Birdwatching

​A naturalist was studying a group of birds.

He eventually concluded that they were a family, as he determined a male, a pregnant female and a "grandmother figure" older female.

What he observed aside from their natural behavior, was that the entire family would fly away just before winter and fly back to the same nest just before summer.

This occurred for multiple seasons without fail.

He understood that the reason for this was that this bird family was trying to forego the cold weather for warmth, but he also understood that this was not as much a "reason" than his own rationalization.

If he was the one making judgments about what he saw, how was he to know what the actual reason was for the birds?