They should be one regular expression, and should be in one string:
"nt|nv" # rather than "nt" | " nv"
f_recs[f_recs['Behavior'].str.contains("nt|nv", na=False)]
Python doesn’t let you use the or (|) operator on strings:
In [1]: "nt" | "nv"
TypeError: unsupported operand type(s) for |: 'str' and 'str'