Here is a compilation of the replies I received to the following question:

O.K. -- you have a VERY limited block of time for a warm-up rehearsal with
orchestra before the performance. What do you consider to be the
indispensable exercises for your chorus (vocal...physical...mental...) and
what is the MINIMUM time you would allow for them??

Ruth McKendress Treen
Chatham, Massachusetts

...........................
I would ask my singers to come "vocally warm" and focus on spot fixes of the
score.
Bare minimum---15 minutes of actual singing (not organizing bodies) time.
20-30 minutes better.
.......................
3 minutes -- I think we can assume the singers arrive pretty much aware a)
of their own enthusiasm (or anxiety) for the program they are to sing; b) of
the limited time; and c) that this rehearsal may not primarily be "for"
them. Given these, many will likely have put themselves in a "just do it"
frame of mind. A few will perhaps have vocalized themselves on the way to
the rehearsal!

* Stretching and relaxing -- they are likely going to tend toward tension in
this rehearsal; might as well help them both focus and relax.
* Tone and sostenuto -- to get them to start thinking musically, as an
ensemble; and possibly this could include a vocalise from the program's
repertoire
* flexibility and range -- we have a favorite, and no doubt you do too; ours
really works the range, and is fun, and it always puts a smile on their
face. Even this work-out relaxes the choir before a particularly
challenging morning service (I work with a church choir) and before special
events.
................
In the evenings, people don't need much of a warmup, since talking all
day has pretty much done that. Still, the extreme ranges need to be
focused in.
1).
1 - 5 3 - 8 5 - 3 1 - 5 3 - 8 5 - 3 1
Kee Yah Zha Kee Yah Zha Kee

Scale degrees 1 5 3 8 5 3 1 5 3 8 5 3 1
3/8 time, quarter, eighth, quarter, eighth, quarter, eighth, etc.
make the eighth notes crescendo into the quarter notes, and lots of
energy throughout. 1 measure = about 56. Make sure the intensity of
the Kee carries into the open Yah.
2) sing through a brief passage of the music- something that demands a
bit from them. Encourage them to have good posture, hold music up, mouth
very open, etc. (We all need to be reminded!) Remind them that most
mistakes (according to John Bertalot) are caused by lack of
concentration.
3) Finish with something quiet and intense. This is most important to
get them to concentrate on blending the sound, following the director
precisely, and using lots of energy. Remind them that singing soft
should take more energy than singing loud.

I would allow a minimum of 6-7 minutes for all of this.
...............................
.....I would try to make the chorus call early enough that you WILL have
adequate warmup time. If it's a union orchestra, you certainly don't
want to usurp time they're being paid for. Warmups can't be hurried,
and they can't be skipped, either, but if you're forced into it, teach
your singers to warm up on their own in their cars on the way
to the rehearsal.
.........................
Indispensable: a half hour together as a group before the concert. Much less
than that is not very helpful. I prefer to have about 45 minutes with the
group, followed by a 15-minute break for drinks, sitting and relaxing, and
quiet preparation.

Start with a good stretch of the arms and torso and neck, and have the
singers give themselves a short neck and face massage (or a group shoulder
massage).

Then: group breathing exercises,

humming descending 5-note scales in mid range (repeating a half-step up
or down),

and other vocalises which may incorporate motives from the work you're
about to perform or words from the text (particularly those with sounds
idiomatic to the piece).

Then fast scales and arpeggios rising to the upper range, followed by slower
descending scales or triads from mid to low range.

Then some quiet tuning and listening exercises (e.g., pure fifths,
S+T on upper note, A+B on lower note).

Start the main sections of the piece, or review a tricky transition,
or sing the fugue subject--whatever will focus the choir's attention.

Then use the sugestions you got about what to say to the choir before they
go
onstage.

Finally, allow a good 10 minutes of silence (or as close to silence
as possible) to think about the meaning of the work, the sound they want to
produce, or some other mental exercise to get them ready for performance
before lining up to go on.

[Based on this advice, I asked the chorus to turn to the front of their
music - John Rutter¹s Requiem - and silently read the translation for the
last ten minutes before we lined up. R.M.T.]
..........
I would ask the choir to stand anywhere in the
performing area and stretch and vocalise a bit.

As for words of warm welcome -- I wouldn't. Start first by delivering a
downbeat at the scheduled time. Go ahead and do the first piece.
Then begin the spot check......

Reminders, etc. should be at the end of the "rehearsal" time -- and should
be brief -- and could include a general thanks to all musicians (and that
includes the chorus who have, after all, worked longer as a group on the
material than the orchestra has).

In general treat the chorus as you do the orchestra, with respect and
efficiency!!
...............
I usually explain to them at the last rehearsal without orchestra that my
attention will be more on the players at the rehearsal and that this is
necessary in order to be efficient, and in order for the orchestra to give
the singers the best "back-up" that they can -- in the performance my
attention is on the singers and doing everything I can to make them
sound great I have even stopped using the baton -- though I have
used in the orchestra rehearsal -- so that the singers get me as they
usually get me!! The orchestra at that point is much more interested in
making beautiful music than in my proving to them that I can conduct.
That I did by using their time efficiently at rehearsal and preparing the
singers well.