Pressure waves in recordings

["Jatinder Singh" duma_js]

Hello All,
I am new to recording nature/ bird sounds.   
I was recording birds at the edge of a large lake, and upon review of the 
recording I notice "pressure waves" in the recording.   It was not windy during 
the recording.  I am using ME66 microphone equipped with a sponge windscreen in 
association with a MZRH1 recorder, the whole contraption is hand held.
Could anyone please educate me on these "pressure waves" and help me eliminate 
them in future.
Thank you
Jatinder

       
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