The oral flanges are an age indicator in birds. They look like enlarged flaps formed by integumentary tissue on the sides of the bill. In nestlings they are very prominent and as chicks grow oral flanges disappear. These yellow marks let us discard the bird as an adult. 

 It is normally yellow and it will be more or less bright depending on how well lit the nest is. It is believed to arouse parents to feed the young and to serve as a food target to place it properly.