Speech Activity Detection- Adhoc Problem with multi source sounds(birds,trees,heavy air) and how to eliminate them completely
This is not aliasing this is not any phenomenon, there are number of birds whose vocal fall in speech range even a frog vocal falls in speech range so essentially a training based speech detection is required to classify whether a given frame is speech or a non speech 

one such implementation is shown here

Speech Discrimination based on multiscale spectro temporal modulations