In addition to the answer by Kerrek SB please consider the following:
Computers (almost always) use something called twos-complement notation for negative numbers, with the high bit functioning as a 'negative' indicator. Ask yourself what happens when you perform shifts on a signed type considering that the computer will handle the signed bit specially.
You may want to read Why does left shift operation invoke Undefined Behaviour when the left side operand has negative value? right here on StackOverflow for a hint.