What does it mean when you hear birds chirping when you start the van?

bird nest in the van anywhere?

It means the ghost of Alfred Hitchcock is there!!!!........ Loose/worn/contaminated belt(s)...........

Birds happy to be going for a ride!

Bird is trapped in the engine compartment and is probably dead by now. That said in the winter many animals desperate to stay warm often crawl up into a warm parked car so PLEASE BANG ON CAR and HONK to startle them out before you start your motor or you could KILL a cat or dog or BIRD without knowing it until it's too late.

Either you need a new belt or could be the pully I would say a new belt to start off but if that don't fix it I would try your other options