That's How It Happened

For all but one platform, Blackfriars train station is a terminus. Every morning I'm one of the few commuters travelling south out of the City. As the trains pull in, heaving with office workers, the pigeons hop on board and peck around for crumbs. Just before the train is due to leave they hop off again. I haven't yet seen one get caught. I have three hypotheses:

1. The pigeons are able to time the stops, and just know that they shouldn't hang about in the train more than a couple of minutes. This seems implausible, since different trains have different turn around times.

2. The pigeons listen for the beeping that signals imminent departure. I think this is dubious too, because that would only give them a few seconds to hop out.

3. They don't care whether the doors shut or not. They just hang about picking up all the crumbs. If they do get caught, they just chill out, then fly home across the river from Elephant and Castle.

I actually have a degree in animal psychology, and I really can't figure it out. Anyone got any better ideas?