Fun fact, birds can see a wider band of the electromagetic spectrum (still appears as violet to red but spaced further apart) which causes the sky to shift in color as the earth tilts on its axis relative to the electromagnetic field and this is currently hypothesized to be how they navigate and tell the seasons apart. So instead of noticing the changes in foliage, they might just prefer the sky to be a specific tone.
Fun fact, birds can see a wider band of the electromagetic spectrum (still appears as violet to red but spaced further apart) which causes the sky to shift in color as the earth tilts on its axis relative to the electromagnetic field and this is currently hypothesized to be how they navigate and tell the seasons apart. So instead of noticing the changes in foliage, they might just prefer the sky to be a specific tone.
I thought they don’t like the cold / can’t find food
It can be multiple things but a lot of birds migrate before major food or temperature changes. Imagine flying 600 miles on an empty stomach.