It`s a personal thing but I like to use 5s for crows and rooks because they are a very tough bird a rook and you can get a few walkers with anything smaller I have found. If your not going to eat the pigeons then I would use 5s for them aswell as it gives a better instant kill rate. Also use some choke at least quarter/half out of respect for the quarry for cleaner kills.
Remember don`t use plastic wads because they leave an awfull mess in the field and can be eaten by livestock, not to mention the farmer may not ask you back again because of it.