I personally think that the answer here would be C, all the birds in the forest.
The first 25 birds that were recorded are not the population.
The species with the greatest number is not the population. It is the most thriving species.
The species that are native to the area don't make up the whole population. They make up part of it, not the whole of it.
I hope this makes sense and it helps.
-Toremi
