"His shot was low. He should have waited until the bird went up a little more."
Pheasants tend to fly low.
I think the reference was toward when the pheasant are going vertical before they go horizontal.
Pheasants really don't go vertical.
They won't get up more than head high, or right level with the cornfield.
Either the hunters weren't staying in line as they worked the field or this guy just wasn't paying attention to where the other hunters were.