Answer:
It is true that studying natural selection in the fields can be challenging. Some of the challenges which Darwin might have faced could have been:
Darwin was only studying one trait for his results. But there might have been many other factors or traits affecting natural selection.
Natural selection does not happen rapidly hence Darwin would have had to wait for many generations to observe the results.
The environment is not such an easy factor to study. It is complex. It might have happened that the environment might have changed before any changes were seen in the population of the birds.
The three most common elements are oxygen, carbon, and hydrogen
the answer is muscularis externa
