I am able to fly my birds out all through the winter - except when the weather is too bad.
The only time we seem to have problems with hawks is in the the months of March and April (sometimes the last 2 weeks of Feb').
Even though our birds did not get fat over the winter, I have noticed a marked difference in their fitness levels. They have started to get fitter over the past two weeks - maybe due to the bit of training we have been giving them. But I really believe, they are getting fitter on their own as well.
All in all - I think if one can let the birds out over the winter, it is better for them.
JMO
Phil