Citizen Science to Assess the Fine-Grain Distribution and Habitat Use of the Middle Spotted Woodpecker Dendrocoptes medius

Abstract. Citizen science (i.e. monitoring or research schemes with volunteer participation in data collection) is becoming an increasingly used tool that may yield reliable estimates of species distribution ranges. In 2017 and 2018, we assessed the fine-grain distribution and habitat use of the Middle Spotted Woodpecker in the Basque Country (N Spain) by combining two citizen science approaches: opportunistic observations gathered from internet collaborative databases, and standardised records collected by volunteers trained with a species-specific fieldwork protocol. Six out of the thirty-four opportunistic observations were located out of previously known distribution ranges. Because the reliability of those observations was difficult to assess, opportunistic observations should be validated to avoid false positives. As for the species-specific approach, Middle Spotted Woodpecker occurrence was examined by conducting itineraries with point counts and audio-stimulation, sampled twice during the pre-breeding season, in 95 UTM units of 1 km2. Woodpeckers were recorded in 29 of those units. In combination with recent species-specific studies, our results show that Middle Spotted Woodpeckers occurred mainly in a core, continuous range (69 positive units in and around the Izki forest) and in two smaller ranges (6 units in Montes de Vitoria and 5 units in Sierra de Entzia). Out of previously known distribution ranges, the species-specific approach showed that woodpecker occurrence was negatively affected by the distance to the core Izki forest and, to a lesser extent, positively influenced by the tree basal area of the forest stand. While the species-specific fieldwork approach allowed to improve the delineation of distribution ranges and the assessment of habitat use, the opportunistic approach pointed out to overlooked ranges but showed limited efficacy to assess the fine-grain distribution of the Middle Spotted Woodpecker.