"Bird's nest" is also cockney
slang for one's chest. A good example usage comes from the movie Lock, Stock and Two Smoking Barrels
'That's fucking it,' says the guy. 'That's fucking what' says Rory. Rory gobs out a mouthful of booze covering fatty; he flicks a flaming match into his bird's nest and the man lit up like a leaking gas pipe. Rory, unfazed, turned back to watch his game. His team won too. Four-nil.