What does 'primary projection' refer to in this context?

Answer

The length of the longest primary feathers extending beyond the tips of the shorter tertial feathers

Primary projection refers specifically to the length that the longest primary feathers extend beyond the tips of the shorter tertial feathers located at the rear edge of the folded wing.

What does 'primary projection' refer to in this context?

#Videos

Chiffchaff vs Willow Warbler. How To Tell The Difference. - YouTube

birdidentificationWillow Warbler