Middle Spotted Woodpecker territory owners distinguish between stranger and familiar floaters based on their vocal characteristics

Acoustic communication is an important part of behaviour in many bird taxa. In most woodpecker species drumming functions in mate attraction and territory defence. However, woodpeckers also produce calls, which function has been less studied to date. Unlike other woodpeckers, the Middle Spotted Woodpecker Dendrocoptes medius, which is the subject of our study, drums very rarely. Instead, it uses a number of calls, which functions have not been tested to date. We aimed our study at the identification of different calls and assessing their function, both by observation of spontaneous interactions between individuals and playback experiments. We distinguished three calls of different spectral characteristics and functions. Call-1 was mostly uttered by individuals sitting on the branch or a trunk. It was an initial response of territorial pairs to any disturbance within a territory. It signals general anxiety and/or arousal of territorial individuals. Call-2 was mostly uttered by the individuals during flight and it accompanied territorial conflicts and trials to remove the intruder from the territory. It warns the intruder about a territory owner’s readiness to a direct fight. Call-3, characteristic for floaters, was a persistent vocalisation without any obvious stimulus. Acoustic analyses of call-3 revealed that it was individually distinctive and the experimental approach confirmed that woodpeckers were able to distinguish between calling floaters. Thus, Call-3 functions as a vocal signature of unpaired individuals.