Population Dynamics of the Avocet (Recurvirostra avosetta) Breeding in Britain

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Abstract
(1) The number of breeding avocet Recurvirostra avosetta L, pairs, eggs laid, chicks hatched and young fledged were recorded annually at Havergate Island and Minsmere, Suffolk, from recolonization in 1947 and 1963, respectively. Data up to 1986 were used in a key factor analysis. Individual mortalities used were the inability to realize maximum egg production (k1), egg loss (k2), chick loss (k3), and overwinter loss (k4). (2) At both sites the number of pairs increased and young fledged per pair decreased following recolonization. (3) Chick loss at both sites was identified as the key factor explaining 86% and 83% of the variation in total loss at Havergate and Minsmere, respectively. Egg loss at both sites and chick loss at Havergate increased over the study period. (4) Egg loss was positively correlated with the number of eggs laid at both sites. Chick loss at both sites was unrelated to the number of chicks hatched. (5) The main density-dependent loss at both sites was the loss occurring from the autumn population in year t and the breeding population in year t+2 since most avocets return to their natal site in their second year. During two phases, ''increase'' and ''plateau'' the strength of density dependence was determined. During the plateau phase density dependence was almost perfectly compensatory. (6) At both sites the numbers of black-headed gulls Larus ridibundus L., which predate avocet eggs and chicks and appropriate avocet nests, have increased during the study periods although they have been controlled at Havergate and Minsmere since 1964 and 1966, respectively. Those at Minsmere declined after intensive control from 1977 onwards. (7) At Harvergate, chick loss was correlated with the number of breeding gulls up to and including 1971, beyond which kestrel Falco tinnunculus L. predation also occurred, obscuring the relationship with gull density. At Minsmere, neither egg nor chick losses were related to the number of breeding gulls. (8) Mortalities were compared between sites. Egg losses were higher in Minsmere than Havergate. Chick losses were higher at Havergate than Minsmere. Possible reasons for the difference, based on hypersaline conditions at Havergate, are discussed.