Are quartzite scree slopes used by birds to promote sound transmission in the Mediterranean forest?

Birds generate vocalisations (songs and calls) to communicate. Acoustic communication may be hindered by habitat features so birds can use several strategies to favour sound transmission. Sound transmission depends on the acoustic properties of their habitats. Scree slopes, also known as ‘pedrizas’, are frequent in the Mediterranean forests of south and central western Spain. As the acoustic properties of these rocky grounds might favour sound transmission, we propose that birds might actively use ‘pedrizas’ to increase sound transmission. We assessed the following prediction of the hypothesis: the number of vocalisations recorded should be higher near the ‘pedrizas’ than in forest areas far away from ‘pedrizas’. Using portable recorders in the Mediterranean forest of Monfragüe National Park, we found that the number of recorded vocalisations was higher near the ‘pedrizas’. As this result was not due to differences in species richness, we consider it supports the prediction of the hypothesis. This is new evidence that birds might use a natural element within their habitat to increase sound transmission.