Too cold to sing: Warbler dawn chorus affected by environmental factors in a high-elevation wetland

Many bird species take part in a dawn chorus, the phenomenon in which the community of songbirds in a particular habitat, show a peak of vocal activity around dawn. Understanding the species-specific extrinsic factors affecting this intensive singing activity is important for us to predict and understand activity patterns of local species for long-term monitoring purposes. This study investigated the effects of air temperature, relative humidity, wind speed, rainfall and moon phase on vocal activity during the dawn chorus in a suite of related songbirds, the African Yellow Warbler (Iduna natalensis Smith A, 1847), Little Rush Warbler (Bradypterus baboecala Vieillot, 1817) and Lesser Swamp Warbler (Acrocephalus gracilirostris Hartlaub, 1864), in a high-elevation wetland in Golden Gate Highlands National Park, South Africa. We deployed acoustic recorders in a rolling-grid layout in the ∼6 km-long wetland from August 2019 to February 2020, recording daily acoustic data from 04:00 to 07:00 Central African Time. Temperature significantly affected the dawn chorus onset of all three species: on cooler days, the dawn chorus started later compared to days with warm temperatures. On days with high relative humidity, the dawn chorus onset of two of the three focal species shifted in opposite directions: one species began singing earlier, while another delayed its onset. Wind speed and rainfall had varied effects on the timing of the dawn chorus across species. Our results demonstrate that meteorological conditions influence the dawn singing of these three birds differently in a shared habitat.