The shape and size of a birds beak would mostly be affected by what environmental limiting factor

The size of birds' beaks are affected by the availability and accessibility of their food supply. For example, if a certain type of bird primarily ate worms, their beaks might be big and fat because they don't need to reach to get their food. However, if another type of bird primarily ate insects that lived in trees (e.g. termites), then they would need long and skinny beaks that allowed them to reach inside the tree.