White against a dark background is a lot easier to see from a distance.
Some predators hunt by eyesight and most of these notice motion before anything else. So keeping the contrast down helps with these predators.
Others hunt by sound, color makes no difference in locating the meal.
Others hunt by smell and once again color makes no difference.
So if you have a white bird moving against a dark background you rang the dinner bell louder for one group of predators than the other groups.
It all depends upon what you have around for predators and if the birds are out where they present a good contrast or not with their background.