mouth call works fine ..

heck.. you don't even need a mouth call .. just use your mouth .. I've shot dozens of crows just by CAW CAW CAWing at them

stay well hidden, if you have a decoy use it, tie black rags if you don't as has been mentioned in other posts

if you REALLY want to buy the tape .. go right ahead, it'll save you a sore throat which I get if using just my voice for to long