A friend or alibi who does fuck all all night let's you do the hard work, then sneaks in the for kill in order to steal snatch you have theoretically acquired. Usually a mute, but gains confidence after much intoxication.
Bloke A: Let me chat to these girls.
Bloke B: lurks in background and agrees.
3a.m Bird Stealer: 'Hey wanna come back to my place and have some more drinks? I got some cheap Rum and a sofa from value village?'