The first hurdle that you need to face on the rehearsal evenings when the turn out is less than expected, is your own state of mind.
Everyone there is disappointed and the disappointment is palpable.
So the first order of business is to state the obvious: “Wow – not as many warm bodies as we were expecting” and then move directly into the warm up. However, it does help the atmosphere a little if people can do a (very) quick report on the chorus members they’ve heard from. It lightens the feeling in the room when we all realize that the absentees have good reasons for not being there. But a certain amount of gloom will persist until the singing starts.
(A note to Directors about your tone of voice during the acknowledgement phase – you need to get over your disappointment immediately, and stay positive, positive, positive. The chorus will read your disappointment as an indictment of their singing abilities – that without the absentees, you feel the chorus’ singing will not be good enough.)
So – acknowledge the cause of the entire group being bummed out, and then start immediately to shake that attitude off – by singing.
Some of my most intensely productive rehearsals have been on nights when illness and people’s family stuff have decimated the ranks. Corrections and vocal coaching can be tailor-made for the group that’s there. And whatever you work on will have a positive effect on the absent members when they start straggling back. The progress may not be as fast as you’d all wanted – but there will still be progress.
And at the end of the evening I’m always elated that so few people can still sound so good.