Below is a list of possible challenges. check all
errors that you mentioned in your answer.
traits other than beak type might affect
natural selection.
it could take many generations to observe
adaptions to a population.
the environment may be complex and hard
to study
changes in the environment may occur faster
or slower than changes to the birds
the birds may be interdependent with other
organisms that may change, too.

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.

it's diluted or concentrated

in a diluted solution, you can add a lot more solute but in concentrated only a little more can be added

answer: 20 percent

the fish culture has become a developing commercial practice in developing countries which is already prominent in developed countries of the world. the major fish protein demand is being supplemented in developing countries is   fishing to meet the needs of the growing population. therefore, in some developing nations, the percentage of animal protein from fish in the diet can be as high as 20 percent.