I have opossums here on a regular basis. In an effort to protect your birds and yourself I'd dispatch it. I've had them become aggressive (well not really. That's an exaggeration. They just like to hiss at me and dare me to do anything) with me before. I have had a lot of eggs that Ive had to toss and they love some eggs, rotten or not. So far they haven't bothered my roosters that like to sleep in the chain link run, but they usually have plenty to eat. If you can't dispatch it tonight and you're planning on doing this, I'd make sure you have something for it to eat tonight (not near your birds) so it's not tempted by your chickens. This is just my personal opinion and I imagine many won't agree.