What is most likely the cause of the differences in birds in the picture

I can't see the pictures but I'm guessing the enviroment

they're in probably causes a difference in their beaks.

It’s a punnets square. You take it and you can either drop it or put it to the side and the capital letter always goes first. If it is a capital letter then it is dominant but if it is a lowercase letter then that would be recessive. So if there’s one capital letter and one lowercase letter it would still be dominant because of the capital letter.