Swerve always unless it'll cause a fatality but a life is a life regardless of species IMO (I'm all for animal welfare) and I also love my car so I'd like not to damage it. A fox ran out in front of me at 50mph, just casually stepped out in front without looking
and was one of those slow motion "Nooooo" moments. Luckily managed to brake and swerve right, clipping it with just the front left side of my bumper. I went back to check it was ok but nothing there so I guess it got lucky.
I've killed only two, a squirrel trying to cross 6 lanes of motorway which was hit by me and a few other cars & then a rabbit which again just bolted in the road 2ft in front of me so there was nothing I could do, I feel so bad still fo both though.
On another occasion I rescued a wounded rabbit from the road hit by someone, it had a broken leg else but sadly it died from shock on the way to the animal hospital.
My winning story was saving a baby squirrel from a busy road while on my push bike. I rode with it home, we fed it, made it a bed and I believe now resides as a tame animal at the local wildlife welfare shelter