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Fig. 10a. Relative background noise level for the first quiet period in the study, May 4-11. May 7-8 is excluded as some coastal stations had surf noise. Data is for seven nights except Station 6 & 7, which show six nights.

Figure 10a shows the consistency of background noise during the first quiet period. The different levels of background noise indicated between stations reflect inherent system gain differences between the stations. In other words, the equipment recording levels were not the same but varied by 0-6dB. This does not affect call detection because the Old Bird call extraction software detects signals relative to average background noise. Therefore, if the recording level is increased, background and bird calls will be equally increased in loudness and the Old Bird detectors would recognize the same relative loudness difference.