Avocet Population Dynamics in England C
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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.