Whatever it was the falcons that were able to find the more efficient path would have had higher survivals rates and thus successfully passed that ability on to offspring.
Early navigation traveling east / west in the Mediterranean faced the same problem. They did not adjust their compass course because they had used the same compass error to create their charts.
From Hutchins:
"If the cartographer uses a magnetic compass to make a chart and the navigator uses a magnetic compass to determine courses, and if both compasses show the same errors in the same places why would anyone care and how could anyone ever notice that the charts put the land in the wrong place?
Of course mariners who had better charts and better instruments would be at competitive advantage over other less efficient navigators. Same as the falcons only with instruments.