'bread' in itself wont hurt your bird- in facgt sometimes it can be quite a fun snack! . Everything in morderation- but bird nutritional requirements are unique- especiialy with a quacker- they live where I study and the majority of their wild diet is actually carbs- lol. Nothing but high carbs veggies and fruit most of the year unless they fall into breeding season.
Just like humans- birds can have allergic reactions.
I always use a balance of H1 and H2 to calm many allergies when a fungal infection is ruled out. For this, I use Moringa (you can brew it as a tea, and use it in the water). I started studying the effects of moringa on early dietary requirements for fids, and progressed to those with liver and allergy conditions.
It helps immensely- but do check with your vet. You know your bird the best. So trust your parental instincts- but also note: moms tend to get over protective
Let us know what the vet says-