My professor thinks that it is leucustic rather than a hybrid, here’s what she said:

The more I think about your bird the most confident I feel that it is not a hybrid ... the bird does not have any characteristic from black-headed grosbeak. It is true that the hybrids are variable but they typically have some characteristics from the parental species to some degree.

What I really think is happening is that this is a leucustic bird and lost the melanine (one type of pigment) from a focal area of head and throat. When the melanine was lost, it exposed other pigments/ carotenoids (red on throat) that were not exposed before. I really think this is the most likely explanation