 1 H '1U
40.25 j 8 2 5
42.25 | Not weighed
4 FOURTH PAIR TO LAY
Egg Clutch wt(g) wt(g)
(4) No weights
W S 1 ".o (12) 39.0 39.0
82.7±2.23
41.5± 1.59
PAIR WHICH LAID
IN SECTION 3
Egg Clutch wt(g) wt(g)
»;S) 78,
Z) ™ JiSI ™ 40.0 j 39.5 J / M"
or'? 1 75.0 36.5 ) 41.0} 42.0 1
79.5±2.9' 7
39.7±1.52
2I> davs after Hedging, and a second was recovered in Spain in October H)/o
visits. Not all clutches we're found on the date of laying, but, as healthy eggs lose weight at a constant rate (Murton et al. 1974), it was possible to calculate the weight at laying for all eggs when date of hatching was known. The mean weight ( ± S D ) of 24 clutches, including those of one egg but not repeats, was 78.96g ( ± 14.5); while the mean of 22, excluding clutches of one and all repeats, was 82.8g (±7 .1 ) . The mean weight of 46 eggs at laying was 41.2g ( ± 3 . 7 ) . In most years (1973-78), the earliest clutches tended to weigh more (mean 87 .9g±8.9) than those laid second (74.6g± 20.3),
oedicnemus in relation to order of laying in season refer to position plotted on fig. 3
300 Breeding oj Stone-curlews
though the difference was not statistically significant (table 4). There appeared to be five distinct nesting areas used by females, the first
pair to lay invariably choosing the same area, except for one year (1977) when the first clutch (15 in fig. 3) was laid about 150m away. Probably many of these sites were used by the same female in successive years (judged from similarity in egg colour and markings). The case in 1977 was probably a new female laying for the first time in a completely new area, while clutch 16 probably belonged to the female which usually laid first. In the following year, the first bird to lay (18) did so in the usual area, while the second (19) laid in the new area.
The pair laying third, and the pair in section 3 which laid at least a week later, all had heavier clutches on average (means 82.7g±2.2 and 79 .5g±2 .97 , respectively) than the pair which laid second. In the case of the Woodpigeon Columba palumbus, heavier eggs hatch more successfully than light ones (Murton et al. 1974), while in the domestic hen Gallus domesticus large chicks grow and survive better than small ones (Skoglund et al. 1952). At Weeting, Stone-curlews which laid first tended to raise more young than those laying later, but whether this was an effect of date or egg weight is unknown. During 1973-78, 12 eggs laid by the first pair produced nine young, whereas 31 eggs of subsequent layings produced only 14 young (repeat clutches excluded).
Hatching and fledging success
Of a total of 74 eggs laid during the eight years, 77% hatched successfully, 131. Stone-curlews Burhinus oedicnemus, Portugal, May 1972 (Kevin Carlson)
Breeding of Stone-curlews 301
Fig. 4. Weights of young Stone-curlews Burhinus oedicnemus at Weeting Heath, Norfolk, to show rate of growth before fledging. Lines join consecutive weights of some young
though the annual hatching success varied from 54.5% in 1975 to 100% in 1978 (table 3). Glue & Morgan (1974) obtained the same average percentage for East Anglia.
Eight eggs which failed to hatch had a mean fresh weight of 38.5g (± 1.02); seven of these showed no sign of development and had probably been laid by the same female. Nine other eggs disappeared, but five of these may have hatched and the young died shortly afterwards.
Young Stone-curlews leave the nest soon after hatching, and, in my experience at Weeting and in the Brecks in general, stay in the close vicinity for at least a week. At Dungeness, Kent, Scott (1965) noted that young were led off the shingle on to grassland and moister areas, a journey which took up to five days; at Weeting, they are unable to move far because of the fencing.
Weights of young are plotted against age in fig. 4 (all years combined). The data, although sparse, indicate a typical growth pattern: a slow increase for the first few days, followed by a more rapid growth as the young develop.
302 Breeding of Stone-curlews
The young can fly at six weeks (Witherby et al. 1943): one aged 42 days could just fly the width of the Enclosure, but was unable to gain enough height to clear the 1.7-m fence. Because they became increasingly mobile as they grew, survival was difficult to estimate, but counts towards the end of the fledging period gave a good indication of the number surviving. In most cases, the young were between four and five weeks old when last recorded, and they probably survived well after reaching that age. During 1977, two of the four assumed to have fledged, the youngest being 20 days old when last recorded, were recovered 26 days and 15 months, respectively, after their assumed fledging date. In all, 27 young were fledged during the eight years. Omit t ing 1972, 39% of eggs laid produced young to fledging. The number of young reared per pair ranged from 0.4 in 1971 to 1.5 in 1978, with a mean over the seven years for which information was available of 0.8 young per pair (table 3).
Discuss ion
Pellets, faeces and pitfall samples from Weeting Heath indicated that earthworms, beetles, spiders and woodlice formed a major part of the diet, while casual observation indicated that larger prey, including carrion, were also taken. During one year when mid-morning sampling was carried out to measure earthworm numbers, none was recorded. Kollmannsperger (1955) found that the number of earthworms on soil at night was positively correlated with temperature, the optimum for activity being 10.5°C. Tischler (1955) pointed out that temperature extremes are greater on sandy soils, and on warm summer nights the number of active invertebrates seen per hour is twice as high as by day, but only half as many are observed on cold nights. As Stone-curlews prefer breeding on light sandy soils, this may be a clue both to their habitat preference and to their nocturnal feeding
132. Stone-curlew Burhinus oedicnemus, Portugal, May 1972 (Kevin Carlson)
Breeding oj Stone-curlews 303
habit. For the Nightjar, a crepuscular species found on sandy heaths and also declining, Berry & Bibby (1981) showed a correlation between the growth rate of the chicks and the mean minimum temperature, and suggested that cold might suppress insect activity.
Adult Stone-curlews may not feed entirely on their breeding grounds at Weeting. They were frequently observed flying off towards the nearby-pastures during the day and at dusk, while, during July 1976, R. South-wood noticed that they tended to spend the day on the breeding grounds, flying westwards at dusk to return at dawn. Lapwings also flew back and forth between their nests on the heath and surrounding arable land. Scott (1965) noted at Dungeness that, whenever he had a Stone-curlew nest under observation, the departing bird at the changeover made for the moister grassland to the north.
Small young are fed by the adults, food often being carried some distance: I have watched adults, during the daytime, walking 200-300m to half-grown young, apparently carrying only one item at a time. Do adults ferry food to their young at nights, from more distant feeding grounds?
As no relationship was apparent between the food supply on the heath and breeding success, factors operating elsewhere may have affected the population. Either way, Stone-curlews seemed to require two distinct areas for breeding, one suitable for nesting and a second for feeding. If so, this could be important for their conservation; it would be of little use keeping a suitable area of heath where the birds nested, if their feeding grounds were destroyed.
Acknowledgments I am grateful to ail those people who have helped me in a variety of ways. B. Spinks, R. Pimm and R. Southwood, wardens at Weeting Heath, provided much help and useful information. M. Adams and M. Lainsbury gave considerable assistance with the fieldwork, and with sorting pitfall samples during the summers of 1975 and 1976. It is a pleasure to thank Dr Ian Newton for his advice and for critically reading the drafts of this paper. The Nature Conservancy Council and the Norfolk Naturalists Trust gave their permission for me to work on Weeting Heath NNR. Finally, Mrs P. Glover typed from the manuscript.
Summary Five pairs of Stone-curlews Burhinus oediemmus were studied on Weeting Heath NNR, Norfolk, during the breeding seasons of 1971-78. The first individuals usually arrived on the breeding grounds during the first week of April, and the first eggs were laid during the last week. Mean clutch size over the years was 1.95; and there was no evidence of a second brood, though single repeat clutches were laid in four of the eight years. The first clutches laid each year tended to be heavier, and to produce more young, than subsequent clutches. An average of 77% of all (74) eggs hatched; of 17 eggs which failed to hatch, seven showed no development and nine disappeared. Breeding success (eggs to fledglings) was 39%, and mean annual productivity 0.8 young per pair.
The invertebrate food supply on the heath was measured; variations in supply from year to year were not correlated with variations in breeding success. Adult Stone-curlews may feed in other habitats at night; these feeding sites may be as important for the species' conservation as are areas of nesting habitat.
References BERRY, R., & BIBBY, C . j . 1981. A breeding study of Nightjars. Brit. Birds 74: 161-169. DUFFEY, E., & MORRIS, M. G. 1966. Breckland invertebrate studies on Grass-Heath habitats.
Rep. Monks Wood Exp. Stn 1960-65: 7-11.
304 Breeding ojStone-curlews
GLUE, D., & MORGAN, R. 1974. Breeding statistics and movements of the Stone Curlew. Bird Study 21: 21-28.
KOLLMANNSPEROER, F. 1955. Uber Rhythmen bei Lurnbriciden. Decheniana 180: 81-97. MIENIS, H. K. 1978. Tkebapisana in droppings of the Stone Curlew in Israel. Argamon 6:61-63. MURTON, R. K., WESTWOOD, N. J., & ISAACSON, A.J. 1974. Factors affecting egg-weight, body-
weight and moult of the Woodpigeon Columba palumbus. Ibis 116: 52-73. PARSLOW, J. L. F. 1967. Changes in status among breeding birds in Britain and Ireland. Brit.
Birds 60: 122-123. R A W , F . 1959. Estimating earthworm populations by using formalin. Nature,Lond. 184: 1661-2. SCOTT, R. E. 1965. Some observations on the Stone Curlew at Dungeness. Bird Notes 31:
261-265. SHARROCK,J. T. R. 1976. The Atlas of Breeding Birds in Britain and Ireland. Berkhamsted. SKOGLUND, W. C , SEEGER, K. C , & RINGROSE, A. T. 1952. Growth of broiler chicks hatched
from various sized eggs when reared in competition with each other. Poultry Sci. 31: 796-799. TISCHLER, W. 1955. Influence of soil types on the epigeic fauna of agricultural land. In McE,
Kevan, D. K. (ed.) Soil Zoology, pp. 125-136. London. WITHERBY, H. F.,JOURDAIN, F. C. R.,TICEHURST, N. F.,& TUCKER, B. W. 1943. The Handbook
oj British Birds, vol 4. London.
N.J. Westwood, Institute oj Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, Cambridgeshire PE172LS
