Are you by chance denoising your recording? When I look at the spectrogram it appears that noise has been removed as there are areas of "clearing" where there should be noise. If you are not running the file through a denoising or dehissing or filtering process then perhaps your recorder is the problem. Have you tried recording with other recorders and do you have the same problem? It is a beautiful fall New England day in Sarasaota, Florida!!