Avocet population dynamics in England C. J. Cadbury and P. J. S. Olney

Emblem of the RSPB and almost wholly confined as a British breeding bird to two of that society's reserves, the Avocet has been much studied since its recolonisation of Suffolk in the 1940s ntil the early igth century, Avocets Recuwirostra avosetta bred regularly U —but perhaps not commonly—on the east coast of England, from the Humber to Kent, and in southeast Sussex. In 1818, they still nested on Orfordness, Suffolk, close to the present-day breeding site on Havergate Island (Ticehurst 1932). Breeding apparently ceased in Norfolk in the mid 1820s (Riviere 1930); at the mouth of the Trent, north Lincolnshire (now Humberside), in about 1840 (Smith & Cornwallis 1955); and on Romney Marsh, Kent, in the mid 1840s (Harrison 1953). The collecting of eggs and specimens may have been ultimately responsible for this temporary extinction (Axell 1977). Then, for about 100 years, there were only two or three breeding records: an attempt in Suffolk in 1882, and possibly another in 1893; and; unexpectedly, one in Co. Wexford in 1938. Re- colonisation of eastern England began in the early 1940s, when public access to coastal areas was restricted. Avocets bred in Norfolk in 1941 (Seago 1967), and in Essex in 1944 and probably in 1953 (Hudson & Pyman 1968). Suffolk was recolonised in 1947, when four pairs nested at Minsmere and four or five on Havergate Island (Brown & Lynn-Allen 1948). Most of the English population continues to breed at these two sites: during the 1968-72 Atlas survey, Avocets were confirmed breeding in five 10-km squares in Suffolk, with possible breeding in Norfolk and Lancashire (Sharrock 1976). The Avocet has a disjunct breeding distribution, mainly in the steppe regions of the southern Palearctic, with pockets in the west, including the Mediterranean. It now breeds regularly in Cape Province, South Africa, [firi'f. Birds 71: 102-121, March 1978] 102 Avocets in England 103 and irregularly in East Africa, where large numbers winter (Voous 1960, Mackworth-Praed & Grant 1962). In 1969, the estimated breeding popu­ lation in northwest Europe was 10,280 pairs, with most in the Netherlands (41%), the German Federal Republic (27%) and Denmark (24%); Sweden held 6%, and 130 pairs in England represented 1.3%, while the German Democratic Republic and the Atlantic coast of France had even smaller proportions (Tjallingii 1970). The recolonisation of England can possibly be related to an increase in the northwest European popu­ lation: in Denmark there was a 200% increase from 750 pairs in 1920 to 2,300 in 1970 (Dybbro & Jorgensen 1971). In Estonia, breeding was first suspected in 1962, and by 1972 there were 42 pairs (Kallas 1974). Apart from accounts covering the first few years after the recolonisation of Suffolk (Brown & Lynn-Allen 1948; Brown 1949, 1950), the only detailed published information on this relatively well recorded population has been by Conder (1962) and Olney (1965). This paper aims to collate the data available for the 31 years 1947-77 on the breeding numbers and fledging success of Avocets in Suffolk, and to consider the dynamics of this population. Havergate and Minsmere are only 21 km apart. The former is situated on the estuary of the River Aide and its embanked lagoons are subject to some flooding with saline water. At Minsmere, the shallow water areas frequented by Avocets are largely artificially created by excavation and manipulation of water levels; the 'Scrape', a series of shallow, brackish pools with islands, has no adjoining estuary.

Methods Since it was assumed that most failed nests were replaced, the annual breeding population was calculated by subtracting the number of failures up to mid June from the total nests. In the three breeding seasons 1970-72, a total of 125 half- or three-quarter-grown young Avocets was ringed at Minsmere; a further 24 were ringed at Havergate-between 1971 and 1973. All but nine were marked with a plastic (darvic) ring above the tibio- tarsal joint of the left leg and a monel BTO ring in the same position on the right leg, different colours distinguishing young from the two colonies and in the different year groups. Almost all fledged successfully.

Breeding population and productivity The numbers of Avocet pairs breeding at Havergate and Minsmere since 1947 are shown in fig. 1 and table 1. The Havergate population increased steadily to 97 pairs in 1957, declined to 48 by 1964 and then recovered to reach 118 pairs in 1969; subsequently, it has fluctuated between 84 and 112 pairs. The initial colonisation of Minsmere in 1947 was associated with flooding of the area; conditions, however, became unsuitable and, although eight birds returned in 1948 (Axell 1977), none bred there again until 1963, following the creation in the previous year of the first 5 ha of the Scrape; thereafter, there were annual increases and the total reached 53 pairs in 1977. The rate of increase at Minsmere ranged from 104 Avocets in England Table i. Breeding population and breeding success of Avocets Recurvirostra avosetta in Suffolk H = Havergate; M = Minsmere AV. YOUNG BREEDING :PAIR S (-(-NON-BREEDERS) YOUNG REARED REARED/PR H M 1Other s Total H M Others H M

1947 4 4 8 8 1 (2.0) (o-3) 1948 5 1 6 13 (2.6) 1949 17 17 3i 1.8 1950 21 21 40 ••9 '95" 24 24 40 1.7 '952 40 40 120 3.0 «953 45 45 67 i-5 *954 52 52 c.70 !-3 »955 66 66 C.IOO '•5 >956 79 79 c.50 0.6 1957 97 97 e.50 o-5 1958 90 90 31 0.3 1959 65 65 44 0.7 1960 67 67 35 °-5 1961 62 62 60 1.0 1962 77 1 78 20 o-3 1963 55 1 56 29 1 o-5 (1.0) 1964 48 i (+>) 49 52 3 1.1 (3-o) "965 52 3 ( + 1) 55 78 10 '•5 (3-3) 1966 65 4 69 124 8 i-9 (2.0) 1967 73 6 79 139 16 "•9 (2-7) 1968 108 7 (+2) 1 116 162 7 1 '•5 (1.0) 1969 118 11 (+2) 3 132 178 21 1 1-5 i-9 1970 102 15 4 121 '75 34 2 i-5 2-3 i97i 93 25 7 125 103 55 3 1.1 2.2 1972 no 35 ( + 3) 4 149 25 66 0.2 •-9 1973 112 38 4 '54 5° 66 1 0.4 '•7 1974 86 40 7 •33 6 57 2 0.1 1.4 <975 107 41 9 157 80 62 4 0.7 1-5 1976 95 5i 3 149 12 46 0.1 0.9 1977 84 53 1 138 1 3 4 0.1 0.1

36% to 67% during 1969-72, but between 1972 and 1975 declined markedly to less than 10%. Displacement from drought-affected areas may have accounted for the 24% (ten pairs ) increase in 1976, when little rain fell over much of northwest Europe during the first eight months of the year. Table 2 shows the number of Avocets breeding at Minsmere compared with the availability of suitable feeding and nesting habitat resulting from developments on the Scrape. The most rapid growth occurred between 1968 and 1973, when the Scrape was enlarged from 10.3 ha to 20.5 ha and the number of islands increased from 31 to 50. Subsequently, there has been little increase in either (Axell 1974, 1977); other possible effects of this are discussed later. Elsewhere on the Suffolk coast, one pair probably bred in 1948 and another in 1962. Since 1968, Avocets have bred at five sites in Suffolk away from the two reserves—at some regularly—but the Avocets in England 105 Number of breeding pairs 1201

Average number of young reared per breeding pair Fig. 1. Breeding populations of Avocets Recurvirostra avosetta, and average number of young reared per pair, at Havergate (dots and solid lines) and Minsmere (squares and broken lines), Suffolk, 1947-76. Average annual rates of change of whole Suffolk population: '949-57, +25% ±17%; !957-64> -8%±n%; 1964-77, +9% ±12%. Average number of young reared per pair: 1949-77, I.I±CJ-7; 1969-77, 1.5^0.7

Table 2. Population of Avocets Recurvirostra avosetta at Minsmere in relation to area of the 'Scrape' and number of islands Under 'Total area', figures are approximate, since earthmoving in some areas took more than one year; number of islands not known for all years

Total no. Breeding pairs Total area (ha) of islands (+ non-breeders)

1962 Started 1963 4-9 6 1 1964 5-7 25 1 ( + 1) 1965 9.0 25 3( + 0 1966 10.3 4 1967 10.3 6 1968 10.3 7 (+2) 1969 12.3 3i 11 (+2) 1970 '3-9 '5 I971 15.6 25 ■972 16.4 35 (+3) '973 20.5 50 38 1974 20.5 40 '975 20.5 55 41 1976 20.5 5i 1977 20.5 53

Avocets in England 107 total number of pairs has not exceeded nine in any year (table 1). By 1975, the Suffolk breeding population had reached 157 pairs. The average annual rate of change was +25% during 1949-57, —8% in 1957-64 and +9% in 1964-77; over the whole period, there was an annual increase of io±i4%. The greatest change between years (ex­ cluding 1948-49) was an increase of 37 pairs (47%) from 1967 to 1968. Details of the total number of young fledged and the average number reared per breeding pair each year are shown in table 1. The mean annual average for the Suffolk population as a whole was 1.1 (1949-77), with 1.1 for Havergate (1949-77) and 1.5 for Minsmere (1969-77). At Havergate, there were two periods (1956-63 and 1972-77) when the average number of young reared per breeding pair did not exceed 1.0; in 1958 and 1962, and five of the six years 1972-77, it was below 0.5. By contrast, at Mins­ mere, the only years since 1969 in which it fell below 1.4 were 1976, a summer of exceptional drought, and 1977, when there was a prolonged spell of cold weather in May and June and particularly heavy predation of the young. Reasons for the differences and fluctuations will be dis­ cussed by Cadbury et al. (in prep.). An average output of 1.1 young per pair has been exceeded at Havergate in only 13 of the 29 years (1949-77) in which more than ten pairs bred, but has been achieved in all but two of nine years since 1969 at Minsmere (table 1).

Ringing results Age of first breeding Details of ringed Avocets seen at the Suffolk colonies in the breeding season are shown in table 3. Of the 140 young colour-ringed in Suffolk,

Table 3. Colour-ringed Avocets Recurvirostra avosetta summering in Suffolk Numbers expected alive are calculated assuming a survival rate of 0.40 in the first year and 0.90 in subsequent years; nb = non-breeders AGE CLASS i-yr 2-yr 3-yr 4-yr 5-yr 6-yr 7-yr No. of ringed birds (no mortality) 140 140 140 140 140 87 22 Ringed birds expected alive 56nb 5° 45 4i 36 21 5 (8nb) Ringed birds observed in Suffolk ionb 13 13 9 9 15 4 (anb) (2nb) Ringed birds observed at natal colony Havergate inb inb 2nb inb 1 — — Minsmere 5nb 9 7 4 5 10 2 (inb) % observed Suffolk expected alive 17-9 26.0 28.8 22.0 25.0 7i-5 % observed natal colony expected alive 10.7 20.0 20.0 12.2 16.7 47.6 35 & 36. Top, part of large flock of Avocets Recurvirostra avosetta in flight, Netherlands, September 1975 {Jan van de Karri); bottom, Avocet, Havergate Island, June 1976 (Michael W. Richards) io8 Avocets in England

37. Adult Avocet Recurvirostra avosetta feeding, Netherlands, April 1973 (Jan van de Kam) only 7% returned in their first summer after fledging and none bred. Unless they had overwintered, these immatures arrived later (end of April to late May) and departed earlier (before mid June) than the majority of established breeders, which tended to arrive between mid March and mid April and depart in July and August; some paid only fleeting visits to the colonies, but others formed a temporary pair bond and even attempted copulation. Evidence of their sornewhat nomadic life xs provided by a Minsmere-ringed Avocet which spent the first week of June with five unringed individuals on the north Norfolk coast, 106 km northwest of Minsmere {Norfolk Bird Rep. 1972). It is possible that six Avocets observed in June 1976 on the Tamar estuary, Cornwall/Devon, normally a wintering area, were immatures (F. Edwards in litt.). The proportions breeding in their second, third and fourth summers were only 8%, 8% and 6% respectively of the total ringed as young; there were also two second-summer non-breeders. Of a minimum of 17 that returned to Suffolk when two to four years old, 11 (65%) bred for the first tlme in their second summer after fledging. At Minsmere, a third-year male paired with a second-year female. Boyd (1962) considered the age of sexual maturity to be between two and three years. Fidelity to natal and breeding sites Colour-ringing demonstrated that there was some interchange between Havergate and Minsmere. Of the Avocets ringed as young at the latter site, at least eight returned there to breed for the first time, while a minimum of four moved to Havergate. In addition, one was first recorded breeding at. Havergate when five years old, but it may have already bred at Minsmere; another bred in a small satellite colony nearby. Of those ringed at Havergate, only one has bred there. One moved to Minsmere for its initial breeding attempt when three years old. On this evidence, Avocets seem not to exhibit strong attachment to iheir natal site when breeding for the first time. Edelstam (1971) recorded the recoveries of four adults in the breeding season 140-310 km from where they had been Avocets in England 109 ringed in Sweden as young. Without markings to facilitate recognition of individuals in the field, it has not been possible to collect much Information on tenacity to breed- ing site. At Minsniere, however, what was almost certainly the same ringed male occupied a particular feeding and nesting territory for six Summers in succession from the age of two. Moreover, on limited evidence, there was little change in the numbers of certain year groups breeding at Havergate and Minsmere in their second, third, fourth and fifth years after fledging. In 1977, however, there was an influx of 11 six-year-olds that had been ringed at Minsmere: only four of this age class had been recorded in 1976, and the maximum for any previous breeding season was seven when four years old. Passage and wintering areas The presence of colour-ringed Avocets in the high-tide roost on Havergate Island between early July and the end of October indicated that both adults and juveniles from Minsmere joined those from Havergate in the post-breeding period to feed in the Aide estuary and Butley Creek. Besides one local recovery of a first-autumri bird, there have been four recoveries from the Continent (flg. 2): from the Netherlands (first- autumn) ; and (in autumn or early winter) from north Spain, south Spain and Portugal. The Netherlands recovery was from Zuidelijk Flevoland, where up to 4,500 Avocets have been recorded during the autumn pas- sage (van Poelgeest & Osieck 1974). So far, there is no evidence that the English population uses the Dollart in northeast Netherlands, where up to 25,000 Avocets—75% of the northwest European population— congregate in late summer and autumn (Braaksma et al. 1969, Spitz 1969). Ringing recoveries have shown that Avocets from Sweden (Edelstam 1971) and the Netherlands (Bannerman 1961, Speek 1973, and annual reports of the Vogeltrekstation, Arnhem, published in Limosa) winter 38. Avocet Recurvirostra avosetta with nest and four eggs, Havergate Island, June 1950 (Eric Hosking) no Avocets in England ic Natal site • Recovery © Sighting

Fig. 2. Movements of Avocets Recurvirostra avosetta ringed as unfledged young in Suffolk (data for 1970-77) mainly on the Atlantic coast of France and southwestern coast of Iberia, although there are a few recoveries from North Africa and one from West Africa: 10,900 recorded by Prater & Grieves (1975) in January 1975 on the coastal marshes of Portugal represented about 80% of those wintering in Europe. It might be expected that some—if not most—of the English population winter in the same region. A few Avocets overwinter in northwest Europe. In the Netherlands, about 100 were recorded in midwinter in the Delta area in the southwest Avocets in England 111 Table 4. Avocets Recurvirostra avosetta wintering in Suffolk At Havergate, in 1972/73 two first-winter individuals, and in 1973/74 one second-winter, had been ringed as young at Minsmere

1968/69 69/70 70/71 7'/?a 72/73 73/74 74/75 75/76 76/77 Havergate 3 3 3 6 9 10 15 20 '5 Minsmere 1 1 Jan 2 1 Nov 1 Nov " & Dec &Dec during 1912-19 (van Oordt 1919); some still winter there (Woolf 1968) and in Zuidelijk Flevoland (van Poelgeest & Osieek 1974)- One ringed as a chick in Denmark in 1941 was recovered freshly dead near the River Itchen, Hampshire, in January 1942 (BTO Ringing Office), but Avocets did not winter regularly in Britain and Ireland until 1947/48, when four were recorded; a trend for the number to increase has been accelerated by a succession of mild winters during 1971-76 (fig. 3). Of the total of 123 in winter 1973/74, 79 (64%) were on the Tamar and Tavy estuaries; 29 (24%) elsewhere in southwest England; two in Sussex; one each in Kent, Norfolk and Ireland; and 10 (8%) in the vicinity of Havergate, where there have been small but increasing numbers since 1968/69 (table 4). Havergate is the only area on the east coast of England where Avocets regularly winter: in 1972/73, two of the nine were in their first winter, and what was probably one of these reappeared in the following winter; colour-ringed first- and second-winter Avocets have been re­ corded on 55 and 77 bird-days respectively between 1st November and the end of February, while a known older individual has been identified only once within that period.

Fig. 3. Post-breeding (upper line) and wintering (lower line) populations of Avocets Recurvirostra avosetta in Britain and Ireland, 1945-76. Minsmere was recolonised in 1963 5001

400

±300 XI m XI 200 E 3 100

1945 47 49 51 53 55 57 59 61 63 65 67 69 71 73 112 Avocets in England Colour-ringing of Avocets in Suffolk has confirmed a link between this breeding population and that wintering on the Tamar and Tavy. A Minsmere-ringed Avocet was observed in the November of its first winter (1971) on the Tamar, and one (if not the same individual, at least of the same year group) was recorded there in February of its second and third winters; in 1971, it was accompanied by two others which may also have been reared at Minsmere, judging from the position of a metal ring on their right legs (Devon Bird Reps. 1971, 1973, 1974). Immigration Between 1911 and 1974, a total of 7,722 Avocets was ringed in the Nether­ lands, the majority as young; of these, 1,722 were marked in the five years 1970-74 (Speek 1973, and annual reports of the Vogeltrekstation). In 1964 and 1965, on Vlieland and in Noordholland, 91 adults were marked with celluloid or anodised metal colour rings above the tibio- tarsal joint. Some colour rings slipped below the joint and many of the celluloid ones were soon lost or discoloured, but, nevertheless, an Avocet with a celluloid ring was observed in the Netherlands in 1972 and one with an anodised ring in 1976 (A. A. Blok in litt.). In spite of the relatively large numbers of Avocets ringed in the Nether­ lands, only seven provide evidence of immigration into the Suffolk popu­ lation. One, identified through a telescope by its ring number, had been ringed as a chick on Vlieland and was breeding at Havergate in 1974, when 11 years old. Another, with a black or tarnished metal ring below the joint on the left leg (a position not used in Suffolk), bred at Havergate in 1972 and was observed there again at the beginning of the 1973 breed­ ing season; it may have been one of the colour-marked birds from the Netherlands, since certain anodised rings appeared blackish in the field (black celluloid rings were also used, but the chances of one remaining in place until 1972 are small). A third possible immigrant, with an orange ring (a colour not used in Suffolk), was observed once in 1972 (red celluloid rings used in the Netherlands discoloured to orange: A. A. Blok in litt.). In 1977, at least four Avocets ringed abroad bred at Minsmere; two had what appeared to be yellow or gold anodised metal rings. The occurrence of what were probably Continental birds coincided with the breeding of additional Suffolk-ringed Avocets not present in previous years (table 3). Apart from an individual ringed as a chick in the Camargue, France, in May 1934 and shot in Essex in August of that year (Witherby 1936), there is no evidence of immigration from breeding populations in southern Europe. Causes of mortality At least two of the ringing recoveries from Iberia resulted from Avocets being shot, while an adult was shot in September on the River Aide. From an analysis of the recoveries of Avocets ringed as young in Sweden, Edel- stam (1971) showed that shooting accounted for 32 of the 33 for which cause of death was recorded, most from wintering areas in France and Iberia. Such shooting pressure may be influencing the English and other northwest European breeding populations. Lack of protection may have Avocets in England "3

39, 40 & 41. Above, adult Avocct Recurvircslra avosetta with two young, Minsmerc, June 1976 (Michael W. Richards) ; below, Avocet chasing Ringed Plover Charadrius hiaticula, Minsmcre, June 1974 (Richard Hunt) ; bottom, interaction between Avocet and Shelduck Tadorna tadorna, Nethcr- lands, July 1972 (Jan van de Kam) ii4 Avocets in England prevented the species from breeding in Britain during much of the 19th and early 20th centuries. At the end of March 1972, a first-year Avocet died of lead poisoning at Havergate, having ingested no fewer than 55 shot pellets. Presumably, it had picked these up when feeding in an area that had been heavily shot over. Olney (i960) and Thomas (1975) have shown that a number of species of waterfowl are subject to lead poisoning in Britain, mostly ducks (Anatidae) which grub for food in mud in shallow fresh water. Birds feeding in generally softer estuarine sediments appear to be little affected, possibly because pellets tend not to lie near the surface. At least three Avocets wintering on the Tamar are known to have had fatal collisions with the 400-kv power lines which span the estuary in two places; another was injured. Overhead wires situated across flight paths used by waterfowl are well known to present hazards to them; most casualties on 400-kv lines appear to result from collisions with a single earthing wire suspended above the more visible conductors (Scott et al. I972).

Population dynamics of the Suffolk population To assess whether the reproduction of the Suffolk Avocets is alone sufficient to sustain the population levels without immigration, the observed and expected numbers of breeding adults are compared (table 5). In calculating the expected figures, it was assumed that 65% of the birds bred for the first time when two years old and the rest when three. Three different pairs of figures are used for first-year and adult survival rates. From 92 recoveries of Avocets ringed in northwest Europe, Boyd (1962) calculated a survival rate of 0.36 between fledging and the end of the first year, and one of 0.62 for adults. Using these figures, the observed population consistently exceeds the expected one to a significant extent; even though most Suffolk Avocets spend five or six months of the year in regions where they receive considerably less protection than in Britain, Boyd's figure for adult survival is clearly too low. A. J. Cave (unpublished),

Table 5. Observed and expected numbers of adult Avocets Recurvirostra avosetta breeding in Suffolk (1950-77) and at Minsmere (1969-77) It was assumed that 65% of the birds bred for the first time when two years old and the remainder at three. * = P

AVERAGE % OBSERVED -r EXPECTED BREEDING ADULTS SURVIVAL RATE Suffolk ist-yr Adult 1950-57 1958-67 1968-77 1950-77 0.36 0.62 (Boyd 1962) 158.8±20.9** 139.2+.26.6** 137.2+.20.7** 144.1^24.1** 0.41 0.78 (Cave unpub.) iio.8+_i5-8* 102.4+.19.3 98.7+14.3 103.5+.16.7 0.40 0.90 m.6±i5.i* 97.8±i8.6 97-o±2i-7 101.5+.19.3

Minsmere 1969-72 1973-77 1969-77 0.41 0.78 i37-7±23-3** 98-6±8.5 115.9+.23.2 0.40 0.90 134.0+. 12.6** 92.6+_94 m.o+,24.1 Avocets in England "5

42. Avocet Recurvirostra avosetta calling in night, Havergate Island, June 1950 (Eric Hosking)

incorporating more recent data for Dutch Avocets ringed as young, calculated a survival rate of 0.41 in the first year and 0.78 in subsequent years for birds found dead; for those shot, his corresponding rates were 0.50 and 0.72. When the first two survival rates (0.41 and 0.78) are used, the expected figures agree much more closely with the observed than do those computed from Boyd's. Limited information from colour-ringed Avocets in Suffolk (table 3) suggests that the adult survival rate exceeds 0.78: the observed numbers of six- and seven-year-old birds are higher than would be expected from 78% survival. Moreover, even at 0.78, each pair would have to rear an average of 2.4 young to compensate for adult mortality over two years, considerably more than the mean annuäl fiedging success of 1.1 between 1949 and 1977. An average of 1.1 would achieve compensation when the adult survival rate is 0.90; when this figure is used, there is again fairly close agreement between observed and expected numbers over the 1958-77 period. Over the 28 years 1950-77, the observed breeding population in Suffolk exceeded the expected one (assuming no emigration or immigration, and first-year and adult survival rates of 0.40 and 0.90) by an average of 1.4% per annum, and in only ten years was the difference significant (P

Comparisons with other waders Site fidelity Soekkeli (1970a) has compared site tenacity in seven species of waders, and subsequently it has been studied in several others, but not the Avocet. This species appears to conform most closely with the Oyster- catcher Haematopus ostralegus, the Ringed Plover Chamdrius hiaticula and the Kentish Plover C. alexandrinus, which do not show a particularly strong attachment to the natal site, but tend to return to an area once they have bred there. In the case of the Little Ringed Plover C. dubius—a species with an ability to exploit newly created habitats—site fidelity is low among young and only moderate among adults. Young Lapwings Vanellus vanellus may breed for the first time some considerable distance from where they were reared. In contrast, several Calidris species exhibit strong site fidelity, at least among males, when first breeding (Hilddn 1975).

Adult survival rate Adult survival rates of waders, determined by studies of marked birds on Avocets in England 117 the breeding grounds, range from 0.59 in the Ringed Plover to 0.90 in the Oystercatcher. In several small Calidris species, it is between 0.75 and 0.79 (Boyd 1962, Harris 1967, Soekkeli 1970b, Hilden 1972). When size is taken into account, the estimated adult survival rate for Suffolk Avocets of between 0.78 and 0.90 is not markedly out of line with that of other waders, and comparisons support the view that Boyd's (1962) figure for adults of 0.62 is too low.

Average lifespan and expectation of further life Assuming an adult mortality of 0.22, the average lifespan of breeding Avocets in Suffolk would be about seven years, but at only 0.10 it would be 12.4 years (average age = [1/adult mortality] + age of first breeding). Since at least four of the breeding birds in 1977 were seven years old, a longer lifespan is probable. Two 13-year-olds have been recorded in the wild, in Sweden (Edelstam 1971) and in the Netherlands (A. A. Blok in lift.). The limited number of larger wader species for which data have been published show quite a wide range in average lifespan: Ringed Plover 3.5 to 4 years; Kentish Plover 4 years; Redshank Tringa tot anus 4 to 4.5 years (Grosskopf 1964); and Oystercatcher 11.8 years on Skokholm, Dyfed (Harris 1967), and 15 years in Germany (Grosskopf 1964). Expectation of further life of an adult Avocet is 4.1 or 9.5 years, de­ pending on whether the survival rate is assumed to be 0.78 or 0.90 (when e = [i/adult mortality] —0.5). This compares with 8.3 years for the Oystercatcher (Harris 1967).

Age of first breeding The Lapwing, Ringed, Little Ringed and Kentish Plovers all breed when one year old, while the Golden Plover Pluvialis apricaria and Redshank do so when two (Boyd 1962). In Suffolk at least, about 65% of the Avocets first breed when two years old, only a small proportion starting at three or four. For the Oystercatcher, Boyd gave the age as three, but Harris (1967) showed that most of the Skokholm population did not breed until four years old.

Relationship between breeding and wintering populations in Britain Although limited ringing recoveries of Suffolk-reared Avocets indicate that their main winter quarters are in Iberia, their relationship with the wintering population in Britain and Ireland, particularly with the flock on the Tamar and Tavy estuaries, is of interest. It may be fortuitous that Avocets were first recorded wintering on the Tamar following the 1947 breeding season in which they recolonised Suffolk. Both breeding and wintering populations have tended to increase (fig. 3). For all but one of the 12 winters between 1947 and 1959, the number wintering in Britain and Ireland represented less than 25% of the total of adults and juveniles in Suffolk at the end of the previous breeding season; for the next seven winters (1959-66) this figure was between 30% and 47%, subse- II8 Avocets in England

43. Juvenile Avocet Recuwirostra avosetta feeding, Minsmere, August 1973 (G. SlJ. Hollis)

quently falling to between 30% and 16%. There appears to be no clear relationship between post-breeding and winter population numbers. The growth in the proportion of Avocets wintering during 1959-66, and again in 1971-75, may be merely a refiection of relatively low fledging success at Havergate. Of 140 Avocets colour-ringed as unfledged young in Suffolk, single individuals have been seen in only three winters on the Tamar/Tavy. Also, although most Avocets have left Suffolk before mid October, the first arrivals on the Tamar and Tavy are not until the end of that month and the flock does not build up until mid November; most have departed by mid February, about a month before the first return to Suffolk. In most years since 1950, transient parties of up to 25 have been observed on the south coast of England, particularly in Sussex, from mid March to the end of June (Hampshire, Sussex and Kent Bird Reps.), but these had probably wintered on the Continent.

The value of conserving the British population The Avocet cannot be considered a threatened species, when its world population is taken into account, and those breeding in England represent only about 1.5% of the northwest European population. It may, therefore be reasonable to question whether the British population merits high priority in conservation terms (Gooders 1976). Drury (1974), however, has argued that, for a species' long-term welfare, a number of fairly discrete populations, even at the edge of its ränge, may be important. Less hypo- thetical is the fact that estuary reclamation schemes could have a con- Avocets in England "9 siderable impact on the northwest European Avocets: Tjallingii (1970) considered that 75%of the Dutch population of 4,200 pairs were breeding in threatened areas. Since then, there have been some major changes, particularly in the Delta area of the Netherlands, but, fortunately, the threat to construct a canal across the Dollart has been averted for the time being. Not only may the relative importance of the British population increase, but Avocets displaced by reclamation on the Continent may increasingly supplement Suffolk stocks. Since there is apparently considerable emigra­ tion from Suffolk, as well as continuing immigration, it is somewhat surprising that the two sites recolonised in 1947 still hold the majority of the British population and that breeding has occurred on few occasions in recent years outside Suffolk. Suitable breeding sites exist on the Norfolk and Essex coasts, if not elsewhere; indeed, one pair bred successfully in Essex in 1975 (Ferguson-Lees et al. 1977), and five pairs nested in Norfolk (two sites) in 1977. The present localisation may reflect how few areas are sufficiently free from human disturbance and protected from mam­ malian predators to support Avocets, a situation paralleled in the present distribution of Sterna terns in Britain (Bourne & Smith 1974, Lloyd et al. 1975)- Although relatively few Avocets reared at Havergate and Minsmere return there to breed, the production of fledged young at these two reserves may facilitate the founding of new colonies in Britain, should suitable sites become available. Indeed, the establishment of a regular breeding population at Minsmere in 1963 is likely to have been influenced by the close proximity of the Havergate colony, the creation of favourable habitat in the form of the Scrape, and the protection afforded by the reserve.

Acknowledgements This paper relies to a considerable extent on the meticulous records of Herbert Axell, warden of Minsmere Reserve from 1959 to 1975, and Jeremy Sorensen, his successor; and those of the late Reginald Partridge, warden of Havergate from 1949 to 1974, and John Partridge, his successor. They and their assistants are gratefully acknowledged for allowing us the use of data collected over such a long period. Frank Edwards kindly provided information on the Avocets wintering on the Tamar and Tavy. We acknowledge the BTO Ringing Office for providing data on ringing recoveries, and we thank Dr A. J. Cav6 for allowing us to use results of his unpublished analysis. We are indebted to Mrs Penny Richards, who drew the figures, and to Mrs Annette Carlisle, Mrs Pamela Garner and Mrs Anne Smith, who typed from the manuscript.

Summary Details are given for the 31 years 1947-77 °f the population size and fledging success of Avocets Recurvirostra avosetta breeding on the Suffolk coast. The average annual rate of change of the population was an increase of 25% during 1947-57, a decrease of 8% in 1957-64 and an increase of 9% between 1964 and 1977. The population in 1975 was 157 pairs, which represented all but one pair of the British stock, but only about 1.5% of that in northwest Europe. The two RSPB reserves, Havergate and Minsmere, where recoloni- sation first occurred, still hold the majority of the British breeding population. The ringing of 140 young with both BTO and colour rings, the latter indicating the year 120 Avocets in England group and natal colony, resulted in a number of field identifications of individuals and four recoveries from the Continent. From these and other calculations, the following conclusions are drawn: (i) About 65% of Avocets in Suffolk breed for the first time in their second year after fledging, although 18% of those expected to be alive when a year old returned to Suffolk as non-breeders. (2) They do not exhibit strong attachment to the natal colony when first breeding: not only is there interchange between Havergate and Minsmere, but about three-quarters of the surviving two-year-olds apparently do not return to Suffolk to breed; once breeding has taken place, site fidelity is probably stronger. (3) Avocets from the two main Suffolk colonies intermingled in a post-breeding flock that roosted at high tide on Havergate Island. (4) British-reared Avocets winter in Iberia, where shooting may be an important cause of mortality. (5) Although Suffolk Avocets may occasionally winter on the Tamar/Tavy estuaries, Cornwall/Devon, there appears to be no clear relationship between the fluctuations of this wintering population and the breeding numbers in Suffolk. It is estimated that the Suffolk Avocets would have to rear, on average, 1.1 young per pair each year to compensate for adult mortality, considered to be 10% per annum. Such productivity has been attained regularly at Minsmere (average 1.5), but in only about half the years at Havergate (average for 1949-77, 1.1). Excluding the effects of emigration and immigration, the Suffolk population has the potential to be self-sustaining. The total Avocet population in Suffolk is increasing by an average of 10% a year. Immigration appears to be more than compensating for considerable emigration of immatures and the relatively low fledging success in many years at Havergate, but in recent years, at Minsmere, both the rate of growth of the colony and immigration appear to have declined, indicating that the area is reaching its maximum carrying capacity for breeding Avocets. Attachment to breeding site, adult survival rate, lifespan of breeding adults, life expectancy, and age of first breeding of the Avocet and other waders are compared.

References AXELL, H/E. 1974. Minsmere (Part 2). Establishment and management of an artificial brackish lake with nesting islands ('The Scrape'). IUCNjIWRB Manual of Wetland Management. Slimbridge. 1977- Minsmere: A Portrait of a Bird Reserve. London. BANNERMAN, D. A. 1961. The Birds of the British Isles, vol. 10. London. BOURNE, W. R. P., & SMITH, A.J. M. 1974. Threats to Scottish Sandwich Terns. Biol. Conserv. 6: 222-224. BOYD, H. 1962. Mortality and fertility of European Charadrii. Ibis 104: 368-387. BRAAKSMA, S., TIMMERMAN, A., KLEVER, E. P., & TEN KLOOSTER, W. P. 1969. De avifauna van het Eeins-Dollardgebied. Limosa 42: 156-177. BROWN, P. E. 1949. The breeding of Avocets in England, 1948. Brit. Birds 42: 2-12. 1950. Avocets in England. RSPB occasional publication no. 14. London. & LYNN-ALLEN, E. 1948. The breeding of Avocets in England in 1947. Brit. Birds 41: 14-17. CADBURY, C. J., OLNEY, P. J. S., & RICHARDS, P. A. (in prep.). The breeding and feeding ecology of the Avocet in England. CHABRZYK, G., & COULSON, J. C. 1976. Survival and recruitment in the Herring Gull Larus argentatus. J. Anim. Ecol. 45: 187-203. CONDER, P.J. 1962. Notes on the management of Minsmere and Havergate. IUCN Book 3: 302-317. DRURY, W. H. 1974. Rare species. Biol. Conserv. 6: 162-169. DYBBRO, T., & JORGENSEN, O. H. 1971. Udbredelsen af Stor Kobbereneppe {Limosa limosa), Aim. Ryle {Calidris alpina), Brushane {Philomachuspugnax) og Klyde (Recurvirostra avosetta); Danmark 1970. Dansk Orn. Foren. Tidsskr. 65: 116-128. EDELSTAM, C. 1971. Flyltning och dodlighet hos svenska skarflackor Recurvirostra avosetta. Vdr Fdgelv. 30: 168-179. Avocets in England 121

FERGUSON-LEES, I. J., & THE RARE BREEDING BIRDS PANEL 1977. Rare breeding birds in the United Kingdom in 1975. Brit. Birds 70: 2-23. GOODERS, J. 1976. Viewpoint: A question of priorities. Brit. Birds 69: 16-19. GROSSKOPF, G. 1964. Sterblichkeit und Durchschnittsalter einiger Kiistenvogel. J. Orn. 105: 427-449- HARRIS, M. P. 1967. The biology of Oystercatchers Haematopus ostralegus on Skokholm Island, S. Wales. Ibis 109: 180-193. HARRISON, J. M. 1953. The Birds of Kent. Edinburgh. HILDEN, O. 1972. Mortality and average age of Temminck's Stint. Int. Orn. Congr. 15: 650. 1975- Breeding system of Temminck's Stint Calidris temminckii. Ornis Fenn. 52: 117- 146. HUDSON, R., & PYMAN, G. A. 1968. A Guide to the Birds of Essex. London. KAIXAS, J. 1974. Estonian Wetlands and their Life. pp. 119-138. Tallinn. LLOYD, C. S., BIBBY, C. J., & EVERETT, M.J. 1975. Breeding terns in Britain and Ireland in 1969-74. Brit. Birds 68: 221-237. MACKWORTH-PRAED, C. W., & GRANT, C. H. B. 1962. Birds of the Southern Third of Africa. vol. 1. London. MINTON, C. D. T. 1976. The waders of the Wash—ringing and biometric studies. Un­ published appendix to NERC report 'The Wash Water Storage Scheme Feasibility Study'. OLNEY, P. J. S. i960. Lead poisoning in wildfowl. Wildfowl Trust Ann. Rep. 11: 123-134. 1965. Management of Avocet habitats in Suffolk. Bird Notes 31: 315-319. PRATER, A. J., & GRIEVES, A. 1975. Wildfowl count in Portugal in January 1975. Un­ published report. RIVIERE, B. B. 1930. A History of the Birds of Norfolk. London. SCOTT, R. S., ROBERTS, L. J., & CADBURY, C. J. 1972. Bird deaths from power lines at Dungeness. Brit. Birds 65: 273-286. SEAGO, M.J. 1976. Birds of Norfolk. Norwich. SHARROGK, J. T. R. 1976. The Atlas of Breeding Birds in Britain and Ireland. Berkhamsted. SMITH, A. E., & CORNWALLIS, R. K. 1955. The Birds of Lincolnshire. Lincoln. SOEKKELI, M. 1970a. Dispersal of Dunlin Calidris alpina in relation to sites of birth and breeding. Ornis Fenn. 47: 1-9. 1970b. Mortality and reproductive rates in a Finnish population of Dunlin Calidris alpina. Ornis Fenn. 47: 149-158. SPEEK, B.J. 1973. Ringverslag van het Vogeltrekstation 1911-1970. Limosa 46: 109-165. SPITZ, F. 1969. Present knowledge of the number of waders wintering in Europe, the Mediterranean basin and North Africa. IWRB Bull. 27/28: 12-14. THOMAS, G. J. 1975. Ingested lead pellets in waterfowl at the Ouse Washes, England. Wildfowl 26: 43-48. TICEHURST, C. B. 1932. A History of the Birds of Suffolk. London. TJALLINGII, S. T. 1970. Inventarisatie van de in 1969 in Nederland broedende Kluten. De Levende Natuur 73: 251-255. VAN OORDT, G.J. 1919. The Avocet wintering in Holland. Brit. Birds 12: 239-240. VAN POELGEEST, R., & OSIEGK, E. R. 1974. Enige gegevens over het voorkomen van de Kluut Recuwirostra avosetta in Zuidelijk Flevoland. Limosa 47: 94-99. Voous, K. H. i960. Atlas qj European Birds. London. WITHERBY, H. F. 1936. Avocet ringed in Rhone Delta found in Essex. Brit. Birds 30: 50. WOLFF, W.J. 1968. Een samenvatting van de gegevens over de Kluten van het Deltagebied in de loop van het jaar. De Levende Natuur 71: 35-42.

Dr C. J. Cadbury, RSPB, The Lodge, Sandy, Bedfordshire SG19 2DL P. J. S. Olney, The Zoological Society of London, Regent's Park, London NWI 4RY