Bird songs, which are transmitted through a natural habitat, will always be degraded in different ways. Used as a holistic term, song degradation will usually include three aspects: attenuation and addition of background noise resulting in a reduced signal-to-noise ratio (SNR), blurring, and elongation with echoes. Degradation may therefore influence a number of different communication network activities. It may, for instance, constrain information transfer, especially over long ranges, and at the same time provide ranging cues. Some of the factors causing or influencing song degradation, e.g. temperature gradients, wind speed, relative humidity and background noise, are likely to vary over the day. The options for communication activities that depend on sound degradation could therefore be predicted to show a similar variation over the day. Normally, they are believed to be best at dawn. We investigated this by transmitting typical sound elements from the species characteristic terminal motif part of the blackcap song in a typical blackcap breeding habitat, a deciduous forest. The experiment was made in Denmark late in April at the blackcap's return from the wintering area and before foliation was complete. The sounds were transmitted at three different times of the day: dawn, morning, and afternoon. All of the above mentioned aspects of song degradation were quantified. The experiments showed a significant decrease in excess attenuation and increase in SNR over the day, where as the measures representing the blurring (blur ratio) and the elongation with tails (e.g. the tail-to-signal ratio) showed no diurnal variation. The results have a number of important implications for different communication network activities. These will be discussed, but it should be clear that the traditional view of sound propagation and communication being most effective early in the day does not always hold true.
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