A sonogram is nothing more than a graphic interpretation of a sound. It is given as an x-y graph, in which time (usually in seconds) is on the x-axis and the frequency (usually in kHz) is in the y-direction. The sound is represented as a black 'blotch'. This is blacker if the amplitude is higher (that is, if the sound is louder), but since this aspect is often hard to judge, it is highly useful to include a so-called wave-form, too. This is another graphic representation, in which the noise signal (the loudness of the sound) is set against the time. It is for instance useful for interpretation of the number of tones in a bird call (the number of 'syllables'). I personally think that a graphic description of a sound should always include both graphs.

Important to realise when judging a sonogram is the frequency range: this is not linear but quadratic: an a-flat is 440 Hz, the next a-flat (one scale higher) is 880 Hz, the next 1760 Hz, etc. Therefore, the difference between a tone of 6 kHz and one of 7 kHz is much more difficult to discern than that between tones of 3 and 4 kHz. Note that a tone of around 5 kHz is already quite high. In the range over 10 kHz it is hardly possible anymore (at least to us humans) to distinguish between tones. Most elderly people won't even be able to hear these frequencies at all.

But back to a sonogram (e.g., Humes Leaf Warbler nr. 1):

In the left picture (the sonogram), you can see some black lines going up and down. That's simply exactly what it sounds like, too. The call first goes up till around 6 kHz, is followed by a slow (less steep) upslurred whistle to about 5 kHz and immediately after the end of that by a more rapidly downward curve. For your convenience, I have given a horizontal grid in this graph, which makes a more accurate judgement of the call's frequencies possible.
In the right graph (the waveform) you can see that the first short call is very soft, but the upslurred part (which actually consists of 2 tones very close to one another) is the loudest, followed (clearly separated) by the final, slightly softer, tone.

If you start analysing calls in this way, you'll notice that many bird calls that seem to consist of, let's say, three tones, actually comprise many more. It's just that the human ear cannot keep up with the bird's pace.

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