The efficiency of three-visit square surveys vs. one-visit line transects in censusing sparsely distributed birds in managed forest landscapes

Abstract

SummaryWe conducted three-visit surveys of 1-km2plots and traditional Finnish single-visit line transects (considering only the 50 m wide main belt) to evaluate these methods in censusing of a predetermined set of 23 target species known to prefer old forests in three regions in Finland. The efficiency of the two methods was compared on the basis of the number of territories recorded per hour. An attempt was made to find indicators of the occurrence of suitable habitats for species preferring old forest in general, including the rarest ones, and so also largely indicating total diversity of forest bird fauna of the study area. The total number of pairs observed per hour and the abundance of sedentary bird species were significantly higher in the square surveys than in the main belt of the line transects. There were significant positive relationships between the densities of relatively abundant (density > 1.0 pairs km−2) and less abundant target species. There emerged five common forest bird species that seemed to form a suitable set of indicators of the occurrence of habitats for birds preferring old forest in the northern boreal zone: Great Spotted WoodpeckerDendrocopos major, Mistle ThrushTurdus viscivorus, Willow TitParus montanus, Eurasian TreecreeperCerthia familiaris, and Eurasian BullfinchPyrrhula pyrrhula. We concluded that sedentary species preferring old forest are good candidates for indicators to characterize some threatened aspects of forest bird diversity.