I'm composing for a competition that is for a string quartet but also has a requirement for a mezzo-soprano. I have very little experience with singers but I'm aware that the range is A3 - A5.
Are there other constraints I should be aware of? Although breathing is an obvious aspect. Does it make sense for me to try and sing the appropriate part to feel it's doable?
Yes, that's the textbook range, though I'd be wary of the top few notes. That's not what mezzo-soprano is all about.
Yes, it's an excellent idea to check that the part is singable by you. Not so much for the range (unless YOU'RE also a mezzo soprano!) but for breathing, tricky jumps etc. And make sure it's melodic and rewarding to sing!
You'll need to suss out whether this is the sort of competition where cutting-edge 'alternative' stuff is obligatory, or whether you're allowed to write tunes :-)
Another thing to consider in your orchestration is the timbre, more specifically that it changes as the part goes towards the extremes. Do you really want the sound of a mezzo-soprano trying to hit an A5 just so that you can fill out your chord? Be aware of the blend you can realistically achieve with the part you've written. The more range you include in the mezzo part, the more difficult it will be for a human singer to produce the tone you desire.
Additionally, you may want to go find a friend who can sing this and ask for their opinion. Even if you yourself are a mezzo-soprano, it's always better to hear others' opinions. The composer may overestimate the abilities of the performers they write for, as they didn't have to "learn" the part, because they wrote it around their own voice (perhaps subconsciously).
