I keep my bunny outside and I'm not sure how it might survive in the winter. Suggestions?
I had a similar issue with my rabbits too.
Firstly I would start with moving the hutch away from any cold winds or breezes, if you can't move it, try putting a tarp over it to block the weather but make sure if there's a sunny day let it out to enjoy the sun.
In the wild, coming up to winter they usually fatten themselves up so their body fat can keep them warmer, you can buy feeds that are higher in calories and protein, like calf-manna, to help fatten up a rabbit. Or you can feed it oats (just plain oats, usually called "rolled" or "old-fashioned") and raw black sunflower seeds. Chaff and lucerne hay, are mainly used for feeding cattle but rabbits, being rabbits, will eat anything and those grasses are really well for fattening them up.
Also with their bedding, it's best to load up lots of hay, they will eat a lot of it, but they will also use it as a source of warmth.
If, however you do notice them looking a bit under the weather, take them inside at nights while the weather is at its most severe. Putting them in a plastic washing hamper with an old towel should keep it happy for a while (be careful it doesn't escape though, that could be quite messy). Giving a sick and/or lethargic bunny a slight bit of honey mixed with water can raise blood sugar levels and give it more hope for surviving if it is struggling to keep up with the cold..