short time energy function explanation
If you're trying to identify birds, you should look into the Mel-frequency Cepstral Coefficients (MFCCs). They are commonly used in speech recognition, and there's actually a fair amount of academic papers on bird song recognition using the MFCCs and various other features.

Short-term energy is definitely useful to extract areas of bird sound, but it probably won't be sufficient to just take an FFT and look for coefficients. For example, imagine the same two  bird sounds, but one is slightly higher pitch, or there is more/different noise present. The FFT coefficients will be significantly different. The MFCCs basically capture the shape of the spectrum, on a scale that emphasizes regions of the spectrum where more detail is needed, and discard some of the unimportant details.