‘Reading’ an spectrogram is fundamental to processing nocmig recordings (indeed any audio recordings). The Sound Approach to birding book provides an excellent introduction to spectrograms and is recommended. Here we summarise some of the main points with specific reference to nocmig recording.
Below is a 30 second section of a spectrogram produced in Audacity using its default colour scheme. The vertical axis shows frequencies (from 0 to 10kHz) and the horizontal axis shows time (from 14:00 to 14:30 mins). Here warmer colours indicate more energy (i.e. louder noise) at a particular frequency at a particular time. Sometimes you’ll see spectrograms in greyscale (e.g. on Xeno-Canto) but the principles are the same.
Here the continuous pink bands at the bottom in the 0–2kHz represents low frequency white noise, in this case the low level rumble of traffic and urban noise. The feature of interest shows a sound that oscillates in frequency – the siren of an emergency vehicle! There are a few other features – some pink shading in the left which is probable wind noise through trees, and some vertical lines which are broad spectrum clicks (interference or other non biological sounds).
The spectrogram below is an 10 second extract from October 2017 and shows five distinct calls. All are relatively high pitch, around 7–9kHz. The calls have distinct shapes, with three being angled downwards (like grave accents) and two being shorter and of more constant frequency. These and other features help to identify the former as Redwing calls and the latter as Blackbird calls. Often superficially similar sounding calls are visually distinct in spectrograms. Examples of spectrograms of calls of common UK nocturnal migrants can be found on the Identification pages.
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