The condition you are describing is molting. Birds shed their feathers before winter and summer to allow new feathers to grow.

No, feeding bread does not affect the bird's molting of feathers. We have heard all kinds of crazy things about bread including that it will cause the swans to be bound or constipated (DOES NOT), that it is the reason that birds are being afflicted by the Pink Feather (Flamingo) Syndrome (DOES NOT). Bread has no nutrient source, just acts as a filler, basically a treat.

If anything, bread will change the blood chemistry of the swans, meaning that their glucose levels or other components may be changed. In order for the swans to be affected by bread feeding, this would have to be the only food that the bird receives and that it would have to be in huge amounts. So, occasional treats and non-exclusive feeding of bread should not be a problem. The Regal Swan