Let's identify Papi's theory. It is introduced in paragraph three. Papi believes that the birds' map sense is based on their sense of smell. They are able to find their way home based on odors in the air.
You asked why D weakens this theory. If the birds' homing ability is truly based on smell, and they are moved beyond the range of familiar odors, we would not expect them to be able to follow a particular odor back home. Yet, D tells us that birds moved this great distance are still able to find their way home. This suggests that some cause other than smell is responsible for their homing ability.
