Seasonal changes in census efficiency of birds at marshes and fen mires in southern Sweden

Abstract

Sixteen fens and marshes with areas between 10 and 285 ha was censused at least six times in the period 11 April to 10 June. The proportion of the number of resident species observed on a visit increased from the beginning of the census period up to the end of May. Thereafter it decreased. Variability in the census of the number of species was lowest in the end of May and the beginning of June. Throughout the seasons studied there were frequent visits of non‐resident species. The proportion of non‐resident, but not the proportion of resident species, observed on a visit, increased as the number of resident species at a locality increased.Census efficiencies per ten‐day periods are given for 14 species. Census efficiency of a species correlated positively with the body weight of the species. The highest census efficiency for most species occurred in the period 21 April to 31 May. It is suggested that if two visits are made in that period about 80% (S.D. = 9.8) of the resident species will be observed.