Densities of Eurasian Three-toed Woodpecker Picoides tridactylus and sap trees correlate across a latitudinal gradient in Sweden

Abstract

Mapping of sap rows on trees has been proposed to detect the easily overlooked Eurasian Three-toed Woodpecker Picoides tridactylus in boreal forests. Sap rows have a distinct appearance that can be detected in the bark for many years. While it is well-established that Eurasian Three-toed Woodpeckers make sap rows on coniferous trees, it is uncertain whether they also make sap rows on deciduous trees, such as birches (Betula sp.), or if these are made by other woodpecker species. We collated data on woodpecker abundance from the Swedish Bird Survey’s fixed route monitoring scheme and correlated this with data on sap tree abundance from systematic searches in three different parts of Sweden. Assuming that species-specific woodpecker behaviours are similar across Sweden, we suggest that Eurasian Three-toed Woodpeckers are responsible for the majority of sap rows on birches in the central and northern parts of Sweden, while Great Spotted Woodpeckers Dendrocopos major made the few sap rows found in southern Sweden and likely some of those further north. With that caveat in mind, our findings might be used to develop a more efficient method of mapping Eurasian Three-toed Woodpecker abundance in Sweden.