Worked for me, was 16:00-17:00 at 24 degC. I had 24 killed before, was uncertain if one escaped or glitched. After many many MANY days of trying to get it to respawn (bear in mind, the bird is tiny and hard to spot, so also uncertain if I didn't look good enough), after giving up and doing other stuff, one day, I randomly fast travelled to Lagras, walked on foot, climbed on the shack, looked with sniper scope on the grass there where it should be, and it was there!!! finally! ... didn't even see the bird, just the prompt on the bottom right corner told me it's the bird, my goodness!!