LONDON -- Britain's first cat, Humphrey, is innocent of charges that he has murdered a nest full of baby birds, Prime Minister John Major told the nation. "I can tell you beyond any doubt at all that Humphrey is not a serial killer," he told Channel Four television news. "Humphrey has been very unjustly accused. There was a small nest of robins just outside the cabinet room. I am afraid they are dead, but it is natural causes."Humphrey, who lives with his master at the prime minister's London residence, 10 Downing Street, has until now had a very low media profile, unlike his U.S. counterpart, Socks.