I'm totally new to hunting so I apologize in advance if the answer to this is obvious..

I'm wondering how to keep upland game birds from spoiling overnight in the field. Last weekend I bagged a few grouse with my .22 on my way home Sunday afternoon after spending a couple of days in the alpine. I was able to get the birds to the freezer that evening. I'm thinking that this weekend it'd be great to stay out overnight, and hunt both days, but the last thing I'd want would be to have birds from Saturday spoil before I could get them into the fridge or freezer Sunday night. Are there any special techniques for keeping the meat fresh? I guess I'd have the same question for hares and other small game like that.

Thanks for any advice!