You will find spectrogram examples from New Zealand bird species here. We also provide examples of other environmental sounds that are common in automated field recordings. The purpose is to help you read spectrograms and identify species in your field recordings. Notice how each sound looks different in the spectrogram. These differences allow you to differentiate species, call types, gender, and even individuals. Please note that the variation between calls even from the same species is large and we are not covering them all here. There are some non-bird sounds that we commonly see in field recordings. It would be useful to get familiar with those spectrograms as well, especially when you work with automatically collected recordings rather than less noisy manual recordings. Before we move on to the spectrogram examples let’s get familiar with some of the essential terms (unless you already know).
Spectrogram is the visual representation of sound. It shows you the strength/loudness of birdsong over time at various frequencies. Thus the horizontal and vertical axises represent time and frequency respectively. A gray/color scale is used to represent the energy in each time-frequency location.
Sound is recorded by sampling the analog signal in order to represent it in the digital world. Frequency tells you how frequent you sampled the signal, e.g. 16,000 samples per second is expressed as 16,000 Hz or 16 kHz. Sampling frequency has a direct relationship with the capacity of the recordings you collect: lower sampling rates produce lighter recordings and vise versa. However, you need to choose an appropriate sampling rate for the species/sounds you record to avoid any information loss. As you can see in the above spectrogram, the frequency goes from 0 to a maximum frequency of 8,000 Hz. This maximum frequency is the sampling rate devided by two. So, if you choose to record with 16,000 Hz, the highest frequency you will end up having in the spectrogram is 8,000 Hz. Importantly, when your target sounds exceed this range (8,000 Hz in this example), you have to choose a higher sampling rate in your recorder (e.g. 32,000 Hz), so that you do not lose the higher frequencies in your recordings.
The lowest frequency produced by the bird is called the fundamental frequency of that bird. It is also the loudest part of the call and can be seen more darker/lighter in the spectrogram (depending on the choosen colour code for the spectrogram). AviaNZ has the option to visualise fundamental frequency as shown below (the red lines). Fundamental frequency can be helpful to differentiate bird species as it differes between species/ call types. In the following example, the fundamental frequecy of morepork is relatively low compared to that of North Island brown kiwi male. Note that when propagating sound in the forest habitat, higher frequencies get attenuated easily but the lower frequencies can travel relatively longer distances. Therefore, even when you have faded calls in your recordings (as a result of having birds further away from the recorder), fortunately you will still have the fundamental frequency recorded.
|NI brown kiwi (male)||Morepork (trill sound)|