Naguib, M., Schmidt, R., Sprau, P., Roth, T., Florcke, C., & V. Amrhein. 2008. The ecology of

vocal signaling: male spacing and communication distance of different song traits in

nightingales. Behavioral Ecology, 19(5), 1034-1040.

This study attempts to assess signal degradation and distortion of nightingale calls at varying distances and microhabitats in order to infer the signals’ functions. Two specific types of songs, broadband trills and whistle songs, were recorded from 11 male nightingales in a wild population in France. The 11 recordings of each song type were compiled into two separate files which were broadcasted from a loudspeaker hidden in a bush and located at normal perching height (~2m from the ground). The broadcast calls were then rerecorded along 4 transects (3 transects along hedgerows and 1 transect across and open field with no vegetation) at varying distances from the loudspeaker (1m, 32m, 64m and 128m). Using GPS technology, the authors marked the typical distances between individuals in the natural population. The authors then used FFT to create power spectra of the rerecorded calls.

The transect type had no significant effect on signal degradation whereas distance had a strong effect. This could be due to the fact that the habitat was relatively open even on the hedgerows which had very low ground vegetation. Generally, the structural integrity of whistle calls was maintained even over distances much larger than the median distance between nearest neighbors (129 m) suggesting that these whistle calls are likely used for long-range communication between individuals. This whistle may be a general advertisement call that announces the sender’s existence to potential mates or competitors. In contrast, the bandwidth of broadband trills significantly decreased with increasing distance from the source even at 32m, nearly 100 m less than the distance to the closest neighbor. The rapid deterioration of the broadband trills suggests that they are used primarily for short-range communication between the sender and approaching individuals. The differential degradation of certain frequencies of the trill calls suggests that these calls may be used by nearby individuals to assess the exact distance/location of the sender.

As we discussed in lecture, signal constraints such as attenuation, absorption and scattering affect signal propagation differentially depending on the amplitude, frequency, bandwidth, modulation type and temporal pattern of the signal. Thus, the same constraints can effectively amplify some components of a signal while substantially degrading others (i.e. broadband trills). We learned that absorption is frequency dependent (as well as distance and medium dependent) and therefore is the most likely cause of the diminishing bandwidth of broadband trill calls. By comparing the range of frequencies heard at a given distance to that present at the sound source, one can determine the location of the sender. Because these broadband trills may be used to assess distance to the sender, its use in short range communication between nightingales may have evolved in order to decrease the number of potential eavesdroppers. Evidently, it is important to analyze the distorted call that reaches intended receivers not just the unadulterated call that exists close to the signal producer. The difference in the structure of these signals may reflect the signal’s function and help to explain spatial patterns between the sender and his intended receivers.